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Make sure the car has a proper 50/50 mixture of coolant and water. Use an antifreeze tester.
The cooling system on the 3.5 has an odd thermostat design that requires hot water returning from the heater core to open the thermostat. A clogged heater core can prevent the thermostat from opening and cause the car to overheat.
Flush heater core separately and recheck the car. If the problem improves but comes back after flushing the heater core then replace the heater core.
I have seen car huslers that drill a 1/8 hole in the thermostat body to improve thermostat operation. This is a desparation move and may have a negative impact on the cooling system by making the engine run to cold.
A bad head gasket can also cause the car to overheat. The best method to check for a combustion leak is to use a block tester, also known as a combustion leak tester, to determine if you have exhaust gases in your cooling system. A combustion test kit can be found at your local NAPA auto parts store. The part number is(NNN) NNN-NNNN The price for this test kit is $50.00. Exhaust gases in your cooling system can suggest a head gasket leak, a cracked block, or a warped head, etc. To do the test, add the blue detector fluid to the (block-tester) plastic container according to the directions, and place it onto the radiator filler neck. The squeeze bulb is placed on top of the reservoir and squeezed repeatedly squeezing the bulb will draw air from the radiator through the test fluid. Block tester fluid is normally blue. Exhaust gases in the cooling system will change the color of the fluid to yellow, indicating a combustion leak. If the fluid remains blue, exhaust gases were not present during the test. The vehicle should be started and gotten hot with radiator cap off and coolant level 3-inches below full before performing the test.
If fluid turns yellow, remove cylinder heads, have them pressure tested and resurfaced if needed, them put it back together with all new gaskets. Head gasket failure on the 3.5 engine is a commom problem. Confirm with block test.
There is no easy way to be sure. The best way to know is to flush the heater core and see if there is improvement of the overheatng problem. If there is improvement after the flush then the core was clogged. Another way to test it would be to see how much water passes through the core but since there is no way to acurately calculate the flow rate this would be subject to your judgement. If you had an infrared temperature run you could measure the themperature of the heater hoses. You would expect to see a 10 degree drop in temperature maximum between the suppy and return heater hose with the heater fan running at high speed. A greater than 10 degree temperature difference between the two hoses indicates the core is restricted. The return hose that runs from the heater core to the thermostat housing needs to register a temperature of at least 192 degrees or more when the engine is fully warmed up.
You really need to do both tests because a bad head gasket can mimic the signs of a clogged heater core by filling the core with combuston gas and preventing the flow of water through the core.
Step one, flush the heater core, step 2 combustion leak test the engine.
That is incorrect. A combustion leak can not be determined by a compression test. A chemical test is needed as outlined above.
Yes, make sure you simply do not have an air pocket stuck in the system, if air becomes trapped then it will continue to overheat untill all air is out of the system, remove the radiator cap and I like using a coolant filling funnel and then start the engine with fluid in the funnel and turn the heater in the car on, then just keep filling untill it no longer burps, sometimes this can take awhile to preform. If the system has good flow then as you stated you replaced the radiator,water pump and thermostat, then you can put a hose into each side of the heater core to flush it and then you can tell if the core does not have good flow. Other then that do a chemical test on the system.
You should also call the GM dealer with your vin number and see if there is an updated thermostat housing for your car. There was alot of trouble with the coolant flow from the heater core issue and they made a change in the thermostat housing to address some of these concerns. I would still do the block test even though the heads have been done just to be certain something did not go wrong.
When the radiator fans come on at high temperatrure is the air coming out very hot?
Does the car produce hot heat all the time or does it come and go?
I forgot to ask, but do the fans turn on at all like they should?
Also check the condensor in front of the car and make sure it is not full of bugs and sludge.
Will it only run hot/overheat while driving or will it also overheat while at idle in park?
You may want to try drilling a tiny hole in the thermostat (1/8-inch) and putting it back together just to see what happens. At this point what do you have to loose.
True enough but it would provide interesting diagnostic information if the problem went away. The hole makes a bypass at the thermostat which helps it open if the heater return flow is insufficient. Seen this done on lots of 3.5 and 4.6 V-8's.