Now that we have covered how information travels between the engine and PCM, it’s time to start processing. Internal combustion gasoline engines only operate within a specific window. Things have to happen in the right order with the right amounts of each input for the desired result. Much like baking a cake, there is a recipe for turning gasoline into usable power. Whether you are making yellow sheet cake or triple chocolate surprise depends upon the engine and parts with which you start. Calibration cannot miraculously add airflow. That is up to the mechanicals in which the limit resides. However, just like forgetting to stir the mix properly or setting the oven temperature too high ruins the baked goods, the calibrator has the ability to prevent an engine from running well. Likewise, a good cook can bake the moistest cake with the best frosting design, the calibrator can polish the engine’s performance to perfection as well.
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All internal combustion, sparkignited engines have a few rules that must be obeyed. Combustion only occurs between 8.0:1 and 25.5:1 air/fuel ratios. The closer to the edge of this envelope the engine gets, the more likely misfire becomes. The stoichiometric mixture is 14.68:1 (l = 1), which would theoretically leave the least amount of leftover ingredients. Slightly leaner mixes result in better fuel economy and slightly richer mixes result in better torque. Excess fuel can be used as a cooling agent to limit combustion temperatures at high load.
Peak flame speed is found at l » 0.9. At this point, less ignition lead is required to reach peak cylinder pressure at the same time in the cycle. Adding excess fuel or air slows down the combustion process. When operating under significant power enrichment (l << 0.9), leaning the mix out increases flame speed as we approach l » 0.9 again. Additional fuel enrichment (to ratios below l » 0.9) slows the flame front’s travel, requiring more ignition advance to achieve peak cylinder pressure at the correct time.
Running an engine properly is much like making cookies. If the proper amounts of all components are delivered and mixed correctly, the results are more impressive. (Nate Tovey)
The importance of efficient manifold tuning can be seen in this BMW design. Long individual runners are sized to improve midrange torque of this engine, and the plenum can clearly be seen as well. (Nate Tovey)
The spark initiates the combustion process in normal operation. For this to happen, the spark energy must be high enough to create an arc across the plug gap. As the air/fuel charge mixture density increases, more energy is required to create the electrical arc in the gap. Charge density is directly proportional to engine load, so it is affected by throttle position, port velocity, compression, and manifold “boost.” The wider the gap, the more energy is required to jump it. More surface area of the arc from a wider gap yields better initial combustion due to an increased number of oxygen and gasoline molecules in contact with the arc. Ignition energy is directly proportional to input voltage, coil winding, and coil saturation time.
Engine speed is a result of the difference between loads (friction, accessories, pumping losses, vehicle movement) and output (current engine power, often a result of throttle position). Any time output exceeds the current loads, speed increases. When output is less than current loads, engine speed drops. Holding a constant engine speed requires making just enough power to equal the current loads on the engine. Idle speed is a perfect example of this delicate balance since load changes from accessories and friction as a function of speed must be offset by careful adjustment of engine power from throttling and spark. Low speed idling is one of the most difficult balances for any engine to make. EFI systems can experience significant trouble due to the relatively large amount of time between power strokes when attempting to make relatively small corrections. If the loads on the engine exceed output by enough, stalling occurs.
Spark advance at most operating conditions should be set as close to MBT (maximum brake torque) as possible to take advantage of maximum engine efficiency. It is difficult to damage an engine from over-spark at light load. MBT may make best power at WOT, but it is often wise to retard timing slightly to provide some safety margin and avoid knock. It is not ideal to operate at MBT at idle. Timing should be intentionally slightly retarded from best torque to allow for spark trimming of idle speed. If an engine is idling with a spark advance equal to MBT, it is not possible to add timing to quickly compensate for any added load or drop in speed.
Most engines are perfectly happy to run at stoichiometric (l = 1, 14.68:1 A/F) ratio 95% of the time. The primary exception is WOT performance, where a richer mixture not only makes more torque, but also allows for more spark lead and cooler exhaust temperatures. Once the fuel mixture is right, adjusting spark for best operation becomes much easier. Some vehicles with excessively large duration camshafts simply do not idle well at l = 1, and adding up to 10% fuel can help idle quality. This increase in fuel delivery yields a slight torque increase which helps stabilize combustion, even at light load. These same vehicles are typically fine at l = 1 for cruise, but require larger amounts of acceleration enrichment (AE) to prevent stumbling on tip in.
The bottom line is that once the calibrator has given the engine the right set of operating parameters, vehicle performance should be a direct result of the parts of the system. Too large of a camshaft and the calibrator has little hope of good idle quality. Too small of an intake runner and total power suffers regardless of what air/fuel ratio or spark lead is run. Keep in mind as a calibrator that it is possible to adjust things to a certain extent in the name of driving behavior, but there are limits. Often it’s best to recognize the situation for what it is and either change parts or deal with the less-than-optimal results.
To begin the actual calibration process for an engine, one of the most important steps is to recognize the instantaneous airflow. This airflow can then be processed to determine the necessary fuel delivery to maintain smooth engine operation. The most important core function of any PCM is this airflow modeling, which determines subsequent fuel commands. To this end, the calibrator must create adequate representations in the PCM code of what is happening in the engine’s physical environment. Modeling of the engine airflow is done in one of two methods: Mass Air Flow Measurement or Speed Density Calculation. Either system is capable of properly controlling an engine, has its own pros and cons, and sometimes both are used together.
Mass Air Flow
Mass Air Flow systems rely heavily upon input from the MAF sensor discussed earlier. These systems take the output of the MAF sensor as a direct representation of current engine airflow. This approach makes for very simple and straightforward calculation of engine load and fuel requirements. In this case, engine load (Volumetric Efficiency) can be instantly shown as:
Changes in throttle position simply restrict airflow. Part load (vacuum) is seen by the PCM simply as a smaller mass flow value. Since forced induction engines end up moving more total air mass per cycle, actual boost pressure is not required to calculate load and fuel demands. Knowing exact air mass flow makes fuel demand calculations simple as well:
Fuel flow rate = MAF x (desired A/F ratio)
The crucial point to making mass air systems work properly is that the output of the MAF sensor must reflect reality. This is where the calibrator must spend some time to ensure correlation between actual mass flow and indicated mass flow. A large portion of the calibration process on a mass air system is spent tweaking the MAF transfer function in the PCM to reduce the variation across a wide range of steady state conditions. Making the MAF data more reliable to the PCM forces many other operating conditions to simply fall into place easier.
Any errors between the MAF output and actual engine airflow directly result in inaccuracy in fuel control and engine operation. The MAF must have adequate resolution, range, and repeatability for the system to work properly. This is particularly important when high RPM, high load flow rates have the potential to exceed the measurement range of a particular MAF sensor. In performance applications, it is not unusual to see the “pegged” MAF reporting a constant maximum value to the PCM. In turn the PCM commands a constant fuel delivery against what is actually an increasing airflow. The result is a progressively leaner air/fuel ratio and often detonation. The solution is either a change in MAF sensor hardware or some creative compensation in the PCM tables.
For a mass air based system, the amount of fuel delivered is calculated directly from the MAF sensor input and desired ratio.
The placement of the MAF sensor in the inlet tract can create some challenges. As discussed earlier, the MAF should be positioned such that it sees laminar flow across its element. This often means that some tight inlet routes may not be conducive to proper MAF placement. Many aftermarket engine packages such as “spider EFI” intakes with a single throttle plate under a central air filter do not even leave any room for a MAF sensor. The only way to run a mass air system on these applications is to extend the inlet to a remote MAF and filter assembly, making for a somewhat more complex inlet system. The primary benefit to the mass air system is that any changes in actual airflow that fall within the MAF sensor’s range and resolution can be instantly accommodated by the PCM.
This means that a change to a larger camshaft or higher flowing intake manifold simply show up to the PCM as slightly higher airflow rates, resulting in slightly higher fuel delivery. Mass air systems tend to be very forgiving of relatively drastic modifications in the name of horsepower. If it is possible to cleanly install a MAF sensor, this is the most desirable method of engine control due to its flexibility and accuracy.
The second strategy for engine control is speed density. In this method, airflow is never actually measured. Instead, incoming air mass is calculated based on temperature, manifold pressure, and engine speed, using a reference volumetric efficiency table. The engine’s volumetric efficiency is a constantly moving value based upon RPM, camshaft design, manifold design, displacement, compression, and instantaneous manifold pressure. The main volumetric efficiency table is usually shown as manifold absolute pressure versus engine speed. The values contained in this table represent a volumetric fill percentage for the current manifold pressure and engine speed. The PCM must then take this volume, the MAP value, and inlet temperature to calculate the incoming air mass. High school physics has conveniently shown us the universal gas law: PV = nRT. Using this equation, the PCM can calculate air mass based on sensor inputs, molar density of air, and the gas constant for air. Once the incoming air-mass flow rate is calculated, a corresponding fuel flow rate can be determined. It is up to the calibrator to develop the volumetric efficiency table for each engine combination. Since there may be similar airflow rates at various speed and MAP loading conditions, it is important to find steady state VE values for all regions of the table.
Temperature correction becomes critical in a speed density system. Unlike the mass air systems that have meters compensated for ambient changes, actual intake air density must be calculated in a speed density strategy. Changes in inlet temperature can have a significant effect on actual inlet air mass for a given volume and pressure. Boyle’s gas law is hard at work here.
For a speed density system, air mass must first be calculated from a reference table before proceeding to the fuel calculation.
The downsides to speed density are a reduction in airflow resolution, increased time needed to calibrate, and reduced flexibility. Building the volumetric efficiency table typically takes longer than modeling the transfer function of a MAF sensor. Special care should be taken to ensure that the volumetric efficiency table is a relatively smooth function in both axes. This table is a representation of the engine’s pumping characteristics and is generally smooth with respect to pressure and speed, just like any other pump. Significant step changes to cam timing events, such as activation of VTEC, may dictate relatively large breaks in the RPM curves, but this is the exception.
Most volumetric efficiency tables are 32 x 32 or 16 x 16; many older versions are even smaller. At wide open throttle where MAP values are relatively constant, this leaves fewer adjustment points across the RPM range to model the engine’s behavior. Although this function is relatively smooth, it still leaves quite a bit of interpolation to be done by the PCM between cells. The more cells available to model the engine’s VE, the smoother overall operation can be, but the calibration process takes longer to optimize each additional column or row of cells. Once the base VE table has been constructed at a steady inlet temperature, testing must be done to develop the inlet temperature compensation curves. These temperature compensation curves may vary from the ideal thermodynamic curves due to sensor placement, construction, or reaction times.
One of the largest concerns to the performance calibrator is that speed density systems are based upon the assumption that engine volumetric efficiency remains essentially unchanged at each cell in the VE table. Changing engine components, especially camshafts, intake manifolds, or superchargers, can have a drastic effect on engine volumetric efficiency. Any change that significantly improves engine airflow requires another change to the base VE table to allow the engine to maintain the same relative fuel delivery. Since spark tables are also usually linked to engine load, speed density systems are especially prone to poor operation after parts changes without recalibration. A simple cam change may require several hours of dyno time to remap the VE table correctly. Additionally, if the range of the MAP sensor is exceeded by increasing manifold pressure (super or turbocharging), it becomes necessary to install a sensor with a wider range and rescale the pressure axis of the VE table to accommodate.
Speed density systems are popular because of their relative simplicity compared to mass air systems. By eliminating a fairly obtrusive sensor requirement in the inlet tract, designs become much more open. Exposed air boxes, filter elements, or individual stacks no longer pose a problem for air measurement. Many OEMs employ speed density because of the reduced cost of the eliminated MAF sensor. Although it may take slightly more development work to optimize the VE table, their economics of scale more than make up for it. An even simpler version of speed density calculation used on some raceonly vehicles is Alpha-N calculation. In this strategy, the base airflow map is a function of throttle position and engine speed. Without regard to actual manifold pressure, the calculation is simpler, but lacks resolution to correct many drivability issues or provide decent emissions control. This strategy is best left to dedicated drag cars where the engine’s duty cycle is little more than idling in the pits and WOT down the track.
The PCM only has an output for individual injector controls. Several steps must be performed to get from the desired air/fuel ratio to injector output. Once the PCM knows the incoming air mass, it is possible to calculate the desired fuel mass to attain the target air/fuel ratio. The first basic calculation is to determine desired fuel mass delivery:
[Intake Mass Air Flow (lb/hr)]
Fuel Mass (lb) = -----------------------------------------------------
[(des. A/F ratio) x 120 x (RPM)]
To calculate actual fuel delivery to the engine, one of the following formulas are used: see Figure 6-1 Once a desired fuel mass has been determined, a corresponding injector output can be calculated. The actual injector on time is adjusted based on static flow rate, rail pressure, and injector opening characteristics to convert desired fuel mass into a binary injector output for a fixed amount of time. Knowing the injector flow rate and coil characteristics is critical in this stage. Much like errors in the calculated intake airflow, errors in calculated fuel flow versus actual result in less than ideal operation and drivability. Whenever injectors are changed on a vehicle, it is important to update the injector variables in the PCM to reflect the all-new operating characteristics. This gives the PCM more accurate control over actual fuel delivery rather than simply assuming that a Bosch 24lbs/hr injector and Siemens 60lbs/hr injector behave identically with exception of their static flow rates. While the trend may be similar for both injectors versus voltage, the actual offset can be enough to cause a rough idle or poor transition behavior on tip in. Keep in mind that changes in fuel rail pressure or delta pressure across the injector have an effect on static flow rate, but not on the opening characteristics.
With accurately modeled airflow and fuel flow, calibration of transient conditions becomes much easier when the tuner no longer needs to chase his tail on small fuel calculation errors. The calibration process itself should be more focused on moving a desired and actual air/fuel ratio simultaneously rather than trying to make them converge on the fly. Worse yet, attempting to reach an actual air/fuel ratio by commanding a different desired ratio makes for a frustrating exercise when trying to calculate minor changes. Calibration is much easier when a 1% change in airflow comes with a 1% change in fuel flow.
Closed loop systems constantly monitor engine gas outputs and adjust future fuel delivery accordingly. This allows the PCM to learn trends in engine performance, maintaining proper engine operation and emissions.
The timing of the injection can have a pronounced effect upon emissions and torque. This sample shows fuel being injected prior to intake valve opening in order to allow for complete evaporation before ignition.
The time at which the fuel is injected during the cycle can also have a pronounced effect on engine performance and emissions. Most OEM sequential EFI systems are timed such that fuel is injected against a closed intake valve slightly before opening. This allows the fuel to evaporate for better mixing and cools the valve simultaneously. The trick is to time this such that the evaporated fuel does not have time to travel back up the manifold runner before the valve opens. Many aftermarket “tuners” make the common mistake of trying to align injection timing directly with intake valve opening. Injecting fuel directly into an open valve can have the negative effects of poor mixing due to lack of evaporation. Low air and engine temperatures make the problem worse, as evaporation is slower at reduced temperatures. The unburned fuel and generally poor combustion yields a loss of engine torque along with excessive hydrocarbon emissions. After prolonged operation, improper injection timing may even lead to washing the oil coating off the cylinder bore and premature engine wear or poor ring sealing. The loss in torque alone from poor evaporation should be enough incentive to the performance calibrator to seek proper injection timing.
Picking a Ratio Now that we have accurate control of the actual air/fuel ratio, choosing what that ratio will actually be for the engine is far easier. The next question becomes, “What air/fuel ratio is desirable?” The answer is, “That depends.” Air/fuel ratio has a significant impact on emissions, fuel economy, idle quality, and power.
As mentioned earlier, stoichiometric mixture leaves the least amount of leftover primary reagents for the catalyst to process and generally gives good economy. Overall emissions are more involved due to the fact that different byproduct amounts vary with respect to air/fuel ratio. Hydrocarbon and CO emissions generally decrease with leaner mixes, with most significant reduction at l = 1, and above. Because of the higher burn temperatures NOx emissions increase significantly above l = 1. Although these leaner ratios can help fuel economy at cruise, modern emissions standards often prevent continued operation above l = 1.
As mentioned earlier, idle is often best done at l = 1 for a number of reasons. Fuel economy and emissions are good, but there is still room for torque control (and consequently speed control) through changes in fueling from stoichiometric if necessary beyond the normal adjustments from the IAC or spark. In cases where camshaft design is too aggressive to support a stable idle at l = 1, slight enrichment often stabilizes things. There is a practical limit to this enrichment from the potential for bore wash. If the idle mixture is too rich, fuel literally washes the walls of the cylinder clean of oil. Bore wash is most pronounced at idle due to relatively low intake port velocity combined with constant injector spray intensity. Without a large enough air charge to disperse and dilute the fuel spray, much of it lands on the opposing cylinder wall, rinsing off the oil layer. The result is excessive ring wear and eventual loss of compression due to blowby. Although the engine often idles smoothly as rich as l = 0.7, anything richer than l = 0.85 runs an increasing risk of bore wash, depending on port design. Conversely, an excessively lean condition at idle tends to exhibit a surging speed as torque changes when the leaner mix brings the engine closer to MBT. Although far less healthy for ring durability, the richer condition at idle is more stable and therefore helpful when attempting to initially calibrate idle speed without stalling. It is not recommended that the engine be allowed to operate at an excessively rich idle for more than a couple minutes without clearing any built up fuel deposits.
At wide-open throttle, the primary concern becomes torque output. Remembering that a slightly rich mixture of l » 0.91 (13.2 to 13.4:1 air/fuel) yields best torque, this becomes a good starting point for WOT air/fuel targets. Some OEMs continue to target l = 1 at WOT to improve emissions, and are knowingly sacrificing potential power to do so. In many cases, knock limits or exhaust temperatures dictate that some additional fuel must be added at WOT. Richer mixtures burn cooler and consequently allow for more spark advance at the same load. These cooler burning rich mixtures also reduce the amount of heat to be absorbed by the cylinder head, piston, and valves. A cooler head and piston are less prone to preignition due to hot spots. Valve and catalyst durability is greatly reduced if temperature increases too much, so high mileage warranty concerns often drive OEM fueling requirements at WOT. For this reason, many OEM vehicles sold today approach l » 0.82 (12.0:1 air/fuel) or richer once a catalyst protection threshold has been reached. Unlocking control of this protection circuit can be the key to properly calibrating such vehicles.
Adding extra fuel is a quick method of maintaining power output without reducing spark advance… to a limit. A little bit of extra fuel can go a long way toward providing a knock safety margin for the occasional bad tank of fuel. Many forced induction engines can simply not operate above l » 0.85 without detonation due to the extra heat present in the intake charge as a result of the primary compression stage of the super/turbocharger. It has been my experience that most properly designed forced induction engines exhibit excellent power with good durability at approximately l » 0.8 (11.7 air/fuel ratio). This usually leaves enough cooling effect to allow for slight fuel and weather changes if calibrated at least 3 degrees away from the spark knock limit.
Going too far in this direction can also have a negative effect. Excessive over-fueling can result in rich misfire conditions or fuel burning off in the exhaust system, increasing EGTs. In extreme cases, fuel collects on the spark plug fouling the leads and hampering ignition performance.
Transients and Modifiers
To more accurately predict the dynamic fuel needs of an engine, EFI systems employ a model to predict the volume of fuel trapped on the manifold wall. This film is known as “Tau” (t). Since no current port injection system has the ability to spray 100% of its fuel into the cylinder from a single injector pulse, part of this spray collects on the walls of the intake port. Incoming air flowing past this “puddle” of fuel on the port walls carries part of it away into the cylinder as well. The more volume of air flowing past the t puddle, the more this puddle is being decreased in mass. Tau modeling systems use instantaneous airflow predictions to attempt to maintain a constant volume of t. This means that increases in airflow require an increase in fuel to maintain constant t volume and decreases in airflow allow for a decrease in fuel delivery to maintain constant t.
Once the engine is performing well under steady state conditions, it is time to look at transients. During tip in, there is a temporary rush of air to the manifold and cylinders. This spike in delivered airflow must be accompanied by a matching increase in fuel delivery to compensate for the loss of wall film. Without this additional fuel increase, the engine experiences a temporary lean condition that is felt by the driver as a loss of torque or “lag” of power onset followed by a surge to what is normal power for that throttle position. The larger the camshaft or higher the supercharger boost, the more pronounced this effect can be. On a carburetor, the accelerator pump is mechanically actuated by primary blade movement. In EFI systems, a separate “acceleration enrichment” (AE) function is often used to model the necessary temporary fuel increase. The net result of this function is either an adder to the delivered fuel pulse or an additional asynchronous pulse during the following cycle. EFI systems have the benefit of being able to make the AE function variable with respect to throttle position and temperature. Initial throttle opening from closed makes a bigger difference in airflow and requires a larger AE compensation than going from 70 to 100% throttle. Cooler engine temperatures also require slightly larger amounts of acceleration enrichment.
Not all fuel injected into the engine goes directly into the cylinder. A small amount collects on the port walls only to evaporate later. Keeping this amount of evaporation from the walls consistent is the function of the acceleration enrichment logic. (Nate Tovey)
Extra fuel is added to the engine at cold temperatures to offset the reduced evaporation rate.
When the driver lifts completely off the accelerator pedal at higher engine speeds, it is obvious that no engine torque is required. To help slow the engine down, many EFI systems employ “deceleration enleanment (DE). DE is a condition where some or all of the fuel is temporarily shut off to the engine under closed throttle. Without any fuel to burn, the engine is left with pumping losses that exceed the (reduced) power production, causing much quicker deceleration of the crankshaft. The engine speed drop can be harnessed through the transmission to slow the vehicle as well. A side benefit that many OEMs enjoy is that DE reduces fuel consumption. The only trick to properly using DE is the transition back into normal operation. Care must be taken to ensure that normal operation is resumed at a high enough speed to prevent stalling. Additionally, some hysteresis must be built into the system to prevent rapid changes between normal operation and DE at low throttle cruise conditions. Reducing fuel delivery during light deceleration also prevents excessive buildup of wall film that would otherwise lead to an overly rich condition when returning to part throttle operation.
Pumping pure air through a catalyst that has recently been deposited with unburned fuel during a WOT run can lead to large exothermic reactions taking place, increasing mid-bed temperatures and shortening catalyst life. The closer the catalyst is to the cylinder head, the greater this effect is. Some high output supercharged OEM applications actually employ enrichment during deceleration to improve close coupled catalyst cooling. The richerthan- stoichiometric mixture burns at a lower temperature and excess fuel can continue to cool downstream exhaust components.
Other more complex systems employ an intake manifold model to predict actual air delivery to the cylinder as throttle position changes. Depending on manifold volume, port design, and sensor placement, it can be two to three cycles before instantaneous MAF sensor changes can be processed into effective injector output changes. Although this is practically transparent to drivers, increasingly stringent emissions standards are forcing OEMs to split hairs to find incremental improvements in lambda control. The intake manifold model is constantly anticipating airflow to each individual cylinder on every cycle. This anticipated airflow value can be verified and updated during relatively steady state conditions and is often used as a rationality check for MAF and MAP sensor performance.
During extreme temperature conditions, it becomes necessary to adjust lambda targets to improve drivability or durability. Extremely cold conditions make fuel evaporation slow and difficult. To make sure that a consistent amount of evaporated fuel is available for combustion, some excess total fuel mass must be added until temperatures normalize. This often means that a significant amount of unburned fuel and HC emissions are passed through the exhaust. Although this is in direct opposition to good emissions performance, the vehicle can sometimes be literally undrivable without some degree of cold enrichment.
Hot ambient conditions reduce the engine’s cooling capability and the hotter associated intake charge temperatures are more prone to detonation. Increasing fuel delivery reduces the total amount of heat absorbed by the engine during combustion and can be used in a limited capacity to provide for a “hot limp mode.” Since hotter intake charges lead to faster flame speeds, a little extra fuel cooling of the charge reduces chances of detonation at high temperatures. Because of their susceptibility to detonation, forced induction engines often have aggressive fuel compensation at high temperatures.
During startup, the intake manifold and combustion chamber are relatively cold. Much like during cold ambient conditions, fuel evaporation is relatively slow. In order for the engine to run, fuel must be evaporated prior to ignition. If only a portion of the fuel injected into the engine has evaporated, the result will be a lean condition at the time of ignition. EFI systems offset this problem by adding a larger total quantity of fuel to the engine, knowing that only a portion of this fuel delivered actually burns. This extra fueling results in the correct air mass to fuel vapor mass ratio inside the combustion chamber when the engine is cold.
As running time passes, the engine warms up and a larger percentage of the fuel being injected evaporates prior to ignition. The amount of excess startup fuel being added can be decreased to zero. This amount of cranking fuel and startup enrichment should decrease with respect to engine temperature to avoid flooding on hot restarts.
Written by Greg Banish and Posted with Permission of CarTechBooks
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