Overheat when AC is on


Old 08-15-04, 08:15 PM
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Overheat when AC is on

I have a 1995 GMC Sonoma truck with shift stick and 1.8L engine. The engine overheats when air condition unit is on. The following are my diagnostics:
- Check engine light does not come on.
- It overheats only when air condition (AC) is on, and backs down when AC is off => means that thermostat is working properly.
- The coolant is full. When the engine is near overheating, I twisted half-way the radiator cap, the coolant is gushing out. This means that there is pressure in the coolant system, also means that the water pump is working (?).
- The belts, and hoses seems o.k.
- I did not notice significant leaks in the coolant.
- When the fan/water pump is turning, I did not notice any unusual noise => water pump bearings are o.k. (?).

Any suggestions what may be wrong and what to check ?
Thank you very much,
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Old 08-15-04, 08:50 PM
Join Date: Dec 2001
Posts: 2,538
check the fan clutch when the engine is warmed up place a rag up against your a/c condensor the fan should be flowing enough air to hold the rag against the a/c condensor if it doesnt I would suspect a bad fan clutch.
check radiater condition if partially restricted can reduce coolant flow and cause overheating usually only under heavier engine loads.
waterpumps usually fail by starting to leak and or bearing noise or excessive play removing the radiater cap and seeing fluid gushing out does not mean its working waterpumps very rarely cause overheating problems unless they are leaking coolant.
the thermostat has nothing to do with the a/c and just meters coolant flow regardless if you are using the a/c are not, and you may want to change it just to rule out the possibility of a sticking thermostat.
Old 08-28-04, 10:31 PM
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Overheat when AC is ON

As far as I can tell, there is only one fan. Once I have the engine running (with or without the air condition ON), the fan is always turning.
The fan belt tension appears to be fine.
I checked the external of the radiator, did not notice any significant object/material blocking the radiator.
I have the radiator flushed and pressure/leakage checked by a mechanic, it was fine. I then changed the thermostat, as well as the top hose connecting to the radiator.
It seems to improve, such that the overheating now is only up to about 75% of the scale instead of previously going all the way up to the red zone.
If the radiator is flushed and pressure checked, does that means it is o.k ?
I would like to improve it further that the temperature stay less then 60% of the scale.
Do you have any suggestions what can I do next ?

Thank you very much,
Old 08-28-04, 11:11 PM
Join Date: Dec 2001
Posts: 2,538
flushing and pressure testing the radiater does not necassarly mean it is ok the cores in the radiater can become restricted or completly blocked by deposits that flushing will generaly not remove which usually requires replacement of the radiater or it to be taken apart at a radiater shop and rodded out where they run a rod through the cores of the radiater to make sure thay are free flowing they may not be able to do this with all radiaters as many are plastic and aluminum.
the fan will turn when engine is running but it is attached to a fan clutch that causes the fan to slip and run at lower rpm than the engine and the amount of slippage will vary with engine temperature on a warm engine it slips less on a cold engine it slips more the clutch can go bad and cause it not to increase airflow when needed, or sometimes they will lock up and cause excessive noise and early pump failure, but wouldnt cause a overheating problem. which is why you should place a rag against the a/c condensor on a warmed up engine to see if the fan is moving enough air to hold the rag against the condenser, if it isnt the rag will just fall and not cling to the condensor.
you should see a shop where a scan tool can be used to monitor the coolant temperature to see if its actually overheating as the guage just isnt an accurate way of determining the engine temp and they would better be able to tell you if the temp is normal or on the hot side and what would likely need replaced to lower the engine temp if needed.

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