99 deville,run normal,overheats when a/c is turned on over 250 degrees ,changed thermostat and coolant sensor,still overheats.

I am sure it is a thermostat for a P2181

How hot does it get in degrees F? Does the radiator fan work? Some fans won't turn on till about 220 degrees F.
Could have blockage in radiator or engine block? Check the water pump. Your serpentine belt ok? Could be gasket issues? Any coolant in the oil or oil in the coolant. Check your coolant temp sensor. Don't replace anything unless your testing points in that direction. Are you losing coolant?

How hot does it get in degrees F?
Have you ever changed the thermostat? Can you checK coolant flow through the radiator? Check for combustion gases in the coolant? Does it overheat going down the highway or at idle?

Hi there:
The engine coolant temperature sensor is a variable resistor with a range of -40 degrees F-265 degrees F (-5 degrees C-129 degrees C).

The engine coolant temperature sensor provides an input voltage to the PCM. As the coolant temperature varies, the sensor resistance changes resulting in a different input voltage to the PCM.

When the engine is cold, the PCM will demand slightly richer air/fuel mixtures and higher idle speeds until normal operating temperatures are reached.

The engine coolant temperature sensor is also utilized for control of the cooling fan.

Fig. 1: Engine coolant temperature sensor location - 3.3L and 3.8L engines



Check this test procedure:

1. Turn the ignition switch to the OFF position.
2. Detach the coolant temperature sensor electrical connector.
3. Using a DVOM set to the ohms scale, connect one lead to terminal A and the other lead to terminal B of the coolant temperature sensor connector.

1. With the engine at normal operating temperature, approximaterly 200 degrees F (93 degrees C), the ohmmeter should read approximately 700-1000 ohms.
2. With the engine at room temperature, approximately 70 degrees F (21 degrees C), the ohmmeter should read approximately 7000-13,000 ohms.
3. If not within specifications, replace the engine coolant temperature sensor.
4. Test the resistance of the wiring harness between PCM terminal 26 and the sensor wiring harness connector. Also check for continuity between PCM connector terminal 43 and the sensor wiring harness connector. If the resistance measures greater than 1 ohm, repair the wiring harness as necessary.

Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.

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250 DEGREES ENGINE GETTING TOO HOT.SOUND LIKE CLUTCH FAN FREEWHEEL.IF CLUTCH FAN DONT LOCK IN OR HAVE RESISTANCE TURNING BY HAND WHEN ENGINE HOT.FAN CLUTCH IS BAD.IF FAN CLUTCH OKAY COOLANT LEVEL CORRECT.I WOULD CHANGE THERMOSTAT FLUSH OUT ENGINE AND RADIATOR.IF ENGINE TEMPERATURE STILL READS 250 DEGREES OVER FLOW JUG OR RESERVOIR NOT BOILING OVER THE COOLANT TEMPERATURE SENSOR FAULTY.

250 TOO HOT FOR ENGINE AND TRANSMISSION. SOUND LIKE COOLING FAN CLUTCH BAD ITS FREEWHEELING NOT LOCKING UP TO DIRECT DRIVE TOO COOL ENGINE OFF BECAUSE THE FAN CLUTCH THERMOSTAT NOT WORKING THE HOT SILICONE OIL CAUSE IT TO EXPAND TO PUT RESISTANCE ON CLUTCH TO PUT DRAG ON FAN CLUTCH CAUSING IT TO LOCK IN GIVE IT DIRECT DRIVE.I WOULD STILL REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP IF TEMPERATURE STILL 250 DEGREES YOUR RADIATOR AND COOLANT SYSTEM NEED FLUSHING.

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.
If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.
The hose goes in to the engine to a crossover and comes out the other side close to the thermostat housing,the blockage may be there.
Also note that the cooling fans don't come on until the coolant temperature hits around 225 degrees.
During normal highway driving the fans will very rarely come on(if at all) due to constant air flow through the radiator.They will only usually come on in heavy traffic conditions on a hot day.
But......if you have the Air Con switched on, the fans will run all of the time.

fan does not generally come on till you reach about 210f. second fay comes on when ac compressor is on. Allowing for gauge inaccuracy this does not sound like a problem. Have you checked for coolant loss inside or outside the engine? If coolant is low, it may not be reaching the sensor unless you make pump run faster. Those engines are known for head and intake gasket problems, which may be the underlying cause of what you described. A partilally obstructed radiator can give some of the symptoms but I don't thind that is it.

did you try replacing the radiator cap and the temp sensor?

If you are not loosing coolant on the ground. You may have a problem of a bad intake manifold gasket or worst a bad head gasket. The two things to look at are coolant in your oil, manifold gasket. Coolant out the exhaust, head gasket.