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Is Your Chevy 348/409 Overheating?

Overheating can come from a number of sources. It’s all about transferring heat away from the system and related parts and assemblies. On 348/409 engines, users  have reported these problems and solutions. Brad from Show Cars says, “The most common problem is the use of a small-block fan shroud, or no fan shroud at all. The 348/409 fan shroud is shorter. To get the best results, it should cover half the fan blade.” Brad goes on to list over-painting the , car or truck conversion from a smaller engine, fan clutch, water pump, piston/cylinder clearance, running a leaner fuel mixture, incorrect timing, bad gas, and even the use of a bad thermostat. All of these ailments are fixable, with many being small, afternoon projects.



This tech tip is from the full book, HOW TO REBUILD & MODIFY CHEVY 348/409 ENGINES. For a comprehensive guide on this entire subject you can visit this link:


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When it comes to a radiator with too much paint, the biggest problem is the paint acting as insulation and not letting heat transfer take place. This is especially true of older radiators that have paint that is lead based. Much like the lead used in soldering a copper/brass radiator, lead in paint reduces the effectiveness of heat transfer that cools the liquid inside. Stripping off the paint is the solution. Sandblasting is not recommended as it takes off too much of the softer base materials. This is true in copper, brass, and aluminum. Chemical dipping is the best solution. Even if the paint is stripped off at home instead of by a professional paint stripper, the biggest concern is cleaning out all of the removed paint from the many small openings between the fins. Clogged fins are another reason for overheating and it is just as bad when pieces of paint cause clogging.

When swapping engines, installers need to check out the radiator to make sure a bigger radiator is used when a bigger engine is dropped in. A simple check of capacity and size is all it takes and if needed, it might be a good time to look at an aluminum radiator as mentioned earlier. The simple cure is to use the radiator that came with the engine whenever possible. Most susceptible to such problems are cars that came with six-cylinder engines receiving not only V-8s but bigger V-8s as well. Likewise, upgrading the cooling system when increasing horsepower should be factored into the build.

Cooling System Pressure

Mechanical parts failing such as the fan clutch, water pump, radiator cap or thermostat can cause almost instant overheating. Fan clutches disengage the fan when a predetermined engine speed is reached. Symptoms are usually overheating at slower speeds but not at highway speeds. A simple test is to turn the fan when the engine is off. If it freely spins, chances are the fan’s clutch is defective. The water pump is another mechanical component that can fail and cause overheating almost instantly.

This state-of-the-art radiator cap is just as important as the rest of the cooling system. It contains a magnesium sacrificial anode designed to attract and absorb electrical energy that may cause electrolysis and shorten the life of the coolant. (Photo Courtesy Northern Factory)


There are many ways to check if a water pump is bad. One is to watch the temperature gauge after removing the thermostat. If the engine takes a long time to warm up, the pump is good. Another way to check a pump is to remove the belt that drives the pump and feel it as you rotate the pump. If there is excessive movement in the shaft or if there is more movement or restriction than normal, it is likely the pump is bad. Yet another way is to check how the coolant acts when drained out of then returned to the radiator. If the coolant level drops when you start it up, that indicates a good pump. A bad pump would not allow the coolant to circulate and fill the voids in the block causing a drop in the level of the radiator. A bad radiator cap can also contribute to overheating if it does not maintain the proper pressure. Caps can be tested for their pressure rating and are economical to replace.

A bad thermostat can cause overheating, and Show Cars recommends, a high-flow thermostat. Operating an engine without a thermostat keeps the engine from getting up to the proper operating temperature. A bad thermostat can be indicated by the engine temperature and how it never gets up to normal operating temperature. This can be partially confirmed by checking the outside temperature of any hoses on a cold engine just after you start it. If the hose from the thermostat is not sending hot water back to the radiator, it’s likely a bad thermostat. A bad thermostat can also cause overheating if the thermostat’s diaphragm is broken and stays shut. This does not allow the flow of heated coolant in the block, causing temperatures to increase quickly.



An in-line cooling system filter is easy to install and allows the coolant to be inspected via see-through glass. It can be installed in a heater hose line much like an in-line fuel filter. This one by Northern Factory is reusable. (Photo Courtesy Northern Factory)



Internally, there are engine considerations that can contribute to overheating. Building the engine with a piston-to-cylinder wall clearance that is too tight will, among other things, contribute to friction and that equates to heat. In that same area, many people suggest the chome-moly piston rings run cooler than cast rings. When setting fuel mixture rates, a lean burning engine can cause it to run hotter. Likewise, improper timing, even when not combined with a lean fuel mixture, can cause an engine to run hot. A more radical cam and even the timing of that cam can affect power and therefore, heat.

Proper control of the cooling system comes down to removing the heat the engine creates when it makes power.

Written by John Carollo and posted with permission of CarTech Books






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