Three weeks ago I replaced the water pump on a '98 A6 2.8 as it was leaking badly. I replaced the timing belt, thermostat and other rollers, etc. while I was in there. Although I didn't realize that I shouldn't have done this, I used Peak antifreeze (not G12) when I refilled the coolant. It said it was safe to mix with all colors of antifreeze, so I thought it was OK. I don't know what was in the car previously (we bought it used), but it looked green so I thought it was the standard antifreeze.

Car ran OK for 3 weeks after the new water pump was installed (during the summer time). Three days ago, after my daughter drove it, it lost a lot of coolant and was steaming. I wasn't home to see it, and she didn't know what to look for. So I wasn't sure what happened. No one drove it on Friday. Friday night, I added more 50/50 and let it run. It never leaked and the temperature gauge remained normal. I changed the oil Saturday morning. No water in the oil. I took the car on an errand. It was fine on the way there, but after arriving home from a 15 minute drive in 108 degree F temperature (max speed 40 mph), a few minutes after turning off the car, I found that the coolant was boiling over through the pressure release cap. Much coolant was lost. I did not notice if the electric fan was still running. I started the engine up right away, and the coolant temperature was normal. Thus I do not believe the engine was over heated.

I started researching the problem on the forums, and got the idea that I really should have used G12 coolant. So I decided to try to flush the existing coolant and then switch it to G12. I drained the radiator and added straight water. I also disconnected the hose from the return line of the heater core to see if coolant was coming back from the core. I ran the car and turned the heat up to the max. Eventually, although it seemed like it should have started sooner, a trickle of water came through the heater core. I was expecting a stronger flow. After a few minutes, that trickle stopped. In addition, the air coming through the vents never got the least bit warm. In fact, it felt like it was still being cooled from the air conditioner. The exterior air temperature was 102 F. I should point out that the climate control settings were flashing for a while after I started the car (I noticed that this was happening before I replaced the water pump, too). The coolant temperature remained normal during all this testing.

I also observed that the o-ring in the coolant reservoir cap is missing.

Current guesses are:
1. The new water pump is not working. It is not leaking, but it is not moving coolant.

2. The thermostat is stuck closed. However, I think the coolant temperature would not remain normal ... correct me if I am wrong about that test.

3. The heater core is plugged. The water pump is not able to circulate coolant. However, in the summer time, does the coolant even flow through the heater core? Is it diverted to the heater core only when interior heat is requested?

4. The current coolant has too low of a boiling point. It was fine when the fans were on, but once at least one of the fans stopped, and with the very high outside air temperatures, it boiled. Perhaps I just need to use G12.

Are there any simple tests I can perform to determine if it is the water pump, the thermostat or the heater core? I do not have a VAG. I'm a driveway mechanic.

Thanks in advance.

 

If your coolant isn't circulating your temperature gauge will always read normal as it has no coolant running through to get a reading. I would monitor your oil temperature to make sure its not over heating or use a vag com to monitor temperature. Also if you did not "burp" the system you could have large air pockets in the cooling system and it will give you your symptoms

 

Do you know where the coolant temperature sensor is located in the coolant system (i.e. is it between the radiator output and the thermostat or somewhere else)?

One thing that doesn't make sense to me is that the temperature gauge reads cold when the engine is cold and slowly increases as the engine warms up, then maxes out at the midpoint just like it always does. Where ever the coolant is being measure, it seems to be absorbing heat similar to the normal running conditions. If the coolant isn't flowing, wouldn't it continue to absorb heat and show an above normal temperature since it is never cooled by the radiator?

I am thinking about disconnecting the hose that feeds the heater core to see if there is a blockage in the heater core. If coolant flows from the engine there, then I think that means that water pump is functionaly, but not actually functioning because the heater core is blocking the flow. If however there is no flow coming out of the engine to the heater core, then probably the water pump is not actually pumping it. Is there any reason to not disconnect the heater core intake hose?


Thanks for your continued help.

 

Coolant temp sensor is on passenger side by the iac valve. I have an a6 and mine did the same after changing water pump and I didn't bleed it. The gauge did the same thing but no coolant was flowing. Open the main bleeder valve located on the drivers side coolant pipe to see if you're just getting air or steady flow of coolant. You could also bypass heater core to see if that is your problem too. Also feel both upper and lower radiator hoses they should both be hot after car is up to temp when coolant is flowing properly.

 

It turns out that I do not have a bleeder valve on the rear coolant pipe. After many attempts to locate it, with very clear pictures from others, I can't find it. I found another post where another person says they don't have a bleeder valve either. There must be some models without one. In any case, I disconnected the hoses from the heater core. The water pump appears to be working and is pumping coolant to the heater core. I reattached the input to the core and verified that coolant is flowing through the core. It seems like I have to get the RPM's up to 3000 to get a heavy flow through the core, otherwise it is fairly week flow. I reattached the heater core output hose but left the bleed orifice exposed. With the engine revved, I had fluid shooting out this hole. I sealed the hole by sliding the hose back on all the way.
I test drove the car. Here are the latest symptoms. The car shows no signs of overheating. It idles without leaking any coolant. As far as I can tell, it also does not leak any fluid while I'm driving, even setting at signal lights. However, after I turn the engine off, the reservoir fills up and within about 15 seconds, coolant comes shooting out the over-pressure vent on the bottom of the reservoir.
I noticed that the electric fan does not continue running after I shut the engine off. On my previous Audi with the same engine, the fan would run for a while after the engine was stopped. This fan does run when the engine is running (I've verified it many times). Could the coolant be overheating after the engine stops and then expanding too much?
What could be pushing coolant back into the reservoir?
Thank you.

 

Should the auxiliary fan run after the engine is turned off (and it's 100 F outside)? Mine is not running at all after the engine is off. It does run while the engine is on, though. If it should run, is it likely to be the coolant temperature sensor that is near the bottom of the radiator on the passenger side, or would it more likely be a relay?

Thanks

 

Light flow through the core is normal. It's in parallel to the rest of the system. It sounds like you didn't burp the air out. Replace the coolant cap since a missing gasket means the system won't pressurize properly. Run the car, driving of necessary, until you get 3/4 of the way up to an overheat, then stop and let it cool. When it's safe, pop the cap to release pressure, and top off with water. Repeat as needed. If its an air blockage, this will fix it.

 

ImTheDevil, thanks for the advice. I've since been able to bleed the coolant system and I believe that I don't have any air pocket(s).

Do you know if the auxiliary fan (secondary fan) on a '98 A6 2.8L normally should stay running for a short while after turning off the engine on a very hot day? Mine is not, and I seem to remember on a previously owned '99 A6 2.8L, that the electric fan often came on after I got out of the car.

I tested the relays tonight for the auxiliary fan, and they tested OK, but this assumes that I identified the relays correctly.

Thanks

 

Honestly I am not sure, but I don't think so only because I seem to vaguely recall threads over the last few years where that was happening and people weren't used to it, and were trying to correct it. I really don't know though.

 

My 97 a6 has never had the fan running after shutting it off not sure if that's normal or not. Also my 96 a4 fan doesn't run with ignition off.