Temp question 98 grand prix

30

Asked by May 18, 2013 at 04:46 AM about the 1998 Pontiac Grand Prix 2 Dr GT Coupe

Question type: Maintenance & Repair

I have a 98 Grand Prix that I replaced the engine with one from a 2001 buick regal.  I put in a new 160 fail safe thermostat.  When driving non stop, the temp gauge barely moves, but if I drive in stop and go traffic, or let it idle, the gauge goes up between the halfway mark and the hot mark.  The car does not overheat, but the fans also do not kick on until the gauge reads about the 3/4 mark.  If I start driving non stop again (expressway) the temp gauge goes down again.  Any suggestions ?

22 Answers

102,135

Thermostat is too cold, that engine is designed to operate properly at around 197 degrees, replace the thermostat with the proper heat range. The cooling fans should turn on at around 210 to 212 degrees, so make sure you have the proper mix of coolant in the system to prevent boil over. You might think about getting a live data scan tool so you can read the actual temps as the system is operating. It will also help you in future diagnosis. Hope this helps

2 of 2 people found this helpful.
10,845

Thermostats suck. If the new one gets damaged even just a bit there can be enough of a gap to flow coolant when "closed". This small amount of leakage will not allow the car to gain temp. Almost every used car that I have bought in the last 2 years has had a bad thermostat. When I get a new one I flip it upside down and put water on the hot side and check for leaks. I then throw it in a pot of water on the stove and verify the opening temp. Then as cools I check that it has closed completely by doing another leak test. I've had new ones that were sealed untill they opened once then would not close. This test beats f*&$in around with the thermostat neck twice. Not sure where it is on the 3.8 but the 3.4 DOHC sucks.

2 of 2 people found this helpful.
145

a 160 degree stat is fine i reckon it hasnt been bled properly or the thermostat is in upside down

3 of 3 people found this helpful.
10,845

If it wasn't bled the air lock would delay the opening of the thermostat and the engine would heat up very quickly (I've cooked a motor). As with installed backwards, it would take awhile for the engine heat to conduct through the thermostat to start to open it and you would run hot. It has to be stuck partially open to run cool.

1 of 1 people found this helpful.
30

ok guys, I tried to bleed it and couldn't get any water or air bubbles to come out of the bleeder screw. I had it unscrewed enough because I could see the whole in the bleeder screw. Also my fans didn't kick on until it got to the 3rd mark, then they shut off at the 210 mark. Any answers ?

2 of 2 people found this helpful.
102,135

You should get some fluid out of the bleeder, if the bleeder is plugged just remove it and bleed it out that way, when it starts to flow without bubbles then put the bleeder back in. Don't trust the temp gauge, get a live data scan tool so you can read the actual temps the computer is seeing and at what temps the fans are cycling at. Did you put in the factory recommended thermostat?

1 of 1 people found this helpful.
10,845

I have three GM cars right now that do not run hot due to the thermostat passing. Pull it and check it. My '94 never passed 50 C untill you let it idle. Quick check to see if it's your thermostat passing without testing it as described is to use something with two rounded surfaces to pinch the flow on your top rad hose. Your car should heat up alot faster than it has been. Your bleeder is plugged for sure.

2 of 2 people found this helpful.
30

do you think that the temp sensor is bad?

1 of 1 people found this helpful.
102,135

Are you getting a temp sensor code?

2 of 2 people found this helpful.
30

no

102,135

Then the sensor should be good. Have you done a live data feed on a scan tool to check actual temps?

1 of 1 people found this helpful.
30

no, unfortunately I don't have one. What does the temp sensor do?

102,135

The temp sensor tell the computer the coolant temperature as it flows through the engine so it can adjust fuel mixtures and timing to optimal settings and it also tell the computer when to go into closed loop. I would really try to rent or borrow a live data scan tool if you can't afford to buy one, then you can watch the temp rise or fall, idle speeds, tps voltage, air flow readings, O2 switching, I would not be without one in my tool box.

1 of 1 people found this helpful.
10,845

Just to add clairity. You will have two sending units. One will feed the temp gauge and the other one is the ECT which feeds the above data to the computer. The ECT is also the unit that operates your cooling fans. If you haven't fixed the initial problem of running cool when moving and heating up when stationary and or stop and go traffic you should take care of that first. Your fans will only kick in when you need them and I believe my car is around 210 too. If your gauge was showing 210 and the fan kicked in the sensors are not the issue. If the gauge sending unit fails it will short (show max temp) or go infinite (show 0 temp), same with the ECT if it failed the fans would be on all the time or never turn on. I guess it is possible to fail at a steady state resistance but I've never seen that. The purpose of the thermostat is to keep your engine at operating temp AND to stop the flow of coolant through the rad to let it cool. If it is constantly flowing your engine will run cool under no load and with wind from highway speeds. Under load (stop and go) your engine will start to heat up. Left unchecked it may get hot enough outside to cause your car to overhead as you will not give the fans enough time to cool the fluid in the rad once they kick in. As I said before just because the thermostat was new doesn't mean that it is good.

1 of 1 people found this helpful.
145

temp guage wont read anything at all if theres not fluid running over it

2 of 2 people found this helpful.
10,845

You bet it will. I will read the air temp inside the engine. Air is an insulator so it just takes time for the engine/coolant to heat the air and give you a reading. If the coolant level is below the sensor you wouldn't make it very far.

2 of 2 people found this helpful.
20

i am having the same problems with my car but the fans get voltage. just enough to give a slight amount of resistance but not enough to turn them. Other times they come on. In what ya'll have commented on. it sounds like i will be taking it to have a doinostics run to see what the computer is reading. I have replaced temp switch, themostat, three fan switches in fuse box. I've insured that the antifreeze is correct, and free of air. I've done everything ya'll have spoke of before i read the comments and I am stumped. some time the fan will come on but most of the time it will not. Computer is all I have left to check out i think. Do you have any other ideas that I can try before I take it to a garage. Thanks.............Sam

2 of 2 people found this helpful.
10,845

I would get a volt meter and test light and go through the wiring before I bought a PCM. I'm not sure what kind of access you have to your relay box under the hood. Easiest method to narrow down on whether its a control issue or wiring issue is to ground out the PCM signal wire at the relay box. With the key on the fan should run with that pin to ground. Try moving the wires/plugs to ensure that it is not a loose connection(s) or broken wire (I have had conductors break cleanly within the wire jacket and make/break the circuit with little movement and with no external signs of the break.) Are you sure that you are exceding the 210 F temp is takes to turn the fan on?

1 of 1 people found this helpful.
145

well i got ocean front property in Arizona

just burp it the old fashioned way.

3 of 3 people found this helpful.
10

Failsafe thermostats are notorius for "failing". I suggest you try an OEM thermostat that opens at the manufacturer recommended temperature. "NOT" a "Failsafe".

1 of 1 people found this helpful.
10

The only thing that has worked for me is putting it up I jack stands. Full the radiator from cap while holding down accelerator. When full hurry and put on cap before releasing accelerator, forcing the pressure I guess. Kinda difficult by self, but I managed

1 of 1 people found this helpful.

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