Q: I have a 1988 GMC pickup with a 350-cubic-inch engine. I'm having problems with the thermostat. The truck runs great without the thermostat...

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Q: I have a 1988 GMC pickup with a 350-cubic-inch engine. I’m having problems with the thermostat. The truck runs great without the thermostat. But, once I install one, the truck overheats. I’ve had two new ones installed.

A: There can be many causes for an engine to overheat, and a faulty thermostat is certainly one of them. But since you’ve renewed this part twice, we should be able to scratch it off the list, assuming they were properly installed.

Thermostats regulate engine temperature by increasing or decreasing coolant flow, as needed, between the engine and radiator. Removing a thermostat dramatically increases coolant flow, often masking the actual problem. Plus, overcooling increases wear and emissions, hurts fuel economy and can cause damage. You are wise to pursue a proper fix.

Start with a cooling system pressure check. You’re looking for an external or internal leak. Even a small leak can cause problems, as the system is unable to hold pressure, reducing the boiling point of the coolant.

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External leaks are fairly simple to spot. Possible causes of an internal leak are:

• A failed intake manifold.

• Or a leaking head gasket.

Milky engine oil is one sign of internal leakage, and a carbon monoxide test of the vapors escaping the removed radiator cap is another.

If the system tests tight, one of these coolant circulation problems is next on our list:

Worn or loose water-pump impeller.

• Collapsing lower radiator hose.

• A restricted radiator.

• Clogged coolant passages in the engine are possible, but rare.

Comparing the temperature of the upper and lower radiator hoses (the in and out) as well as before and after the thermostat can provide useful clues. A non-contact pyrometer (infrared temperature gun) is a neat way to perform these tests.

A radiator is only as good as the air flowing through it. If the overheating condition occurs primarily at lower vehicle speeds, check for:

• A faulty fan or missing/damaged fan shroud.

• A faulty thermostatic clutch.

• A buildup of debris on the radiator’s fins or the A/C condenser mounted directly in front of it.

• A damaged or missing plastic air dam beneath the front bumper (not an issue on your truck).

Even more possible causes:

• A restricted exhaust, due to catalytic converter failure or a kinked or collapsed exhaust pipe.

• Incorrect ignition timing.

E-mail Brad Bergholdt at under-the-hood@juno.com