I just bought a 97 STS and it overheats on me whenever I accelerate hard at high speeds. If driving normal or at park, I can accelerate as much as I want and it drives fine, but as soon as I accelerate at high speeds it overheats immediately. I flushed all the fluids, got a new radiator, new thermometer, new sensors, tested the engine for cracks or leaks, and it still does the same. Has anybody had similar problems or have any idea what it might be?

 

Have you checked for hydrocarbons in the coolant?(head/gasket problem)

 

Yea, that was the first thing we did. By now we have done it a few times and it comes out clean every time. A mechanic said that it might be coming out clean because maybe the crack or leak is on the exhaust side and not the intake side of the engine

 

How about the waterpump impeller?

 

check your heater core hoses when your car is at operating temp. if one is much hotter than the other one your heater core is plugged. I had 1998 deville 4.6 that no one could solve the overheating problem in after changing water pump, radiator, thermostat, had it flushed it still over heated. everyone said it was more than likely the head gasket, but it wasn't using fluid. as my last resort I unhooked the lower heater core hose and bypassed the heater core to see if it would solve my problem, IT DID ! For some reason if the heater core is plugged it doesn't allow the coolant to circulate properly in the engine, causing it to overheat. You can bypass the heater core by removing both hoses off of the heater core and connecting them together with copper pipe or plastic coupler, it's the easiest way to see if it solves your problem, if it does then replace your heater core.

 

read above posts - they have merit.

regarding the hydrocarbons ...
how did you test for leak of gaskets? i be getting the dye test in the coolant
and running the car like you said you do when it overheats,
[align=left] pull over AFTER it overheats
[/align]and check the coolant for change in COLOR.
THEN you will know if it is a yes / no

engine pressure changes not only with rpms but with LOAD - WIND LOAD ...
gaskets may do fine on the driveway, even at 4,000 rpm, not much demand on
power there but push the 4,000 lbs into the wind, now we got real loads.


look at the FAQ Forum
HEADGASKET LEAKS

https://www.cadillacforum.com/m_7193/tm.htm