# Engine temp warms while idling, but drops while driving



## JettAffair (Jan 7, 2004)

As the topic states, my car seems to warm up normally if it's idling, but when I start driving it cools back down. If I'm on an interstate or expressway, the needle will drop down into the white. This isn't a problem in warm weather. My first thought is the thermostat, but I've had that replaced in the last couple of years. I do have a green top sensor. Is there anything it could be other than the thermostat, or should I replace it again?


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## JettAffair (Jan 7, 2004)

Bump. I need heat.


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## got_boost (Dec 5, 2001)

*Re: (JettAffair)*

if you have 0 heat then id look into the thermostat,if the gauge reads cold but you do have heat then id look at the sensor.
i never seen a bad waterpump cause it to go totally cold while driving but it is the cold season







just for humor,when was the timing belt or more important the waterpump,in this case, last done? if ever?


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## JettAffair (Jan 7, 2004)

*Re: (got_boost)*

The timing belt, water pump, and thermostat were done in July of '08. That's why I'm not jumping on the thermostat, although it could still be the problem. 
I don't have heat like I should, so I guess it's not the switch.


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## ritchic (Oct 1, 2005)

I have this problem right now
New temp sensor so Im thinking thermostat I'll replace it soon heopfully fixes the issue


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## Buschwick (Feb 12, 2007)

*FV-QR*

replace your thermostat


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## engineerd18t (Dec 12, 2007)

*Re: FV-QR (Buschwick)*


_Quote, originally posted by *Buschwick* »_replace your thermostat

What he said. When you drive you cool down that coolant with the air flowing quickly through your radiator. Not the case when idling. I had a similar issue and found that my thermostat was stuck open.


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## Vdubed13 (Jun 6, 2008)

this has been happing to me the past couple days ive got an 01 jetta 1.8t with eurojet fmic and everything was done at 90000 and it has 120000 on it now but it only drops from like 190(red notch) to like one line below it and its only when im cruising on the interstate...then of course if i get on it it goes to normal again....ITS COLD AS CRAP OUTSIDE so i just figured...its cold...lol


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## JettAffair (Jan 7, 2004)

*Re: (Vdubed13)*


_Quote, originally posted by *Vdubed13* »_this has been happing to me the past couple days ive got an 01 jetta 1.8t with eurojet fmic and everything was done at 90000 and it has 120000 on it now but it only drops from like 190(red notch) to like one line below it and its only when im cruising on the interstate...then of course if i get on it it goes to normal again....ITS COLD AS CRAP OUTSIDE so i just figured...its cold...lol

That's not what's happening to me. If I'm on the interstate, the temp needle drops all the way to the left. It ONLY warms up when I'm at idle.
Does the fact that it heats up normally in warm temperatures rule out a stuck thermostat? In other words, would it overheat in the summer if it were stuck open?


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## screwball (Dec 11, 2001)

My sticky thermostat did the exact same thing you're describing.


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## rissa422 (Jan 17, 2009)

I have the same problem. Its the thermostat. I mean if your over cooling and its not a faulty gauge reading could it be anything else? Extra wet water?


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## kroutbrner (Nov 2, 2009)

*Re: FV-QR (bbeach)*


_Quote, originally posted by *bbeach* »_
What he said. When you drive you cool down that coolant with the air flowing quickly through your radiator. Not the case when idling. I had a similar issue and found that my thermostat was stuck open.

X2 replace thermostat and problem fixed. good luck. hope things heat up for you soon.


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## terri1977 (Dec 17, 2009)

*Re: FV-QR (kroutbrner)*

Having the same issue with my Jetta. Had a feeling that it might be the thermostat. How do you change it?


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## dbn23quattro (Apr 8, 2008)

*Re: FV-QR (terri1977)*

I agree with everyone else, its your thermostat. You should be able to find a write up on how to change it, but its not to hard of a job.


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## kroutbrner (Nov 2, 2009)

*Re: FV-QR (terri1977)*


_Quote, originally posted by *terri1977* »_Having the same issue with my Jetta. Had a feeling that it might be the thermostat. How do you change it?

find where your coolent hose enters the engine block (this is where the thermostat is usualy) 
remove hose
take out thermostat
replace w/ new one
put hose back on
Im not exactly sure where the thermostat is but the job is simple. just make sure to get some coolent and something to catch the spilt coolent with. im sure you could find out where the thermostat is by calling a local repair shop or if anyone klnows off hand maybe they can chime in.
hope that helps. Good luck!!


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## drewnashty (Jun 3, 2009)

*Re: (JettAffair)*


_Quote, originally posted by *JettAffair* »_
That's not what's happening to me. If I'm on the interstate, the temp needle drops all the way to the left. It ONLY warms up when I'm at idle.
Does the fact that it heats up normally in warm temperatures rule out a stuck thermostat? In other words, would it overheat in the summer if it were stuck open?

You don't have to worry about it overheating if it's stuck open ... this just causes it to take longer to warm up and makes it difficult to maintain optimal operating temperature.
I had the same T-Stat stuck close issue just this last summer here in Phoenix. I had to pull over like 5 times because of stop and go traffic; turn the motor off, pop-up the hood and turn the AC on to get the FANs to come on







and wait like 10 minutes just for her to get a little below 190. With it being winter obvious you aren't running into this problem with it not being 105+ degrees outside.
After letting the motor cool down, I removed that vacuum sensor crap n249 or w.e. sensors at the intake manifold, removed the coolant hose to the t-stat housing, then removed the t-stat housing and dropped the t-stat in boiling water which is 200+ degrees and it did not open. Replaced T-stat and it ran great and what do you know my fans started working normally


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## rob251 (Mar 29, 2007)

*Re: (drewnashty)*

hope that help you 
I post the same question...we put new Gaf water pump and German T-stat (2nd t-stat changed it last year also) from world impex in wifes passat and car still doesnt heat up. I know about the Passat heater core plugging up and we flushed that . the trouble is the motor never goes above 90-100 when driving, it will warm to 190 when at idle. and once you start driving, it cools way off, I even have the radiator blocked off with a piece of coraplast


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## bmp20th03 (Sep 11, 2009)

*Re: (rob251)*

Scan the car, if the thermostat is stuck open you should have a "cooling system performance fault". This does sound like a thermostat and we do replace alot of them, especially when it gets cold out.


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## JettAffair (Jan 7, 2004)

*Re: (rob251)*


_Quote, originally posted by *rob251* »_hope that help you 
I post the same question...we put new Gaf water pump and German T-stat (2nd t-stat changed it last year also) from world impex in wifes passat and car still doesnt heat up. I know about the Passat heater core plugging up and we flushed that . the trouble is the motor never goes above 90-100 when driving, it will warm to 190 when at idle. and once you start driving, it cools way off, I even have the radiator blocked off with a piece of coraplast 

I hope my problem is the thermostat, but it sounds like you have the same issue and the t-stat didn't fix the problem. I'm going to replace mine next week and see what happens.


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## screwball (Dec 11, 2001)

you try and testing the thermostat? I've had bum ones before that were new.


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## rob251 (Mar 29, 2007)

*Re: (JettAffair)*

no scan faults showing
its been in the teens here . wife said gauge barley moved close to 90 when she drove it yesterday
so the colder it is the less it comes up...and with the radiator blocked off it should be heating up


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## Gordi (Dec 5, 2009)

*Re: (rob251)*

As everyone said, its your t-stat. You can have a brand new one not work. Even if you block off your radiator with the really cold temps it wont heat up, the coolant will just keep recirculating through the radiator and cooling off. At least your stat stuck in the open position.


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## spain4065 (Nov 28, 2008)

Or it could be that the foam on your heater box flap has erroded away...thus allowing the cold air just to blow thru and keep the engine cold when driving and heat up w/ idling.


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## PimpMyRide (Apr 30, 2004)

*Re: (spain4065)*


_Quote, originally posted by *spain4065* »_Or it could be that the foam on your heater box flap has erroded away...thus allowing the cold air just to blow thru and keep the engine cold when driving and heat up w/ idling. 

wut? its your t-stat. Look at your intake mani, it is inbetween the 1st and second runner on on the passenger side. Follow the hose down to the block, remove both bolts, dump the fluid into a bucket, pull the t-stat and o-ring out, replace. You will need extension and a pivoting end to get to the bolts.


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## burble X2 (Dec 9, 2009)

*Re: (spain4065)*


_Quote, originally posted by *spain4065* »_Or it could be that the foam on your heater box flap has erroded away...thus allowing the cold air just to blow thru and keep the engine cold when driving and heat up w/ idling. 

stay the hell out of the tech section.


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## spain4065 (Nov 28, 2008)

Haha...funny guy! Ever heard of the heater box fix? Maybe you should spend a bit more time in the tech section...oh i see you just joined the vortex about 3 weeks ago...the ignorance section is that-a-way ----->


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## mrb5_supra_wrx (Jul 1, 2012)

burble X2 said:


> _Quote, originally posted by *spain4065* »_Or it could be that the foam on your heater box flap has erroded away...thus allowing the cold air just to blow thru and keep the engine cold when driving and heat up w/ idling.
> 
> stay the hell out of the tech section.





PimpMyRide said:


> _Quote, originally posted by *spain4065* »_Or it could be that the foam on your heater box flap has erroded away...thus allowing the cold air just to blow thru and keep the engine cold when driving and heat up w/ idling.
> 
> wut? its your t-stat. Look at your intake mani, it is inbetween the 1st and second runner on on the passenger side. Follow the hose down to the block, remove both bolts, dump the fluid into a bucket, pull the t-stat and o-ring out, replace. You will need extension and a pivoting end to get to the bolts.





spain4065 said:


> Haha...funny guy! Ever heard of the heater box fix? Maybe you should spend a bit more time in the tech section...oh i see you just joined the vortex about 3 weeks ago...the ignorance section is that-a-way ----->


What's funny is you guys being idiots about it even tho this is a almost 10 year old thread just want to say that at least the guy is trying to help instead of just saying "hey it's ur thermostat" I've had similar issue as the o.p and replaced three thermostats and had the same issue still. Only thing could still think of would be maybe issues with clogging heater core or something idk but trying to still figure it out . But it funny for you guys to just dog the guy and then a solution still not be found

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## IHK11 (Jan 15, 2021)

Hi i dont have a VW but ive got a 1990 Audi coupe 2.3e B3 and im getting exactly the same problem.i started off flushing my heater matrix as the heater in the car has never been good since ive owned it. It was pretty shocking the coolant that came out like a brown colour with small bits of rust in it. I ithought if it was cleaner i might get better heat. Ive flushed it in both directions, replaced a leaking hose from the engine to the heater matrix and cleaned every single hose and the expansion tank which had brown sludge in it. After flushing with a hose pipe Ive replaced the coolant with a red af12 antifreeze which is used in newer audis and vws at a 50/50 mix with distilled water. Ive replaced the thermostat twice and bled the system. Both cercolli thermostats but somethings not right. When i start my car and leave it idling its slow warming up but does eventually get to half way on the gauge 90 degrees C. At this point the heater is fairly warm but as soon as i start driving and get up to speed my temp gauge drops to the top of the cold and the heater temp drops also. If i hit traffick or im going less than 10 mph my gauge rises again and my heater gets warmer. There are no warning lights on my dash and my car NEVER overheats. Its like the cooling system is working too good. My thermostat is suppose to open at 87 degrees C and the hose next to the thermostat appears to not get warm until the gauge is just a notch below half way which is close to 87 degrees C. Today i looked at the temp sender and the temp sensor but both appear to look in very good condition. Im totally puzzled as to what the problem is. Ive also done some research on circolli thermostats and they dont seem to get positive reviews. Im just wondering if perhaps my thermostat is opening at the correct temp of 87 degrees c but not closing completely allowing too much cooling. I previously had a 1993 Audi coupe 2 litre 16v B4 which had a great heater and the temp gauge always used to stay between a notch over quarter and a notch over half way on the gauge. The expansion tank is always cold as well whether engine is hot or cold which i find strange. The water pump is about 2 years old and ive replaced the seal on the thermostat and the housing is in good condition. Does anyone have any ideas what the problem could be? Im considering changing the thermostat again but not sure which brand to go for. Any help or advice would be much appreciated. Thanks for reading Ian!


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## Vegeta Gti (Feb 13, 2003)

heater core, blockages from age and debris and coolants being mixed, etc,etc


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## IHK11 (Jan 15, 2021)

Hi Thanks for replying. Do you think a blockage could cause the temperature to drop? I thoroughly flushed the heater matrix and engine and radiator with a hose pipe and then put an airline through everything making sure there was no coolant left in the cooling system before i put red coolant in there. I also left some lemon juice in the heater matrix for 24 hours before flushing and a lot of bits came out. It seems pretty clean in there now tho. Do you think a blockage somewhere could be preventing theexpansion tank from warming up? It is always cold. Thanks Ian!


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## Vegeta Gti (Feb 13, 2003)

Who knows. Pressure is the best way to check and clean. I would say though, that the thermo isn't operating(opening) correctly and probably isn't flowing correctly.


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## IHK11 (Jan 15, 2021)

I think il change the thermostat again and hope for the best. Which brand of thermostat do you think would be the best to go for? Thanks Ian!


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## Vegeta Gti (Feb 13, 2003)

Test it before it goes in and drill an extra hole or two. 

Remember, distilled water only, never tap water.

Microwave water in a large cup/Pyrex measuring cup, drop thermostat in and see if it fully opens

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## IHK11 (Jan 15, 2021)

Hi ive had the thermostat out and put it in a pan of boiling water and it opens as it should. Its suppose to open at 87 degrees celcius so i think it opens ok but it does seem very slow to close again. What do you mean when you say drill an extra hole? Wouldnt that allow water to flow when the thermostat is closed? The brand of the thermostat is Circolli but the reviews dont seem too good. Could you recommend which brand you think is good? Thanks Ian!


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## MiltDoggie (Jun 30, 2020)

IHK11 said:


> Hi ive had the thermostat out and put it in a pan of boiling water and it opens as it should. Its suppose to open at 87 degrees celcius so i think it opens ok but it does seem very slow to close again. *What do you mean when you say drill an extra hole?* Wouldnt that allow water to flow when the thermostat is closed? The brand of the thermostat is Circolli but the reviews dont seem too good. Could you recommend which brand you think is good? Thanks Ian!


Drilling a tiny hole in the thermostat allows air in the system to escape preventing airpockets which potentially cause overheating. No real cooling benefits or performance benefits.

Since you're in the UK, I would look at Febi Bilstein. I use parts from them as they originate from Germany and some people claim them to be OEM. (If they're not, they're damn close to OEM)


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## IHK11 (Jan 15, 2021)

Hi thanks for your reply. Ive seen some thermostats with a single hole in them to allow air to escape. At first i thought my thermostat had one and id installed it wrong causing an airlock but it doesnt have one. Im not having any trouble with overheating but having problems with my coolant temperature getting too cold which is making the heater temperature too cool. When im idling and the temp gauge hits half way my heater is really warm. I just need the temperature to stay around the middle of the gauge when driving as it keeps dropping really low when driving. Ive bled the system so many times and even at 3000 rpm so i dont think ive got air in the system. Thanks for the advice about the Febi Bilstein thermostat. Il give that a try and hope for the best. Thanks Ian!


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## MiltDoggie (Jun 30, 2020)

Have you tried replacing the Coolant Temperature Sensor or swapping it out with a known working one? You mentioned that they appear to be in good condition however they may be the culprit of your problem.
When I had problem with my Temp gauge a couple months back (gauge would drop and rise and drop and rise randomly with no significant pattern. Sometimes wouldn't register at all) the Coolant Temperature Sensor ended up being the culprit as it was still the original one from the factory 20 years ago. It also appeared to be in good condition.

This may be something to look out for... Just my 2 cents...


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## Vegeta Gti (Feb 13, 2003)

also log it to see voltage with VCDS. But milt is correct, those green tops are the best yet but they don't always last.


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## IHK11 (Jan 15, 2021)

Ive bought a new temp sender so i think il change that and might as well do the temp sensor then i can rule both of them out. Its strange because the car runs really good but has become a lot heavier on fuel lately. Ive gone from getting about 30 mpg to about 23 mpg. I think its worth doing both sender and sensor as the car is 30 years old. If its still the same after then il try a Febi Bilstein thermostat. Thanks for all your help Ian!


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## Vegeta Gti (Feb 13, 2003)

Your ecu uses coolant temp, intake temp and other data to create a target afr/boost/ timing point in a table, then make adjustments to obtain that target. If you have a boost leak, bad sensor here or there, etc it's getting incorrect data, therefore it will be adding or subtracting fuel/timing/boost accordingly. 

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## IHK11 (Jan 15, 2021)

It does sound like one of the coolant sensors could be faulty. Although they look ok you never can tell for sure. Do you think a faulty sensor could be preventing the expansion tank from getting warm? Its always cold whether the car is warmed up or not.. Ive never had this problem on any car before. Its been bugging me for a while. Thanks Ian!


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## Vegeta Gti (Feb 13, 2003)

That's a thermostat or blockage in the system. Clogged heater core can do fun stuff, sometimes they burst.

These cars are 22-17 years old now. 

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## IHK11 (Jan 15, 2021)

Its a real mystery. The hoses at the heater matrix seem quite warm so i think coolants definitely flowing through there. I reverse flushed it as well. The bottom hose next to the thermostat housing does seem to stay cold until the temp gauge hits just under 90 degrees celcius then it warms up and the radiator and all hoses going from it do seem to get warm. Although coolant flows through the heater core the car is 30 years old so the metal could be so worn maybe it doesnt conduct heat as well as it should. I imagine its a huge job to replace. Way too big a job for myself i think. Thanks Ian!


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## Vegeta Gti (Feb 13, 2003)

Who's Ian?

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## IHK11 (Jan 15, 2021)

Im Ian. Just thanking you for your reply. I wasnt calling you Ian. Sorry for the confusion!


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