Headache after headache after headache after headache ...

[seadoo2006]

Ah, the never-ending joys of Audi ownership ...

History to this tale:

After purchasing my Ultrasport Avant this past January, it decided to blow the seals on the 5000 mile, 3 month old water pump halfway through Texas. After a looooooong 24 hours of driving back to Ohio having to fill it back up with coolant every 100 miles, I replaced the water pump and the car ran right as rain for a few months. Then, on my cross-country road trip this summer, the car developed a leak in the coolant line to the turbo, draining the entire system of coolant on the side of the road while the residual heat of the block popped the head gasket. After determining that the car had low compression on all cylinders, rather than risking replacing the head gasket on a 181k mile car, I just replaced the engine from another 1.8t for the same price.

This new engine seems to be under-heating now ...

1) How much pressure should be built by the cooling system of a stock 1.8t after driving for an hour? My recent experience has all been overheating situations, so when opening the cap, should it hiss for 30 seconds and boil over if opened quickly? Or should it let out a light "pop" and then unscrew with little or no drama? Because my car is doing the latter. Maybe 1-2psi of pressure in the cooling system.

2) My B6 is still undercooling. Just replaced the thermostat & CTS and she still gets cold (to about a 1/4 on the gauge) going down long downhills. When my R32 did this, it was because someone put the wrong FCM in the car causing the coolant afterrun pump to always cycle. I do not believe this is the case with this car. The fans run normally and cycle on and off as they should.

3) I am losing no - ZERO - coolant and my heat only seems to stay warm when driving with load. It gets colder at idle and if I'm cruising downhill.

4) I have no CELs or codes for anything regarding the cooling system.

5) I've bled both the heater core and the hard pipe on top of the engine. Doesn't seem like there is air in the lines, but seeing that I'm not making hardly any coolant pressure, I don't know how effective bleeding is.

Any ideas on what to do? The last time this happened to one of my VAGs, I spent $2500 trying to fix it by replacing every component of the cooling system and it ended up being a FCM. Trying not to fire the parts cannon, but, it's a VAG, so what's my next step? I've replaced the t-stat and CTS, so what else could be causing it to underheat in low-load situations?

TIA.

[Rocs]

Id gladly take that USP Avant and those problems off your hands

[MetalMan]

Generally speaking this sort of issue is over-cooling, caused either by a stuck-open thermostat or excessive fan cooling.
You mentioned already replacing the thermostat, so hopefully you can rule that out... did you use a Genuine Audi part or aftermarket?
And can you confirm the radiator fans aren't running constantly?

[seadoo2006]

Generally speaking this sort of issue is over-cooling, caused either by a stuck-open thermostat or excessive fan cooling.
You mentioned already replacing the thermostat, so hopefully you can rule that out... did you use a Genuine Audi part or aftermarket?
And can you confirm the radiator fans aren't running constantly?

T-Stat was OEM from Audi dealer.

Radiator fans do not come on when the car is cold (first start in the morning) and remains that way until car is to temp. They do not seem to shut off once the car is warm though.

99% of the time, the car's needle is straight up on normal. Only seems to be an issue when coasting downhill or driving on level ground with low load (1500rpm, 6th gear).

Heat is hot when I'm driving, but will blow cool when stationary or when the gauge dips in the previously stated circumstances.

[seadoo2006]

Guessing I may have gotten a bum tstat based on the responses so far. Does anyone have a good diagnostic algorithm before I spend another $300 getting it fixed ... again?

[MetalMan]

Can't help with T-stat diagnosis... but can you confirm whether or not the fans are on while the "underheating" scenario occurs? There is a thermal switch in the lower radiator hose that may be worth looking into if it hasn't been replaced. Should be cheap enough to replace but coolant has to be drained.

[coffeysm]

Maybe take the thermostat out and stick it in some boiling water?

[old guy]

Read posts 14 and 22 in this thread for an explanation as to how the mapped cooling system functions and how to diagnose a bad thermostat: Clicky click®

[seadoo2006]

Can't help with T-stat diagnosis... but can you confirm whether or not the fans are on while the "underheating" scenario occurs? There is a thermal switch in the lower radiator hose that may be worth looking into if it hasn't been replaced. Should be cheap enough to replace but coolant has to be drained.

I'll check that out. My gut feeling is that this is something mechanical and not electrical as there are measurable effects in addition to the gauge not reading normal - ie., heat gets cooler and cooling system not building pressure even after a lengthy drive.

Maybe take the thermostat out and stick it in some boiling water?

Easier said than done. I live in an apartment complex, so I must use the services of a mechanic. Unfortunately, for some reason, warranties don't ever cover the labor portion of this fix. I spent $300+ on the last tstat replacement. Just trying to avoid dropping another $300 to fix a part that is actually working correctly in search of another part that has failed.

Read posts 14 and 22 in this thread for an explanation as to how the mapped cooling system functions and how to diagnose a bad thermostat: Clicky click®

You, sir, are a god amongst men. I was hoping there was a diagnostic procedure for these cars' tstats. I will do just that and report back. Thank you.

[Steve Bassen]

Could always be a flaky temp sensor, as well, that's causing it to read low. A no-name "assembled by ECS" sensor ran a good 15 degrees low vs a genuine OE sensor on my car.

[seadoo2006]

Alright, so ambient temps this weekend were in the 70s again, so I wasn't able to replicate the problem on any of my usual downhill stretches. I will update this with a log when I see the problem return.

Here is the first 7.5 minutes of logging on a cold startup ... not sure if anything is out of order here. Gauge hit straight up on normal mark at around the 210.0s mark and never deviated from that position with my logs today.



Also, here is the log from the downhill stretch during my normal commute home. Again, I did not see a temp gauge drop off today, but this is the stretch of road where I normally see that happen:

[mobilesuit818]

I had a similar situation. Car gauge wasn't going past the middle but was actually overheating with bubbles etc.
Thought it was head gasket but took the time to troubleshoot. Changed the thermostat assembly and FCM with no change and really thought it was the CCM.

It eventually was found to be the rear coolant temp sensor....so check that.