open thermostat...how do you tell?

Yes, this is a stupid sounding question (at least to me) but how can I tell if my thermostat is stuck open? The temp gauge barely moves in the summer time and in the winter, forget it. I do get heat and it's hot but it takes a LONG time. I finally put a cover over the upper grill and the gauge moves two clicks and I get heat a little bit faster but again it's not all that great.

Also, when I say getting heat faster I'm talking about driving. I know better than to idle the car and attempt to get heat

I don't know if it's just because the engine is that much bigger but my 7.3 powerstroke seems to heat up (to the point where I get heat in the cab) much much quicker.

I don't know if this is just the nature of the VW TDI or if my t-stat was open. As far as fuel mileage goes, I've been consistently getting higher mileage in the 48-50 mpg range in the summer and 43-46 in the cold...just if that helps in the diagnosis.

Thanks for your input!

When mine was open, I wouldn't get much heat around town. On the highway, it would heat up going up hills, and drop quickly on the other side.

With good one, it goes to 190 in about 15 minutes driving around town.

I suspect you have an open t-stat.

rdkern said:

When mine was open, I wouldn't get much heat around town. On the highway, it would heat up going up hills, and drop quickly on the other side.

With good one, it goes to 190 in about 15 minutes driving around town.

I suspect you have an open t-stat.

Click to expand...

Repeat.

well that pretty much says I'm not crazy!!! I just always assumed the t-stat was fine since it did get heat (and when I do get heat it's hot, not just warm). I'll have to attempt this when it warms up.

Hey on a side note but similar topic, what happens if I don't use G12 coolant in cooling system? I think my brother (the previous owner) said he used regular orange coolant. I'm assuming he drained the cooling system and replaced it all. Just curious.

As long the old coolant was flushed it will not cause a problem. Hopefully he didn't use Dexcool...

On the thermostat question, the way I do it:
Start the engine cold, touch upper and lower hoses at the radiator. (Not always an easy task)
With proper thermostat operation the upper hose will get warm/hot whereas the lower will be cold until the thermostat opens then they will be the same temp. (Coolant recirculating).

If they are the same temp during warm up then you have a stuck open thermostat and coolant is recirculating constantly.

You will have massive fluctuations in engine temp

FYI. The dash gauge comes off the starting point at about 135°F, and shows 190 from actual 165 to 235°F.

mctdi said:

FYI. The dash gauge comes off the starting point at about 135°F, and shows 190 from actual 165 to 235°F.

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I'm pretty sure mine doesn't. I'll hook up the VCDS to test, but the rapid, but smooth motion it had with the bad t-stat would be difficult with that much buffering. I've heard the A4's do this, but cannot confirm this from personal experience.

You 100% have a weak thermostat. I've found they don't fail open, they just get weaker and weaker over time, opening at a lower and lower temperature.

On my mk4 I had a weak thermostat, replaced it and got heat MUCH soon, but the replacement was stuck open after that. When it failed open the car would not heat up at all. On the highway the gauge didn't move.

On my b4 I had the same issues as you (not as bad), thermostat replacement fixed it.

i just replaced a 6 month old OE thermostat with a cheapy one and i get heat faster but still didnt get above 175 at any point yesterday... it was below zero the whole time though

hoping it will be ok after this frigid weather ends, but mine did as skatanic said, got weaker over time but it only lasted 6 months before i was unable to keep the coolant above 120F.

My engine temp would not go above 160. I had a new stat put in and it imeadiately went to 198 and holds there all day long 365 days a year.Now I need to get the heat/defrost blender door repaired so I can bring some of that wonderful heat into the cabin area. I forgot do not use a cheap aftermarket stat cough up the bucks and put in OEM

mine is still only at 160 - 175 while cruising even with a new 195 stat. it is warms up way faster and holds temp better than the one i just took out but it still cannot get 190+.

not sure what else could be my problem... i can get decent heat inside but it takes a while and is nowhere near as good as it was before when i could get 190 engine temp while cruising.

i only went with a cheap one because i had a gift card and the OE one had failed so soon after install... however i kept the OE one, just in case.

Test the OE one in boiling water. If it doesn't react in the 190 range, (ie, always open) then get a new one. Sounds like yours is opening at 160.

thats why i saved the OE stat but i need to find my thermometer, or get a new one.

i just wish the new stat had fixed the issue completely, but its only marginally better.

IR Temp gun is a favorite tool, point, shoot and read temp. Trick is interpreting the readings correctly.

well this sounds exactly like my problem. I just assumed this VW diesel engine ran cold.

So just out of curiosity, what will this do for fuel economy when I put in the new t-stat? I would imagine right now the mileage should increase as it's not burning at the proper temperature? Like I said, I'm just curious. My mileage definitely went down a few points since the temp dropped below 45* or so.

Thanks Again,

I doubt you'll see a difference.

I think I saw a difference. Highway driving mpg dropped between when I got the car and when the t-stat was going. Perhaps other things affected this too, but highway is in upper 40's-50ish now and when I first got the car, but with the bad t-stat I was lucky to get 45.

Around town even worse. Low 30's prior to t-stat change, about 40 now (generally short trips).

well i'm on the track to replacing the thermostat as soon as I get my car back from the alignment shop. To add injury to insult the car slid in the parking lot at work and I ended up bending a tie rod. On top of that I've been stuck driving my old ford pickup and it too has some kind of heat issue. Go figure.

Anyway, I've read it's best to just buy a new t-stat housing. Dealer only wants $18 for one. Not too shabby!

The almighty question is then do I spend $23 and buy a dealer "OEM" thermostat or just stop at advance auto parts/Autozone and pickup a Stant superstat? What temp range should I be looking at for this engine?

Thanks again.

Stant 238 from NAPA is my preference. It's a little hotter than normal and works great.

When thermostats fail, do they usually fail by staying open?

Yes..