hawkeye1956

My son has a 95 Grand prix 3.1 with a rebuilt engine done at 135,000 miles. (Thanks to GMs notorious internal leak of intake manifold gaskets on the 3.1). Everything was replaced at rebuild and he has only put 8k miles on it since. With a 195 degree thermostat, the temperature guage runs at 165 on the highway; 200-210 during normal city driving, but can climb as high as 230 while idling for extended periods in traffic. Any ideas on why the wide fluctuation in temperature readings? I have asked him if the driverside fan turns on when it goes to 230 but havent gotten an answer yet. I know it comes on in normal driving as I have seen it running. What would be considered a safe high end temperature? Does running as low as 165 cut into gas mileage? He is only getting about 22mpg highway and I know it should be better. I appreciate all replies.

TheNewShape

I have the same car and the same problem. My temperature cant seem to make up its mind and shows that it about to get to hot. According to my mechanic, my temperature guage is old and messed up. It needs to be replace. My guage reads 15 to 20 degrees off. So maybe your guage is just too old and needs to be replaced.

hawkeye1956

Thanks for your reply. Nice to know I am not the only one with this problem. In my case, the guage is a new externally mounted guage which was necessary because I no longer trusted the in-dash guage. This was replaced at the time of the engine rebuild. The indash instrument panel would have costed an arm and a leg.

twstice

I have this same problem on my 95 GTP with the 3.4L DOHC. I think im just gunna replace the thermostat and see if its fine after that. Let me know if u figure it out.

Sp3cops70

I think bigl or kustomkid said on a previous thread that it was normal for it to do that......

jon95se

i have a 95gp and mine only goes as high as 210.
i think he should really check the fan relay or the motor for that fan, cause you may be hearing the fan when the a/c kicks on when you have your heat on defrost. i would definately get that looked into. if anything, you can get it tested to see if the guage is accurate.

hawkeye1956

Thanks to all who have replied. In my case it is definitely not the thermostat. What is interesting is that I have seen several posts where people say 225 is normal and several others say that their GP runs at 210. I saw on one post where someone said that the driver side fan is not supposed to turn on till the temp hits 225, which is exactly what I am seeing in my particular case. The fan stays on just long enough to drop the temp about 5 -10 degrees then cycles off till it gets back to 225. Of course if this is by design, everyone should see their GP run at 225, especially in warmer weather. The inconsistency is what is driving me nuts. Perhaps this is dependent on the year of the GP or engine size? A local mechanic told me that these cars are designed to run hotter to help reduce emissions. Don't know if I buy that one or not but 225 seems borderline hot to me.

In most cars, turning on the AC tends to increase temps but in this GP, the second fan turns on and actually drops the temp which is our workaround for the time being.

The less important part of this problem is why on earth this same car would run on the highway at 165 degrees? This would imply an open thermostat but the car warms up quickly and blows good heat in cold weather. Thanks again for all replies!

jon95se

simle. it stays colder on high way speeds for the sake of more airflow. ever clean the area out between the air conditioning and radiator?

hawkeye1956

I will certainly have a look between radiator and AC cooling. I never really thought of that. Thanks.
Even at highway speeds, I didnt think the radiator would play a big part until the thermostat opens up at 195 degrees, then open and close as needed to keep it there. Perhaps once it reaches temp, it never really closes again after that?

jon95se

one more thing, get the radiator cap tested. as theat may not be holding the right pressure. ran into that recently as well.