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I have tested all 3 (original and 2 new) with a heat gun and they open. Is that not a sufficient test method?
Testing it in water would be a more accurate test. I would imagine it's hard to accurately regulate the heat with a heat gun. Some of those can to 1000F degrees plus. If the thermostat only opens above 300F, for example, that does you no good.
 

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Discussion Starter #102
I bit the bullet and bought a VW factory thermostat today. Because $58 for a $12 part makes sense. At least I can eliminate variables.
 

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If the engine is up to temp and the fan can be stopped easily, it means there is virtually no airflow through the radiator when the car is sitting still. It regulates the radiator cooling more than the electric fan.
 

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Discussion Starter #105
I honestly believe that is not your problem. Rule out the fan first.
I do not believe the thermostat is opening up. How is that linked to the fan?
97894

This is the hose connected to the thermostat housing. And I'm currently running steady at 225F. Shouldn't the thermostat be open by now?

Also the fan is still easily stopping. I ordered another but that's just another $100 thrown at this. I guess it's worth trying...
 

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I just have a hard time believing that three different ones are failing to open at the right temperature. The foolproof way is to drop it in boiling water (or just off boil) and watch it open. That way you know it works at ~200º. With a heat gun, who knows what temp it is opening at?
 

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Discussion Starter #108
I agree with you. It's ridiculous. But the thermostat is not opening and I do not believe it is a faulty part. Maybe I have a bad new water pump? I don't know. All I know is I'm getting really fed up. I do not believe I have any clogs, I replaced every hard part I could, I bled it 458 times, including the second time now on a vacuum bleeder. 3 thermostats. I don't see how the fan could help but I'll replace that too. Why not.

Sorry, just getting really annoyed with this thing...
 

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In the fan video you posted - was the car up to temperature, or was it cold?

I wonder if the coolant temperature sender is having issues? Maybe try comparing your infrared thermometer (pointed at coolant flange) reading with the temperature gauge?
 

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Discussion Starter #110
Coolant temperature sensor is brand new and is matching what the old one said. And the readings match what it's saying.

Yes the car was running at 225 at that point.
 

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Discussion Starter #112
Ok so I'm getting fed up. Here's where I'm at now.

Today I changed the thermostat (again) and used a VW oem piece. I drained the system very thoroughly. I then used my vacuum bleeder to refill it. The temperature stayed around 225F, but it took forever to get there. The electric fan never turned on and the mechanical fan clutch never seemed to kick in.

I do not believe the thermostat is opening. Or if it is there's no actual flow from the water pump. The top radiator hose, the offshoots to the overflow tank and turbo, the input and output on the heater core, the thermostat housing in the block itself, and the tube going from there to the oil cooler are all getting hot.

The bottom hose out of the radiator, the radiator itself, the thermostat housing, none of it is getting hot.

Remember, new radiator, new water pump, new thermostat and housing, thoroughly flushed (many times at this point).

So what's going on? Please someone have the answer! I'm going insane. I picked up a new fan clutch and another water pump. I don't want to do the water pump again but it might be inevitable at this point. And for the fan clutch, I haven't been able to properly test it because that area hasn't gotten hot enough for it to engage, I believe.

My brain hurts.
 

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With the engine running, can you see coolant going back into the reservoir (may be difficult if your reservoir is old and cloudy)? If so, the water pump is pushing at least some water. Same, if you are getting heat.

I assume that you are putting the thermostat in the correct direction (spring in the block). The shop manual calls for the part you can grab to be vertical when installing, but I dont' see what difference that would make.

If I followed your story correctly, I don't see how the water pump would be bad, if the temperature was okay (not overheating) with thermostat removed.
 

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Discussion Starter #114
I put in a new reservoir so I can kinda see but I'm only using water for now. I'd have wasted so much coolant! I tried and it looks like water might be trickling into there?

Thermostat is installed right. I'm an expert on that now.

It wasn't overheating without the thermostat in, but it was hotter than expected. 199F and that's without driving it.
 

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Discussion Starter #115
I'm going to try one more flush with some better chemicals and let it sit in there a bit longer. Maybe I can knock loose whatever might be causing a restriction if anything. I'll also be bypassing the heater core with some clear hose to check flow.

If none of that works I don't know what to do. I guess if I am not getting strong flow I could pull the timing belt back off and check to be sure the new water pump is good.
 

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Discussion Starter #116

Looks like the water pump is moving coolant. Right now I'm flushing again with the thermostat out.

Also after changing the fan clutch I realize it absolutely needed to be changed. That can't be me problem though, right? The fan has nothing to do with circulation...?
 

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Discussion Starter #117
97902

Been running 35 minutes now. Heater core bypassed and no thermostat. I don't know if I changed too many things at once to determine what might be my issue.

Bypassed heater core
Changed fan clutch
Took out thermostat

I took the thermostat out before and got 199F idle. So maybe that's not my issue. Maybe? The heater core can't be blocked cause I'm getting good heat when it's not bypassed, right?

The fan clutch in my mind can't be the issue because the radiator never got hot enough to make any difference, right?

Come on you guys. Y'all are better mechanics than me!
 

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Hopefully this makes sense as I am a bit off due to illness.
With the coolant flow chart listed previously, the coolant goes into the thermostat housing from the radiator. Since that temp is warm, but not hot, it is either not opening (which I doubt) or it is opening and the coolant is cooled to roughly 95 (externally) after passing through the radiator (the coolant inside the housing is probably much hotter). Since there was no actual airflow through the radiator with both fans broken, the coolant was not cool enough to drop the temps inside the engine. I expect this is strictly and airflow issue due to the fans, but it could have been a plugged radiator as well. Once you put the T-stat back in, see if the temps return to normal. Then work on the electric fan problem. My car never overheated in 4 years of driving without an electric fan. The a/c sucked, but I finally fixed the fan a few weeks ago.
 

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Discussion Starter #119
Yes it goes through the thermostat from the radiator when it opens. In the closed loop it runs from the pump, past the back of the thermostat and when that gets hot enough it should open. I believe somehow it was not opening.

As for my fans, the electric one works. I verified it day 1. I verified it by pulling out the fan switch from the bottom hose of the radiator and put a heat gun on it. Fan started. The lower hose has not gotten hot enough to make it turn on. The viscous was faulty as I am getting so much more air flow now.

I'm doing test 2 now. I put the thermostat back in. We'll see how we do with the thermostat, new viscous fan and no heater core. I feel like I'm getting somewhere.
 

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Discussion Starter #120
97903


30 minutes. Thermostat in, heater core bypassed. I may have found the culprit. I think I'm looking at a heater core replacement! 😣
 
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