Grand Prix Running Hot

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.

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

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.

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).

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.

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...

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).

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.

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.

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.

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.

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.

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?

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.:p

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.

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?

where did you get the temp sensor and are you sure it's like the old one? did you try another thermo stat after you replace it also?

I got the temp sensor at Advance Auto Parts. It is BWD brand. I did replace the thermostat after installing the new temp sensor

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.


not 100% sure with the 3.8 turbo but..
when you bleed the system did you bleed both bleeders??
on my 96 3.1 there are 2,on my sons 97 3.8 non-turbo im 90%
sure there are 2. this problem would make sense if you bleed the 1
at the thermostat,but not the 1 at the water pump.if it has one....

I believe the 97 3.8 only has the one on top of the thermostat housing from what I have read in other posts. I had the entire cooling system apart the other day and did not see any anywhere else. I would like to thank everyone for their input. It is greatly appreciated.

np i no it bugs the shit out of you when things like this pop up for no reason
im sure someone will chime if there is one around pump.

Good luck

maybe the new sensor is reading hot , use a meter to check resistance

I am concerned about a possible gauge problem as, according to the gauge, the thermostat doesn't open until a temperature much higher than the 195 degrees the thermostat is rated at. How does the gauge get its input? Is it from the computer module, or is there a direct line from the coolant sensor itself (possibly the new sensor is not reading properly)? The thermostat is new. Thanks again.

I am going to measure the resistance across the temp sensor. I would use the green and black wire pins right? (as there is also a yellow wire and associated pin). Also if I measure the voltage across the green and black wires coming from the PCM with the engine on, I should get 5 volts right?

I am going to measure the resistance across the temp sensor. I would use the green and black wire pins right? (as there is also a yellow wire and associated pin). Also if I measure the voltage across the green and black wires coming from the PCM with the engine on, I should get 5 volts right?


if im reading correctly to check the ect sensor you would measure
ohms across the 2 pins on the sensor.
@ 210 degrees ohms should read 185
@ 100 degrees 1800 ohms
@ 40 degrees 7,500 ohms
hope this helps,as i stated earlier i've seen after market brand
sensor that will raise the temp you see on your gauge but fall
into factory specs,just the higher end.
getting an OEM from dealer may correct your problem/concern

The sensor has 3 pins, as 3 wires (green, yellow, and black) come from it. I measured the resistance at the connector across the front of the engine near the fuel rail that comes directly from this temp sensor (as it is a PIA to try to get at the actual sensor itself without taking a lot of stuff out) and I got a very very low resistance reading (well below the chart I attained elsewhere on this site). I am thinking the sensor is bad or not accurate. It is an aftermarket one, and so I think I am gonna have Advance Auto Parts get me the AC Delco replacement.

Found this info very interesting, thought I'd share.

The thermostat is much maligned, often misunderstood, and seldom credited for its mostly reliable service.

Thermostats in proper working order, control only the minimum operating temperature at which engine coolant is allowed to heat. A properly operating thermostat has absolutely nothing to do with how hot the engine becomes. If you are experiencing an over heating engine, changing to a colder thermostat will not correct the condition, it only allows the coolant more time to over heat. A thermostat once fully open can do no more to control temperature. The overheating cause lays somewhere else.

Some believe that removing the thermostat completely can remedy over heating concerns. This can be deceiving, with out some flow control coolant can pass through the radiator without slowing enough to cool, returning to the engine hotter with each circuit of the liquid.

The temperature stamped or marked on the thermostat is the "start to open" rating. This is often referred to as when the thermostat just "cracks open" and allows some coolant to flow. This is in contrast to the popular belief that the temperature specification identifies when the thermostat is fully open. Most applications have a thermostat that is designed to "crack" at about 180 to 195 degrees F. If the thermostat opening is set too low or too high engine damage can be the result. High coolant temperatures can cause abnormal combustion, loss of power, damage to bearings and other moving parts, etc. Low engine operating temperatures lead to excessive fuel consumption, dilution of lubricating oil by the addition of unburned fuel, the formation of sludge from the condensation of water in the crankcase along with a host of other concerns.

Thermostats are designed to be fully open 20' degrees F after cracking.

So 215'+ on a 195' would be considered "Normal" as on my 2006, and the fans dont even come on till 230'

Don't know how hot would be considered Too Hot, seen other post asking that, Maybe someone more knowlegable will chime in on that. So long as the pressure is holding the temp can go pretty high before there is a real problem.

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.
Check to make sure there isn't any restriction in front of your radiator. If you have air conditioning, there may be leaves between the condensor and radiator. Now that you've replaced the radiator cap, does the system pressurize when it warms up?

Where is the solution to this problem!?

Because my car is doing the exact same thing, I'm in Canada so 112.5 Celsius roughly , 2 bars from the red, it will go cooler at idle, but always runs that hot, never use to, always much cooler, i turn the AC ON now and there is no fan response from AC but they will turn on and it keep it 2 bars red, sometimes 1

I replaced my LIM gasket, coolant temp sensor, t stat, a bunch of other stuff up top that wouldn't effect it

3.8 non S/C

Should start your own thread instead of a year old one with no resolution.......

You have a 3.8...what year?

Your vehicle is overheating? Are you losing coolant?

What was original symptom? Did you replace all this stuff after the problem first started?

Was system properly burped(the funnel with adapters that fits in place of the radiator cap works great on these engines).......check cooling fan operation?

When you replaced the lower intake gaskets, was the EGR opening in the upper plenum checked for degredation?

Hey, so yeah this was a year ago, but after checking everything (water pump, all coolant lines, radiator, thermostat, etc, and breaking a bolt from the power steering pump off in the engine block), it ended up, if I remember correctly, being the coolant temperature sensor. It was falsely reading high. If you end up replacing it, go with the AC delco sensor.

Sorry I figured he might of got it fixed in hope of an answer back, its just such an identical problem to mine

Coolant level is good, I have bled the system, but not with the funnel system,

my original symptom was a bad LIM gasket, which led to
new plugs, new wires, new LIM gaskets, GM sure fix upper intake kit, replaced the egr pipe in the LIM, new MAF, IAC, ECT , TSTAT, fuel injector o-rings, coolant elbows in idle, Throttle body Clean

I have a code for a EGR stuck open or closed, going to replace that,

the egr hole seemed alright, nothing I remember sticking out, but the pipe going back in got kinked a bit going back in (the part of the pipe that going into the LIM)

As of now I am going to replace the EGR, and the one pipe, replace TSTAT and ECT with AC Delco parts and bleed system again with funnel

Check all relays for the coolant fans, the PCU will trigger one of them under a certain condition to give a battery to the coolant fans.

The PCU will control the low speed coolant fan, high coolant speed fan, HVAC cool max, and engine off.

Low speed = 223F
High speed = 230F
HVAC = High cool max
Engine off = 284F

The Coolant fans turn on, i have swapped relays and checked fuses, the high speed comes on 2 bars from red, which from what i read I guess is accurate, AC now is not turning on the fans though, it will drive for hours 2 bars from red, never over heat, but so close to that red, never ran up there before

Doe's the HVAC high heat work?
Doe's it only over heat with the air cond on high max cool?
Doe's the coolant fans work with engine off @ 284F ?
Is your new ECT sensor a AC Delco? Yes

What controls or sensors tell the coolant fans to turn on high max with HVAC on high max cool?
PCU/relay/ sensor?
Air cond pressure switch!

Consider getting a Non-Contact infrared thermometer with laser targeting to check temps @ Amazon.com for $42.00

Not sure if the heat works, I meant to try that today but didn`t about that,

Overheat, it never goes to red, only close to it, and when you say high max cool, I have digital climate control, but on the coldest temp its usually set, and no it doesn`t seem to be the culprit

have not tried them off, but if I converted that right thats about over 130 celsius , my car doesn`t go that hot,

New ECT wasnt AC Delco

I changed nothing today, and i noticed when i started it the ac turned the fans on, unlike yesterday, and it ran cooler all day, around 106 Celsius.. so.. 220-230 F not the first time for this to happen