/// Thermostat issues or something else ? ///

Hi All,

The other day, the temp reading on my scan scan gauge went to about 230 degrees and hovered pretty high....215+ degrees to 220 degrees for a good long while...

The factory gauge was still in its "normal" range and no temp lights came on.

I did not smell anything funny and when looking under the hood nothing smelled, sounded or seemed funny..

Today, I took the car for a short ride to a friends house (it started cold) but for whatever reason, the car was almost, to 195 degrees in a VERY short time (normally I have to drive almost to my parents house before it would be still in the 180's in temp)....by short time I mean literally a few minutes...

After the car sat for about 7 hours, the car started off at 66 degrees on the scan gauge but warmed up much quicker than normal but did not get into the 200's like before (admittedly I did not drive it far enough to see what would happen although the last time it was well into the 200's nothing "bad" seemed to be happening.

What gives ? Could it be a faulty temp sensor ? A partially sticking thermostat ? A combination of both?

I can't recall the last time I switched the thermostat out BUT I do remember reading that a 205 degree thermostat may net better fuel mileage (where would one get such a beast....I read where one was custom made or is it now commonly available ?).

Would the higher temp thermostats shorten the life on anything?

My current thermostat seemed to stay in the 190 degree range before yesterday.

Thanks in advance for any and all help.

Andrew

It's time for a new thermostat. Get a new housing as well. Oem is best but you make that call. Look for a thread by josh8loop on the hybrId thermostat. If you want to try that at a later date after more research would probably be best. My thinking is get your temperature down proving it was the thermostat then experiment with a higher temp opening thermostat.

OK, gotta ask...

How's the coolant level? If you're loosing coolant then things can for sure heat up a lot quicker.

Thermostats generally stick in the open position. Coolant temperature sensor could be flaky (perhaps in addition to something else).

Oh, yeah, MAKE SURE THE WATER PUMP IS OK! Is coolant being circulated well?

Another good reason to have a separate gauge like an Ultragauge. As these show what the engine's [ECU's] real temperatures are (dash gauge is like a stupid side-kick, can't really be trusted).

I would start by replacing the thermostat or at least testing it in a pot of boiling water with a thermometer.
I'd change the coolant temp sensor while everything was drained too.
BUT,.....it could just as likely be a water pump impeller slipping on its shaft.
While you have the thermostat out you can reach in and feel the impeller to see if it slips.

As posted above, VW thermostats usually stick open. So you'd see a cold engine, the reverse of your experience. I think UhOh is on to something-failed water pump. Impeller isn't turning any longer. Or possibly a failed head gasket, but I'd suspect water pump first.

If you're replacing the pump you might as well do a timing belt service unless it's been completed recently. And if it has been done recently and your water pump is failing, I'd suspect the comprehensiveness of the timing belt service.

Another thing: Cooling fans should come on at 96C, or 204F (IIRC). Were they running? If not, you may have another issue.

One more vote on pump, sounds logical. Make sure your replacement has a steel impeller. Definitely think about where your belt and idlers are in there life cycle. Are they as old as the pump? Effort to change the belt is not much more than doing the pump. Also, if that pump were to sieze, SOL

I'd start with checking the coolant level, then see if the fan is coming on. If that checks out, then I'd move on to the thermostat, water pump check, coolant temp sensor, and radiator condition.

Does the lower radiator hose get hot?

HI All,

Thanks for the inputs....I was called out of town for work today so I drove the minivan getting comparatively horrible mileage...

Ill spend some time before going into work tomorrow and see what I find...

Again thanks for the inputs....I'll report what I find.

Andrew

Check the flow of the return hose to the coolant reservoir.

I saw my temp go to 213 on my ultragauge once. It was a hot day and a pretty significant engine load. It didn’t stay there long and was back to 190 or so fairly quickly. When that happened the needle on the heat gauge never moved. My needle reaches the 190 position at 164* and stays there till at least 213*.

There is a small hose that leads to the water resivor near the top. Start the car and unscrew the resivor cap. If your water pump is working a steady stream should be flowing from that small tube into the resivor.

If you are going to replace the thermostat and I definitely think you should as others have said. While the housing is off reach your finger in the hole and try to turn the water pump impeller. Just to be clear the water pump is hooked to the timing belt as you know. That impeller should not move at all. If it moves even a little you need to replace it and the timing belt and related components as well.

If the water pump is good and a new thermostat, thermostat housing and new coolant sensor doesn’t solve the problem then you probably have some type of gunk in your coolant system. Have you ever mixed vw coolant with the green stuff or other coolant of any kind?
If so then you may have some jello like substance in your coolant system.

VW coolant doesn’t react well with other coolant.

HI,

Thanks for all the replies....

After looking at all the posts here and having a mechanically inclined friend over to look over the car, we ended up replacing a temp sensor (it appeared to be seeping a bit also) and that seems to have solved the issue.

I will look into replacing the thermostat soon since I think the last time it was replaced was when we did the timing belt and all before (It's due for another one in about 20K miles and since I mostly drive all anyway it should be soon).

Thanks again for all the input.

Andrew