*posted this to the 100/A6 forum but realized this would be a better place.
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Opinions wanted!

My 97A6Q Wagon was starting to climb the temp gauge at freeway speeds, (only at 70mph+) to the point the AC cut out. I just flushed the system, which showed little debris.

Doing other maintenance. I've taken the intake plenum off and and have now cleaned the throttle body, the ISC valve, and the MAF. Also cleaned out the breather hoses, one was pretty gunked up.

What else should I replace while I have it apart?
I see some vacuum lines are crakced/bad, will order today, blauparts has feet for cheap. I'm also going to do valve cover gaskets as they seem to be leaking.

I thought about replacing the thermo switches just as a precaution, and I can actually get to them easily.
Thermoswitch in radiator runs $16. The "Sender Water Temp" one runs $55 and according to blau: controls the temp gauge, overheat warning light, & a/c overheat shut down functions.

Opinion on if I should replace these? Also do a thermostat? It was done about 50K ago when the timing belt was done.

I also see the lower intake manifold is leaking some oil, but I don't have time for that big of a job just yet.

Other sensors back there that go bad I should replace?

She was using some coolant and I was afraid a head gasket was in order, but I found the upper rad hose I replaced was not quite tight and leaking slightly. Still not sure why the temp was climbing, maybe some blockage in radiator, but figured I'd try all the cheaper things.


Thanks!

Oh, this Audi has 206K on it and running strong.

 

Even seasoned Audi mechanics find it a PITA to bleed all of the air out of the cooling system. In the A4, it has a couple of bleeding locations.

I would try cleaning the combustion chambers to get rid of carbon deposits that can definitely cause engines to heat up. Interestingly, the water method (i.e., sucking water into the engine via the vaccum hole on the intake manifold or on the TB) works well, but it can prove tricky due to the odd internal shapes in our intake manifold. You could easily suck too much water in an kill the engine if you try too hard to clean # 3 & 6. However, just make sure that you pull the water into the engine after you have it hot. The water will not steam clean the carbon off the piston head and cylinder head unless those surfaces are hot.

Most importantly, do you have the proper heat index spark plugs installed? Plugs do more to control combustion chamber temps than any other device in the car. If you have too hot of a plug it could definitely cause loss of power at full engine load.

Your EGR system exists to do nothing more than lower combustion chamber temps to reduce the formation of NO gasses (which, of course, form at high temps). Perhaps you should try to clean it out also while you have the plenum off--Just open the large valve on the TB and get a flexible tube with which you may spray something down the EGR hole in the IM to dissolve carbon deposits.

Aside from that, your coolant thermostat may have failed. While they rarely fail due to Audi's mechanical and basically bullet-proof design. It does happen. Only replace it when you do a full timing belt service; why go to all that effort without fixing everything at the same time?

 

Supposedly it's very effective and relatively safe. Water is a byproduct of combustion anyway. A few guys here used Seafoam (CRAPA, I mean NAPA carries it) through the brake booster hose and they report good results with smoother acceleration, improved idle, and lower oil consumption (means ring/groove-land was cleaned).

 

good idea moribundman, I have some seafoam on the shelf. I've used the water cleaning on several miatas with good results (I do know of one person who let too much in and hydrolocked their engine though)

Ross

 

The regular stuff and another one called Sea Foam Deep Creep. I've used neither one, but they appear to be very popular cleaners that avhieve good results. Water should work too, though!