Grand Prix Running Hot

[duke_winchester]

Sorry to post a new thread on this, but I thought it would be best. I have a 97 Grand Prix GTP with 169000 miles that last week threw a code for low voltage input from the engine coolant sensor. I replaced the sensor and the code went away, but ever since, the car is running much hotter (around 217 degrees at idle warmed up, and higher under power). Yesterday, I went through all of the cooling components. I removed the water pump. It looked to be ok. I removed both the upper and lower radiator hoses, and they looked ok. The radiator is less than a year old. I flushed through the engine block and the radiator and both seemed to flush through ok. Today, same thing. It is still running hot. I am at a loss, as it was running just fine before the sensor went bad (normal temp was around 165 to 170, and normally the temperature would increase when stopped (which is the opposite of what is happening now: the temp climbs while driving). Also, it seemed that the temperature climbed much more quickly today (from completely cool) when I started driving today after the car sat overnight. Please help.

[duke_winchester]

Could there be air in the system? I bled it, but maybe not as completely as I should? Could the radiator cap be bad?

[richtazz]

what brand t-stat did you install? If was a Duralast, it's at least part of your issue, as they are JUNK! The AC-Delco t-stats now come with a jiggle valve that helps bleed off air from behind the stat. Cheaper aftermarket ones don't have this feature and can be darn near impossible to bleed.

[duke_winchester]

The T-stat is from Stant (from Advance Auto Parts). I have had this problem with two thermostats, as I replaced the thermostat yesterday thinking that was the problem, but it turns out it doesnt seem to be. Also, I opened the t-stat housing just after turning the car off, and the t-stat was open. Also, what is the coolant capacity of the 3.8 S/C engine? Is it about 3 gallons (12 quarts)? I don't think I have been able to get more than 2 gallons in (Maybe about a gallon and a half).

[duke_winchester]

Added a new radiator cap last night, but the car is still running hot. Also, the heater seems to be operating normally and blowing hot air.

[brcidd]

Why do you think 217 degf is hot? 165-170 is definitely too cold...and how are you measuring temp? with scan tool? Most systems do not even enable the cooling fans until about 227 degf-(this is with a/c off)- Are you measuring this temp with a/c on or off? If a/c is on, then fans come on with a/c not because of engine temp.. You may be used to seeing engine temp with fans on (with a/c on) thus the lower engine temp.

I get about 3 or 4 of these "too hot" complaints this time of year from folks who are enjoying the weather with windows down- and no a/c-- they tell me it never ran this hot all winter (increase guage temp), or last summer-- truth be known, they had the a/c on (fans on) last summer and it had been cool all winter-- and all of a sudden their engine is "overheating"....If all they had was an idiot light, they would never complain.

I assume you've had no boil overs, no check guages lights etc...now that is what I say is hot...

[duke_winchester]

No boil overs, no lights, but the car has always run in the 170 range, always. I have never had it run this hot. I am measuring temperature using the dash gauge, which seem so to be accurate because the cooling fans are coming on around the right temperature according to the gauge. Also, my gas mileage has decreased markedly. When the car is at idle, it decreases markedly, to around 12.5 mpg (which it did not do before at idle).

[bungea]

Just want to add from the previous poster that my 97 grand prix gt with 138,xxx miles runs at 170-180 while driving and when i sit to long it stays at 210. so i don't believe its not normal for these grand prixs to run at that temp.

I have four buddies that have grand prixs ranging from 96-97 and they all run around 180.

[duke_winchester]

So I am pretty sure I have bled the air out of the system. Still running hot around 215 at idle. With A/C on, it idles around 210. How much pressure is the cooling system under? When I open the air bleed valve on top of the thermostat housing, is coolant supposed to shoot out or just dribble? The upper radiator hose is hot, but when I squeeze on it I don't feel a lot of resistance or like there is really much in it? Also, I noticed today that while on the highway my gas mileage got to like 19.6 mpg, but as soon I would come to a stop it would drop significantly. When I got home and let the car idle for a while, the mpg dropped into the 12 mpg range.

[inafogg]

i really think your ok
i'v seen replacements turn on the fans later(higher temps)
you may try 1 from the dealer,after you ask what brand they are(gm or not)
as far as the bleeders.when you open them you should see a steady flow
also no matter what you drain its not the entire coolant capacity
so normally you will get a gallon to 1 1/2 back in.

[duke_winchester]

When driving the car today, I noticed that while on the highway (about 65), the temp would climb to like 235, fans would come on, temp would drop back down to just above 210, and then keep going up and down like this while on the highway. At a stop, it would stay around 210. This seems like the opposite should be happening.

[Scrapper]

Originally Posted by duke_winchester

When driving the car today, I noticed that while on the highway (about 65), the temp would climb to like 235, fans would come on, temp would drop back down to just above 210, and then keep going up and down like this while on the highway. At a stop, it would stay around 210. This seems like the opposite should be happening.

look at your rad hoses and see if there colapsing when you think it's hot.

[duke_winchester]

They do not appear to be while at idle. This just doesn't make sense that it would start running hot and have temp fluctuations like this after only replacing the temp sensor. At what temp are the cooling fans supposed to come on?

[brcidd]

What engine size is yours? Cooling fans usually come on at low speed at 227 and high speed at about 238 or so....Fans are usually not on while driving 40 mph plus. How do you know fans are on while driving? I usually watch scan tool to tell- otherwise hard to hear them.

An engine requires more cooling capacity at higher rpms, you may be missing an air dam, or have an air dam blocking air flow.

[duke_winchester]

The engine is the 3.8 L supercharged. I am not certain the fans are coming on at high speed, but what leads me to believe that the fans are coming on at highway speed is that the temp climbs up to like 235 or so, then the temp comes back down (which I was thinking was because the fans come on) to like 215 to 210. Then it holds that for a little, then goes back up (which I was attributing to the fans shutting off). Then it gets up to like 235 again and the temp comes back down. Maybe my gauge is just off, and this operating range is the normal and the original sensor I had was just bad all along and giving me false readings? Maybe I should like into getting a scan tool to monitor the actual temp at the computer.