I have changed the antifreeze and thermostat heats up 2 notchs at 75 mph Help

 

Sounds like a possible headgasket. It was common on the earlier northstar engines and not as common on the newer ones but still does occur. It is a hard one to diagnose. I had 2 97 caddys both with bad headgaskets. They would heat up the most on the highway. Sorry.

 

no .. nuh .. maybe ...
answers may be forecoming in our FAQ section under the subject topics.



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So, do you notice antifreeze leaking from anywhere? This problem could be a quite a few things and it would help to know as many details as possible, like, what is the ambient temperature outside, how many miles are on the vehicle, is there a discoloration of the engine oil, does the heater give out good heat, are the radiator fans working, etc.

 

No leaks. 97000 miles. 85 degrees car has been in Florida all its life never had this problem. Oil is fine. Fans are working.

 

Was it running that warm BEFORE your coolant change?

May I assume that by "no leaks", you also mean that the coolant never needs refilling?

Did you burp the system?
A water pump with air/liquid mix cant do its job at high speed as it burden by cavitation (air/gas and liquid mix).

IF you headgasket was failing, I believe that the overheating at that speed would climb fast and into the danger zone over 250F, not stop at the two notches over the middle. However, if you are concerned about that, do the exhaust test or the compression test, or both.

One last note .. with all the talk of WARPED heads over the years, I have yet to actually see one on a V8 Northstar.

Again, have you read our FAQ on the subjects?


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How do I burp the system???????