What's up with $&#*!^% thermostats?

Two years and 34,000 miles ago I replaced the thermostat in my 2000 Golf. Bought the thermostat and flange from one of our trusted vendors, put them in myself, and thought I would be done with that particular task for quite a while.

But recently I've started to notice some symptoms that made me think the thermostat was sticking: slower warm-ups than I'd expect even with the cool fall temps, and occasionally overshooting and overheating when warming up.

Then the coolant level started dropping... first slowly, but in the last couple weeks it needed a pint... looked around for the leak and it looks like it's the thermostat flange gasket. Grr. So new parts are on order now. Hope the weather's kind to me next week.

Do they just not make thermostats (and flanges and gaskets) like they used to? Have other people had problems with thermostats not lasting very long?

Mine's doing it too. Tried aftermarkets (which all three different brands were way under temp) before I bought the Wahler, it has lasted about 4 years, but now its dropping a few degrees and a random big drop now and then. Certainly not rocket science, but sort of odd that something as basic as a stat for a VW (including OEM ones) can't last.
At least no leaks, and I've reused the flange every time and saved the spare.
Not a big deal, but the stat in my '99 Suburban is original and still fine.

Are you 100% sure it is not the water pump leaking and having its impeller separated from the shaft?

What brand was the thermostat?

In any case, when you will remove the t-stat for replacement, try to reach the WP's impeller with your fingers and see if it doesn't spin.

My suggestion would be to go with BEHR brand.

I used a Behr t-stat and it seems to be functioning fine. The housing and o-ring, however, did not. I ended up replacing them using dealer (OEM) parts and all's well. On inspection, the first set were clearly not VW. Others here swear by Behr, though Wahler is also mentioned as "good".

When you replace the gasket, I suggest treating both surfaces with Permatex no. 2 non-setting gasket sealant. It's never failed for me. If it's a gasket, that is, not an O-ring.

Yea, I changed mine a couple of years ago also. I got this one from the dealer along with the flange and gasket, and my Ultra Gauge hovers around 194 to 192. The lower radiator hose is always cold; so I don't know what to make of this. My former replacement did not run as hot. I think they are both Behrs since they lock into the flange like they're supposed to. I have to admit though that I did modify my cooling system hoses when I added coolant GP's to my auto equipped Golf. This required a T connection from the ATF cooler directly into the heater core. The bypass function on my EGR cooler has been eliminated on my BEW engine. I seem to get faster cabin heat this way.

In my experience all the aftermarket o rings leak. Get an oring from VW and you can use the aftermarket thermostat...

FlyTDI Guy said:

Others here swear by Behr, though Wahler is also mentioned as "good".

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Indeed it is good but Wahler's installation is tricky. So why to go a complex way when the price is the same?

Henrick said:

Indeed it is good but Wahler's installation is tricky. So why to go a complex way when the price is the same?

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Nothing "tricky" about the Wahler install, other than the silly clips won't hold it on the flange like an OEM, which if you reuse the flange typically get broken upon removal anyways.
I have yet to need to use a new oring or flange, however. Still have the replacements as spares.

Aftermarket flanges SUCK. Get an oe one.

Henrick said:

Are you 100% sure it is not the water pump leaking and having its impeller separated from the shaft?

What brand was the thermostat?

In any case, when you will remove the t-stat for replacement, try to reach the WP's impeller with your fingers and see if it doesn't spin.

My suggestion would be to go with BEHR brand.

Click to expand...

I've already ordered a "genuine VW" thermostat and flange from Bora Parts. I'll see who's making them for VW these days when they get here

That's a good thought about the water pump. I'm pretty sure it's leaking at the flange, it's wet right under there, and the skid plate had coolant on it right below. But now that you've mentioned it, I'll pull the timing belt cover tomorrow and make sure there's no coolant in there.

I had the timing belt changed not long before I put the thermostat in, and it was supposed to be a good one with metal impeller, but I'll remember to check that when I change the thermostat.

BuzzKen said:

Aftermarket flanges SUCK. Get an oe one.

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Yeah, the extra cost for real VW parts is pretty insignificant. It wouldn't surprise me if my whole problem is because the non-VW flange warped

EVERY aftermarket flange I have installed in my cars have had to be replaced due to warping which caused them to leak so now I only use OE flanges and none of them leak.

jettawreck said:

Nothing "tricky" about the Wahler install, other than the silly clips won't hold it on the flange like an OEM, which if you reuse the flange typically get broken upon removal anyways.
I have yet to need to use a new oring or flange, however. Still have the replacements as spares.

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Yep, that what I was referring to. You will need to use rope to install Wahler.

BTW, I have reused the flanges (even with broken clips) multiple times - never had a problem or a leak.

MidnightBlue said:

I've already ordered a "genuine VW" thermostat and flange from Bora Parts. I'll see who's making them for VW these days when they get here

That's a good thought about the water pump. I'm pretty sure it's leaking at the flange, it's wet right under there, and the skid plate had coolant on it right below. But now that you've mentioned it, I'll pull the timing belt cover tomorrow and make sure there's no coolant in there.

I had the timing belt changed not long before I put the thermostat in, and it was supposed to be a good one with metal impeller, but I'll remember to check that when I change the thermostat.

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"New" and "good one" water pump means nothing. Sometimes things just break, sometimes there are manufacturing defects. A buddy of mine has bought a brand new Vauxhall/Opel Zafira and it leaked transmission fluid at the selector shaft seal from day one!

In order to see is WP is leaking, you will need to remove the bottom timing belt cover. I think it's not worth of that. Just wiggle the impeller while you have the t-stat removed to see if it is not separated (but assuming it is a metal one - the chances are really low).