My thermostat gauge in the car is going up and down, what could be the problem

Asked by Oct 02, 2014 at 02:50 PM about the 2002 Chevrolet Venture LS

Question type: Maintenance & Repair

My thermostat starts on cold and when i get in and drive it goes from low to almost hot., and it jumps back and forth when I drive the car. Do I need to change the thermostat, I add water and coolant but it does the same thing. And  what will it cost to take it into a shop

17 Answers


If this is gradual and it get's hot while sitting in traffic, cooling when moving, or goes very hot after coming off the Interstate, then it's a "stuck open" (or possibly missing if previously owned) thermostat. Replace it and be happy it wasn't stuck closed. If the change is rather quick and jumps around the gauge for no apparent reason then it may be a faulty temperature sensor. If this is the case, replace it because it will put undue stress on an electrical cooling fan with frequent starting and stopping. Most likely a corroded and stuck-open thermostat.

5 out of 5 people think this is helpful.

My guess would be a temp sensor going bad. Have someone with a temperature gauge gun shoot the car and see if there are any hot spots in the area of the thermostat. The thermostat is cheap to replace, time is the only thing. It will require draining the coolant from the radiator, the removing the thermostat housing, usually two bolts. Some thermostats can be put in upside down, so pay attention to the arrows on the thermostat. The water should flow from the engine towards the radiator. The temp sensor I think is in the rear passenger side of the engine on top. Remove the wiring, and using a deep 1 1/16 socket remove it and replace it with a new one. If none of these work, see if anyone can hook up a second gauge to make sure the one on the dash is working correctly.

9 out of 9 people think this is helpful.

My thermostat is going up when I'm going then when I stop it goes down what could be the problem I done replace the temp. sensor .

5 out of 5 people think this is helpful.

Pam, your problem seems similar to the original question. If the engine's temperature will not stabilize around 160° to 180°, regardless of whether you are moving or not, this usually is caused by a stuck open thermostat, or a thermostat that was replaced incorrectly. One check you can do without unbolting anything, (ONLY DO THIS WITH A COLD ENGINE) is to remove the radiator cap, make sure the radiator is completely full, start your COLD engine and see if the water in the radiator begins to move immediately. I CANT STRESS ENOUGH TO ONLY DO THIS WITH A COLD ENGINE! REMOVING THE RADIATOR CAP ON A HOT ENGINE CAN CAUSE SERIOUS BURNS FROM BOILING WATER.- besides this check will not work on a hot engine. If the thermostat is working as it should you should not see moving water for a few minutes. If water begins circulating immediately inside the radiator then the thermostat needs to be replaced. There are a few other major problems that can make the engine temperature fluctuate as you describe like a blown head gasket or a cracked head. If you are sure the thermostat is working properly, and if you are not doing the work yourself, have a mechanic check your radiator for exhaust gasses with a "sniffer". You can also look for white smoke or steam in your exhaust. If either or both of these check positive then I'm afraid your problem is a little more expensive than a bad thermostat. Don't let anyone tell you that unless you have water in your oil you do not have a cracked head or blown head gasket. Neither have to necessarily allow water to flow into the oil, but can allow exhaust to pressurize the cooling system causing it to circulate improperly.

9 out of 9 people think this is helpful.

Dear all , i need help here , i am having the following problem : Temperature gauge goes up and down from 90 to 120 , completely randomly sometimes it doesn't happen and sometimes it does, no difference if i drive , or stand at a traffic light , or drive smooth or hit the road hard. My car is an aud s6 2007 model 5.2 V10 engine , radiators have been checked , flushed , and reconditioned perfectly, thermostat is brand new , temperature sensor is brand new , expansion tank is new too. Than problem occured after i had a blown gasket due to a malfunction of my thermostat couple of month ago. Gasket has been replaced at that time but after 600 hundred km without any issue , i am facing this constant problem of the temperature gauge going crazy and for whatever reason it comes from 90 steady up to 120 in about 30 to 40 seconds and then it comes back down while im still driving, and then all the cycle again , up and down up and down , sometimes more than 120 and soemtimes less. The engine is not over hitting , coolant is not boiling ,cooling system pipes are not hard and can be pressed bare hands. The system has been already bleed properly and filled back with a special vacuum to avoid any air in the system. Cylinder head have been checked by pressure test and have shown a perfect health condition , no cracks no damages . Cylinders have been checked the same way and are also in perfect condition , there is no air lick and no crossover in the system from the cylinder to the cooling. The only thing we are thinking about now is maybe the wiring system. As well please note that this problem has happen just after i had a severe overheating above 120 degrees due to the thermostat that was not working and which caused my fans to not properly cool my engine at a certain point and consequently overhitted the full engine ( cooland boiling) After than incident we were not aware that the gasket was blown and we just changed the thermostat. At that point the car was doing that up and down with the temperature gauge , so we replaced it , ..result was nothing changed it was still doing it again, so we thought of a problem such as air in the cooling system, so we did the bleeding and again nothing changed. Result : same problem , so we opened the engine and found the blown gasket , we thought that it was the cause and we replaced it . Result after 600 km , the shit happened again . Temp Gauge is going up and down again . Do you guys have any clue of what the fk is happening to my car?


You've mentioned just about everything that can make this happen; cracked head - blowing exhaust into the coolant lines; faulty temp sensor; bad electric fan; sticking thermostat (make sure the new thermostat is not put in backwards. A backwards thermostat can cause all kinds of symptoms from overheating to random temperature fluctuations ); etc. You mentioned the electrical system which actually can cause needle gauges to read incorrectly. If you can thump the gauge with your finger and cause the needle to jump to and remain at a different reading then suspect a loose or dirty connection at the back of the gauge. Turn on your lights and watch to see if the needle moves to a different position - if it does then there is probably insufficient voltage on the "+" terminal of the gauge, a bad ground at the gauge or a bad connection at the sensor - could even be caused by a weak battery or an alternator putting out less than 12 volts.

1 out of 1 people think this is helpful.

Dear CaptPkelly I sorted the problem out following the advice of one of the person on this forum which was to reset the ECU's by disconnecting the battery . So since yesterday the gauge stays steady at 90 and no fluctuation anymore . I will keep you posted and thank you for your answer and the advices .

1 out of 1 people think this is helpful.

Pfffffff the shitt happened again . I noticed that it happens when i after i race the car and when i slow down for example at a traffic light the gauge will start to co crazy . I reset again the ECU by disconnecting the battery , Thursday my mechanics will check the ECU connectors witch are probably responsible for that shit i think. Maybe some pins are loose and it would explain why it happens after racing the car due to the hit and vibrations .


It could be in the cooling system also. Is there a possibility of air in the system? Do you have a overflow bag and is it full after racing? The faster you run the engine, the hotter the engine gets. Is the radiator clear? Have you modified the engine? The more HP you have, the hotter the engine runs. Is it timing? Something may be causing the engine to overheat if it only happens when the engine works hard. Now by "racing" is this a sanctioned street race, race track or just having fun on highway Route 66? I'd check and make sure the timing works as it should at higher RPMs. Have your mech. do a coolant pressure test, timing test, and sensor check. If it only does this when you "race", than I'd roll out the temp sensor.


Dear califcarson , So the overheating is coming only after lets say using my car in a sport way ,( as it is meant to be ) using sport mode , in the normal streets not on a track , so basicaly , i am driving fast and an nothing will happen until i stop to a traffic light and then the temperature will start ti rise up to 120 degrees ! So i would stop y engine , and wait for 3 or 4 min and it wil come back to normal... then i will happen again and again...


Z_Marshall: This is a sure sign of a stuck open (or missing) thermostat, a weak water pump (but rare if the water pump is not leaking) or a heavily rusted radiator, or a bad fan switch if you have an electric radiator fan.

1 out of 1 people think this is helpful.

CaptPKelly but bro the thermostat is brand new, the water pump is brand new too , the radiator have been cleaned and checked and is 100 percent fully functional, and all my fans i can hear them and see them running properly when the car is hot... i just dont get it.. and the thing is the temperature will rise and then after switching off and on with a 2 or 3 min gap it will be back to 90 normal. And then it will do the same thing if i stop at a trafic light . it is happening only when i am in the traffic or stopped. i doesnt happen at high speed on the highway

1 out of 1 people think this is helpful.

Do you guys think that some weak piston rings could give a back pressure into the oil chamber and cause that overheat ?


If you can get your hands on a heat sensor, try to shoot the engine when it is doing this and find out where the hot spots on the engine is. That may help start your search for the causation. Have you pulled off your heads on the engine recently? If so did you put on new gaskets? Some gaskets are universal in the sense they fit several engines or set-ups. The water jackets on the motor / cylinder heads are different on some engines. Make sure the gaskets are not blocking a water hole. I had a friend that had constant overheating issues. We pulled the heads and the gaskets were blocking 12 water holes on each head. Poked the holes in the gaskets and problem solved. If you have not pulled the heads, that is something you can keep as a reminder if you do change gaskets. Have you checked the timing or checked if the ignition advances at high speed? I'd check the radiator for pressure into and out of the radiator to make sure there is no blockage. this is going to be a lot of work, but it is better than blowing an engine. Any parts replaced, even if new should be rechecked. Is the correct amount of antifreeze in the system? It would not be a bad idea to spend a few bucks and put in a different thermostat. It may be that you picked up a bad one. Little things that are so simple to fix can cause a lot of problems. When the car is at running temp, are the radiator hoses hard and solid or mussy? I know they are new, but something is not allowing water to flow fast enough to cool the engine. It sounds like either the thermostat is stuck open, or it was forgot to be put in.

2 out of 2 people think this is helpful.

Why not just disconnect the temp sensor to make the fans run full time? if the engine stays cool then you know its not internal but just faulty electronics.

3 out of 3 people think this is helpful.

Yes my moms car when she starts it up it goes to cold to little hot but when she turns her air off it goes down what can be the problem

4 out of 4 people think this is helpful.

For my problem with my audi s6 , we found the problem , after dismantling completely the engine , and fitting it back the problem was still here ! so we decided to try new injectors on the car , and guess what ? no more over hitting ! I cant believe that it was the injectors fuck my life.,.. after all this time chasing our tail ! So basicaly a lean mixture in the chambers creates overhitted exhaust gas and consequently some weird overhitting of the engine ! Specially that here in Dubai the Gas is not well refined injectors are subject to faster degradation. I hope this will help you guys ! So if your engine is shaking , and over hitting it means probably that you have injectors problems like i did !

3 out of 3 people think this is helpful.

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