The only definitive test is to take it out and look at it - same as for the thermostat. Not what you wanted to hear, but everything else is just inference.

 

If the car overheats on the highway but not on low speed driving, 99% chance the water pump is bad (plastic impeller slipping on shaft). If the water pump has over 70K, 50% chance it is leaking. If the coolant is leaking down from the lower pulley onto the oil pan, 100% chance the water pump is bad.

The only other way to be more certain is to remove the lower timing belt cover and inspect the pump for leaks. To see if the impeller is slipping, you need to remove it and put it in boiling water and then see if the impeller can be turned while the pulley is still.

Either way, that is 40-50% of a timing belt change.

 

If the WP is totally shot, but the thermostat is good:
The cold engine starts and warms up normally, but the lower rad hose doesn't get hot for a very long time. Reason: the only fluid circulation is by convection.

You will not sleep well until you know, so go ahead and change it out ! Sorry, that is so easy to type and so much harder to do (

 

Thank you for your post. I had these symptoms. Driven carefully, the car maintained its normal operating temperature according to the temperature gauge, so long as it did not have to go up a hill. The outside temperature was about 15 C. I should also add that the heater would not heat the car even when on full blast which made me think I had water circulation problems.

The first time the car got to hot, there was a big red STOP on the dashboard and a red temperature indicator. I had just reached the top of a hill so I coasted to a lay by. The water level in the coolant header tank was normal, which puzzled me so I added some oil to the engine, as the level was low.

After that I watched the temperature gauge more carefully, and stopped if I thought the car was going to overhead.

When I dismantled the car I found the water pump plastic impeller had shattered. It is made from quite brittle plastic. If it had been metal no doubt it would have lasted longer. The car has done at 90,000 miles and is 12 years old.

 

Thanks for all these suggestions! Helped me to make up my mind to replace it anyway. Especially from PZ's suggestion, it seems indeed that my water pump is failing.

The first overheating occurred on the highway on a hot day, preceded by intermittent stopping of cool air from AC. After that, the overheating usually shows up after a few miles drive, especially after an acceleration. There seems to be no leaking of coolant. No clear drop of the level in the tank.
But the wired thing is that the water pump is a new one with metal impeller. I replaced it together with my TB change early last year, when it was at ~76000 miles. Unless it was a defective one. I bought the kit from mjmautohaus . I didn't replace the thermostat though, because I didn't know how on my AUG engine.

Anyway I think I can't escape doing all the pains again.

 

That info would be good to know beforehand.

Defective metal water pumps are very rare. You may have an air pocket in the coolant. I would try bleeding the system via the heater hoses. I would also change the thermosat.

If that does not work, I would run a compression and leak down test on the engine to rule out a bad head gasket. Only then would I replace the water pump.

 

hmm... so the water pump is less likely to be bad...
I feel that it's not very likely to be a big bubble in the coolant. Because I flushed the coolant following the TB change over 1 year ago. There was a big bubble back then, and I think I got it out and topped off the tank. Everything had been fine in the past year.

Doesn't changing thermostat require similar steps or amount of work as change water pump?

 

I don't know, it sounds like a thermostat to me, especially since you said that was the one part you didn't replace. The first time you overheated on the highway, were you cruising at 70 mph, or stuck in traffic?

 

The first time overheating was when driving in a highway maybe 50~60 mph, but not when idling.
Later it usually started overheating after several miles highway drive. I had to pull over and wait for a few minutes. Then it came back much sooner, and usually after an acceleration.

Would all these seem to be consistent with a bad thermostat?

 

Btw, with regards to your earlier question of how hard it is to change the thermostat, it is VERY easy! Takes about 20 minutes. Remove the throttle body for easier access, the t-stat housing is at the end of the coolant hose. Dust off your Allen wrenches
when my wp went, I tried changing the t-stat first just to confirm.

 

That simple to replace t-stat on an AUG?

 

On AUG? Not sure.... I can only speak for AWM's

 

AUG and AWM are the same block.

 

Yeah, AUG.... Everything seems to be less straight forward on it.
But it's still good news that thermostat is could be easier. I ordered both water pump and thermostat from ecstuning. I'll definitely try TS first before taking everything apart...

 

Hi ning,
Did you ever get this issue resolved? I'm facing what I think is the same issue. At highway speeds, when pressing the engine, the car overheats, but around town it seems fine. What's really troubling is that when the car is overheating, even the heat coming into the cabin will cool down if I put the fan at max. What I think is happening is, under stress the WP impeller is slipping and not circulating much coolant. So none is getting pushed into the heater core either.
What's the real pisser is that I have the WP replaced recently in September of 2013 by Black Forest in Cary, NC. It's not that old, and I'm pretty sure they installed a WP with a metal impeller.

Anyway, I plan to replace the thermostat myself this week, but I don't think that will solve the problem. After that, it's back to the shop to get the WP replaced (again!).

If you see this thread, and remember how it all shook out, please let me know.

Thanks,
Brad

 

I would take it to a VW shop.

 

A slipping Impeller can be verified when changing Thermostat by reaching inside cavity to see if it free wheels, as long as TBelt is still attached. At least for AWM.

 

Where were you 4 hours ago before I'd replaced the t-stat? What you're implying is that the WP impeller is hard-cogged to the timing belt, right?

Anyway, replacing the t-stat didn't fix it, and I don't think its the head gasket, so it is probably the WP. :-(

 

Yes, it is likely a faulty WP.
NOTE: When they first start slipping, the impeller can be quite tight when cold.

 

Sometime ago I had what I thought to be overheating issues cause I was loosing coolant. I kept thinking the car was releasing expanded coolant through the recovery tank overflow. So at this time I suspected a bad WP and for a temp fix hooked up a inline coolant pump from an earlier passat. I attached this pump between reservoir and Tstat hose and also introduced 2 small holes into the Tstat in order not to constrict the flow while engine is warming up cause of great flow and closed Tstat. Got to say this was a great fix if you are not sure whether you have a bad WP or not. This idea was posted by some other on this forum when he had troubles with heater core not getting flow. Pretty clever who ever it was. This could buy you time while planning a WP replacement.

 

The original VW pump with shattered plastic impeller is on the right and the non VW replacement with a metal impeller is on the left.

VW should have made the part properly in the first place.

 

The original VW pump with shattered plastic impeller is on the right and the non VW replacement with a metal impeller is on the left.

VW should have made the part properly in the first place.

View attachment 52057

They aren't the only ones. BMW did too.

 

I installed Hepu brand metal impeller water pump from FCP about 75k miles ago. Running with no problem at all. I am going to do another TB change soon.
https://www.fcpeuro.com/products/audi-vw-volkswagen-engine-water-pump-a4-passat-hepu-p518

 

I'm having the same issue with my 00 VW Passat now, got it a few days ago with 156k. I test drove for a hour & the temp gauge stayed at the 190 mark... Well next day I go purhase car & drive about 50 miles back home & my last 10 miles the temp gauge starts to move up to the 220-260 mark. I had just made it to work, parked & popped the hood. No smoke, coolant was full but there was a light burning smell! The car is a beauty itself.. I'm changing the thermostat today and also the CTS, hoping this fix my problem. When driving on street car does good even on highway till I get into traffic then I'll have to just watch the gauge.... Any help would be great