jesseusmc

Hey guys I have a 2001 a8l 4.2 recently i noticed it got about 3/4 of the way on the temp gauge. I turned it off and let it sit. Checked coolant and it was a little low so I filled it up. Now the problem is that sometimes I can drive it and no problems at all then sometimes it will overheat. And another weird thing is I'll be driving and it'll be about halfway and I'll pull in to get gas or something and turn it off and when I turn it back on its saying it's overheating?. Also I noticed coolant keeps getting low. Also no visible leaks at all. And oil looks fine so I don't expect a bad headgasket. Please help me I'm stumped!

silverd2

99.9999% of the time, it's the thermostat...no one wants to admit it, especially on a 40V engine (really "fun" to get to) ...COMMON failure...I was on my 3rd one at just 100K miles.

1st, the "normal" hash mark on the gauge at left of top is actually part of Audi "idiot gauge" system. Thanks to the cluster processor, the gauge reads consistent up to and above that mark, BUT (although the mark is supposed to mean 90C..."normal" ) it reaches that mark at 81C and holds dead steady until the temp gets above 100C....so, if the gauge moves off that mark at all, you are already at 101C plus, well above "normal". It should never climb above that mark except in hot weather stopped in traffic...and not much above then.

As for low coolant, if it's bleeding off hot pressure through the reservoir cap, there will be coolant trails (chalky light color) under/behind the right rear corner, down the fender liner.

Shutting off an engine while overheating will send the temp even higher...so no surprise there.

Typical test of the thermostat while driving:

- While overheating and moving, shift to lower gear to raise rpms...temp should slowly come down some.

- Set heater and fan to max...temp should come down some fairly fast...eventually down to normal.

There's an outside chance the coolant temp sensor is bad...also a common failure. But that's ONLY if you really don't believe the coolant is hotter than normal. When the sensor goes bad, it won't necessarily mimic overheating...can go cool, hot, up and down quickly or not move at all. The coolant temp sensor has NO effect on actual temps, though...ONLY the thermostat.

dan87951

A cooling system that has air pockets in it will also call intermittent over heating. I would start with bleeding the cooling system than move on from their.

silverd2

That's usually only a problem after a drain and refill...But, personally, I have never had a car that did not completely self-bleed after the first drive, including my present D2 and Allroad after I did the timing belts on both.

I always bet on the thermostat, esp in a D2, because there tends to be a tendency toward failure in these engines, seemingly regardless of the thermostat brand.

BrianC72gt

I resolved a recent overheating situation by cleaning the contacts on the electrical connector to the electric fan and the blade connectors on the variable fan speed controller located under the driver's side front of the car. It is an odd looking silver colored plate contraption that I'm guessing somehow makes internal connections based on temperature, to adjust the electric fan speed.

If you turn on the A/C and the electric fan does not come on, that's a problem. Also, the engine driven fan has a viscous clutch that firms up as the radiator gets hot. If it fails the pencil test, then it is freewheeling and not cooling the radiator and needs to be replaced.

Brian C.