Passat TDI running cold any ideas

[Hillbilly]

This morning went from Sunny Coast down to Bribie Left at 8am Temp 14.5 C Noticed getting down the road car was running cold by about 15degrees. Was still under at 9 am when I got down there 90km later. Temp was up to about 16 by then. Coming home at 1pm it got up to 90 ok but temp was up to 24C by then. On the way down it was fluctuating between 75/85/90/ 75 . All over the place.

Rang the dealer suspecting something might have got disturbed when had belt and water pump done a few weeks ago and not used car much since. It has all the symptoms of a stuck open thermostat but they said it may be in the heater area. Has climate control and stays set on 22 all the time and was working ok today.

Anyone had similar problem and has a fix.
TIA

[Umai Naa!!]

Thermostat.

If the one for the DSG doesn't sort it, the one buried behind the alternator will.

If you set the Climatronic to 22deg, it'll still be cooler than engine temp, even with a stuck open T/stat. Also, the gauge isn't accurate. Ever.

[Hillbilly]

Ok rang the dealer who said just monitor it so today I turned it on in the garage for about 40 mins and it didnt come up to temp. Took it for a 10km run and eventually it did then dropped back to 80 and fluffed up and down. Put it back in garage and left it running and it dropped then sort of stayed nearly hot Turned it off for 15 mins and then it read 90.

I had a look around but pipework is a bit bewildering. Took top off overflow bottle and it gave a puff of air like it was pressurised. Normal????

Wonedered if it has an air bubble trapped after they did the water pump

[Umai Naa!!]

System is meant to be pressurised to raise the boiling point.

Definitely sounds like a thermostat issue.

[Hillbilly]

System is meant to be pressurised to raise the boiling point.

Definitely sounds like a thermostat issue.

Thanks will take it in to them How much labour to change thermostat

[Umai Naa!!]

Depends which one they deem it to be.

If it's the one for the DSG, it's fairly easy.

If it's the main one, a few hours, as it's quite buried behind the alternator and everything.

[Hillbilly]

Im struggling to get my head around why a transmission thermostat would make an engine run cold Radiator is hot hoses are hot engine seems normally hot when I open bonnet but gauge says otherwise
Also dont remember any other cars overflow bottles being pressurised. They just sucked or blew according to how the radiator cap was feeling. This let go like a fuel cap being removed

[Umai Naa!!]

The cooling system for the engine and transmission are one in the same. If the trans thermostat is closed, the loop in the system for the transmission is just bypassed until the desired temp for it to open is reached.

I don't entirely subscribe to the transmission thermostat being the main culprit either, but there's some history of it if you search. I've replaced the main one on a few of these, and that did solve the issue.

It's an expansion tank, not an overflow bottle. It's in the cooling system, that's why it's pressurised like the rest of it.

[Transporter]

This morning went from Sunny Coast down to Bribie Left at 8am Temp 14.5 C Noticed getting down the road car was running cold by about 15degrees. Was still under at 9 am when I got down there 90km later. Temp was up to about 16 by then. Coming home at 1pm it got up to 90 ok but temp was up to 24C by then. On the way down it was fluctuating between 75/85/90/ 75 . All over the place.

Rang the dealer suspecting something might have got disturbed when had belt and water pump done a few weeks ago and not used car much since. It has all the symptoms of a stuck open thermostat but they said it may be in the heater area. Has climate control and stays set on 22 all the time and was working ok today.

Anyone had similar problem and has a fix.
TIA

Since you mentioned that you didn’t drive the car much after they’ve replaced the water pump,
I’d make sure the the cooling system has no trapped air in it, before replacing thermostat.
Though, the thermostat maybe coincidentally failed at the same time or was already failing.

[drjekl]

Hmm this thread is interesting because Ive had a new waterpump and timing belt fitted a few weeks ago (continental) and since its my wife's car I dont drive it that much.

But when I did drive it the other week, it took something crazy like 40 mins to get up to temp at highway speeds!??! fuel economy is fine (always sits around 7L-100km) but I'm positive it use to warm up much sooner then this. I asked a work colleague about this and he said he experienced similar issues in his TDI B7 passat but he cant recall what resolved it (I'm guessing possibly air in the system)

In my BMW I can hold down my accelerator for 15 seconds which activated the coolant bleed cycle, is there a trick like this with the VW B7's??