trapp

was cruising at 60mph yesterday in 80 degree weather in my 2005 when suddenly the temperature got up to 3/4, then down to 1/2, then up to 3/4 again, cycled like this until I got home. I let it cool after I got home and then before driving again checked my coolant level which was slightly below minimum, so I topped off a bit with the pink stuff, I mean slightly like half of pint.

Ran the codes, nothing, so I took it on a ride to see if I can replicate the issue. Everything's fine until I hit the highway, after a bit of driving it gets up to 3/4, but this time instead of dropping back down it continues and I get overheating warning. Pull over to let it cool down and turn heat up, run the codes, nothing. Drove it home with heat on and it managed to stay at 3/4 level. Ran the codes again, nothing.

I mean I'm getting code like 00470 combination comfort databus in single wire open circuit but nothing concerning this overheating issue, no stuck thermostat, nothing. Seeing there's no leaks and coolant is still at consistent level as before, all points to a thermostat. I just found it on odd that Audi wouldn't throw up a code for faulty thermostat, I have an old Kia minivan that would throw up a code for thermostat so it's surprising that Audi wouldn't.

MP4.2+6.0

You have to use standard diagnostic techniques (as in, old school) for port motor 4.2's. Even with the electric assist MAP ones like in the FSI 4.2's, S8's and W12's, the codes you see are often vague and require some guessing and elimination of other possibles like sensor.

Other possibles can be the cooling sensor is failing (not uncommon on Audi's, though maybe more likely to see codes) and the fan speed components (which leads to fans with integration).

What I often do next is check std. fan operation. With car cold, turn off A./C should have nothing. Then A/C on, should have soft fan even with motor cold. Next, at halfway point cooling fan should kick on with A/C off. If not, can still suspect any of the parts mentioned. If kicking on AC and its fan can't keep temp in check (and fan eventually should go to "jet engine mode" if it gets really hot), suspect thermostat. If it can keep temp in check with AC kicked in, suspect the other parts more.

Of course, look at your timing belt history. If it has been close to or more than 75K miles since the last time (or new), you have that whole job to do anyway, and now just be sure to do thermostat with a quality replacement (dealer, Behr, etc.). Typically water pump too if you go for the "full job." The pisser can be is it was done only 20K miles ago or some such and it was the minimal job of belt only, and now the other parts act up. Some of the "cheap" kit stuff can bite badly here too--cheap in more ways than one.

dvs_dave