I have an 86 5000cs. This how it happened. About 2 months ago I was driving and I noticed the engine was smoking(vapor that is). I pulled over and I noticed that my I forget the name of this part. Its the control valve that allows the coolant to enter the heater core broke apart. I walked to a gas station and when everything was said and done I bypassed it by running a hollow plastic piece where the control valve use to be. Since I did this the heat would be running non stop. Well that would be the least of my worries. Well two days later while first driving with no traffic I found myself at a stop light and once again the vapor cloud starts again. I pull over and notice one of my hoses (which was old) broke. I dont want to make this too long but whenever I drive the car at higher speed nothing bad happens. When the car is stopped and runs for maybe 5 minutes the coolant literally blasts through the reservior cap. Even though it is on tightly. I think it may be a variety of things such as the cap may be losing pressure, I may of cracked the head gasket(although when I drive it there is NO giant white cloud coming out of the exhaust ) or and can someone or everyone with knowledge about coolant issues help. Tonight I drove it 25 miles at 90+mph. I stopped afterwards with the car running not one leak but while letting it sit those 3 minutes I noticed that the radiator fan never started up. Can this also be a cause. If a radiator fan quits can it cause this to happen? Any help would be greatly appreciated. Thanks Again for reading this very long post.

 

The problem is quite obvious. You are running too hot as the cooling system has major deficiencies due to small failures that have accumulated over time. Your gauge should reed the first thick bar located ¼ from the C. That is if your multi function sensor is working properly. When you are moving there is some air going through the radiator, when you stop temperatures shoot up but the cap keeps the pressure anywhere from 17-22psi. The radiator, expansion tank and heater-core have plastic parts. These plastic cooling system parts eventually crack, split and leak. It also sounds to me that the cooling fan is not turning on when it is supposed too. Check the relays, thermostat, thermoswitch and the fan itself. It took me a while to troubleshoot my cooling system and discover all these little tings that added up to one big headache!

Just like anything German the Audi cooling system is overly complex and failure prone. Having worked for a German manufacturer I can say that this philosophy of over complexity goes beyond just automobiles. While the radiator is more then capable of removing the heat, it is placed behind the headlight assembly so there is little direct airflow. Instead air is redirected through a cheesy deflector/air-box that doesn't survive the elements, expensive and practically impossible to replace. I tried to find one at local junkyard, I looked at 10+ Audi 5000/200/100 and they were all disintegrated! To change the stupid thing you have to disconnect the AC hose, remove radiator, remove condenser, install the shroud, reconnect the AC line, refill with R-12, etc.

Forget the $$$ Audi piece of cr@p shroud and fabricate a new one from either sheet metal or fiberglass cloth and resin. Replace the thermostat with an 80ºC version and the fan thermoswitch with a new 85ºC/90ºC version. You are probably going to have to replace your radiator, expansion tank, heater core and hoses. If they don’t leak now they will!

 

I would make sure the radiator is not plugged. Just run you hand over the face of the radiator after the car is warmed up and feel if there are cool spots. Don't put your hand on the fan or you might end up with hamburger.

On older cars the radiator gets plugged in some areas and the cooling efficiency drops. There are three possible fixes
1. cheep - might work - radiator flush from auto parts store
2. more expensive - professional radiator flush - bring cash and cross your fingers
3. Expensive -sure to get rid of blockage - new radiator - bring credit card !

 

5000,

First, get your fan working. That is the most important thing. Audis have a pretty small block, and a minimal amount of coolant, 7 litres for the 4 cylinder and 8.1 for the 5 cylinders, compared to the 12 or so we used to have for the big block US cars and trucks.

The coolant fan has to run, and, with the electrics, they may run for what seems an inordinate amount of time after a hard run, with the engine shut down. Thermosiphoning cools the water in the rad itselfand draws new, hot, water into the rad to be cooled.

If the fan does not run, the only cooling you have is the force of air at speed. And, as the above state, that is **** poor, considering it has to force its way through the air conditioner condensor before it even reaches the rad. Major obstruction, there.

Cheers,

George