2000 A6 Avant 249,500 miles

Been fighting a mild overheating problem since last summer. Only truly overheated (2:00 on gauge) once in stop & go traffic on a hot day. In the fall/winter it just runs hotter than it should - between the usual 12:00 position and 1:00. So I'm trying to solve it before warm weather returns and without spending much given the age of the car. Fundamentals of the car are good (e.g. 180 psi on the compression tests I recently ran), but after 19 years I'm starting to get ready for a new one, so limiting expenditures as much as I can. It runs between 12:00 and 1:00 both after idling a long time and on the highway. Worst is climbing a big hill. Gives good heat (within 6 minutes). If it gets at or above 1:00 on the coolant gauge, starts to burble in the overflow tank and drip coolant out below the cap.

Stuff I've already done:
- replaced coolant temp sensor (just in case)
- replaced overflow tank cap (just in case)
- pressure tested at overflow tank; held 14 psi for about 3 minutes
- flushed coolant
- ran liquimoly coolant flush through
- bled air from top heater core return until it spurts fluid (multiple times)
- replaced fan clutch (just in case)
- confirmed that both fans work well. Electric fan does low speed whenever climate system is on, then high speed when coolant is hot.

Water pump and thermostat haven't been changed in close to 140k, but I'm trying to avoid the cost of the whole timing belt etc. job and given the symptoms I'm not suspecting those are the issue (see more below).

So today (42 degrees out) I pulled the bumper cover and AC condenser and monitored the radiator with an infrared heat gun. While the engine was warming up, the top radiator hose and radiator stayed at outdoor temp (so the thermostat seems to close well). After awhile at the 12:00 position the top radiator hose and adjacent part of the radiator start to show some heat. After lots of idling, including at 2000-2500 RPM, and turning off the climate control (which disables the electric fan) got it to go up to and past 1:00. Top right of radiator was hot (like 180F) . Working across top, it drops to 95F by left side. Doing the same about 1/4 of the way down, starts at 160F and drops to 80F by left side. Half way down starts at 140F on right and is down to 60F by middle (seems too fast), then stays at or above that temp to the right. 3/4 way down starts at 90F and drops to about 50F by middle. and this is while the overflow tank is burbling and releasing overflow. Squeezed the top radiator hose (which was very hot), and it didn't resist much, making me think the radiator isn't blocking the flow.

All told, when the car starts to overheat, upper radiator hose is about 180F and lower radiator hose is about 100F. So on one hand, the radiator and fans seem to be doing their job of cooling. But maybe the flow rate through the radiator is too low?

So, change the radiator since maybe it is restricting the flow? Or figure that the thermostat is closing and opening, but only opening part way when it does?

Thanks for any help.

 

It does sound like your water pump impeller could be shot, I see what you're saying about the cost of parts as the car gets older - my A6 is a 2001 so I'm in a similar predicament (I love the car though!).... Can you get hold of an electric auxiliary water pump? You could fit that up near the heater core purely to increase water flow and help the water impeller out a bit? Make sure you get the flow direction correct. VW have an auxiliary pump on some of their cars as a cool down feature when the car is switched off after a long hot run - maybe get one from a breakers if new are too expensive.

- could be a cheaper option than a complete new radiator..... especially if you can get a cheap pump....

 

very cool idea - I'll look into it, thanks!

 

'cool' idea - like it

 

It might be your thermostat, but I'm a little concerned about the bubbling in the over flow tank... Is the system pushing coolant into the tank when it's hot? That could be a symptom combustion leak in the head gasket... Which would also cause the over heating since the coolant is not able to circulate properly..

 

I agree the bubbling doesn't seem right given that it's under pressure, so shouldn't be boiling at anything close to 212 degrees. The bubbling only happens when it's getting hotter than 12:00 on the gauge. That said, I recently compression tested cylinders 1,2,3,5 and they all came out great (180 psi). I got lazy with 4&6 because the test hose just didn't want to screw in and I was mainly doing it to check out cyl 1 for other reasons. Not sure how likely it is to have a leak just in 4 or 6 if 5 reads 180. Also, I don't see signs of oil coloring the coolant and the coolant level isn't dropping. On the other hand, the engine had cooled off quite a bit by the time I got past the first couple of cylinders given that it was only warm to start with and it was cold outside. Does a compression leak only when it is hot sound likely?

 

Tried kjopen's idea. Even managed to nicely fit an Audi aux water pump inline with the heater core hose. Sadly, it doesn't seem to help.

Can anyone reconfirm that on the A6 C5 2.8 the bottom heater core pipe is the supply to the heater core? That's the flow direction I based the new pump install on.

Thanks

 

Hi there, on my 2001 A6 C5 the bottom hose is the supply. Sorry to hear the aux pump didn't help -

 

I had an overheating issue a few years back were she was getting really hot and it turned out to be one of my fans, I didn't mention it on my first post as you said your fans were working fine. It was my electric fan that had failed, I seem to remember it would work intermittently at first then stopped altogether, replacing it solved the problem.

I hate to say it but I think its either:

1 Failed fan (although you say they're ok),
2 Thermostat has failed (which means timing belt strip job)
3 Or water pump impeller failed (I've been told the impeller can break up and disintegrate) - again timing belt job

If you are sticking with it possibly being a problem with the radiator, could you rig hoses up so you can see how good the flow is? - before you replace the radiator - just as some kind of test...