I put overheat in quotation marks because I'm not entirely sure if it is overheating.

This is the situation:

I was on a long, 1000 km drive and 3/4 of the way through, when I stopped to get gas, I had tons of coolant bubbling out of the resevoir (temp gauge in interior was only reading slightly above halfway), I let her cool down for about an hour then took the thermostat out (thinking it might have siezed closed)topped up the coolant, then set back on my way. A couple hours later I pulled over again to check the coolant level, it was slightly low (had to let the new stuff recirculate) but when I shut off the car, the coolant violently bubbled out of the resevoir! This continued to occur everytime I stopped. Coolant level would be low , and I would shut off the car, and it would boil over. Then it occured to me, I have NEVER, since I bought the car, heard the fan on once the car was shut off. Even in quite hot weather, with city driving and A/C on, the fan has never come on when the car has been off. It comes on when the car is running though. I'm thinking that since the only thing to cool the car once off is the fan, and since it isn't coming on, that something is wrong with the coolant temperature sensor, a relay, or the fan itself. Does anyone have an idea how to find the exact problem? Would there be a code thrown into the OBD-II system without a check engine light coming on? This is a very weird problem and the retards at the local dealership didn't give me any help and tried to tell me it was the thermostat (sure guys, that makes sense since the thermostat is out!)

I agree with MadeInJapan, the scenerio that you describe is consistent with a cap failure.

Besides, it is easy to check.

This is in addition to a fan that is not operating properly. If my car is warmed up on a hot day, the fan almost always runs after the car is shut off.

I think that when you shut off your car, a failed reservoir cap allows the pressure in the system to drop to atmospheric, and if your coolant is over 212 degrees, it will boil out. The coolant usually gets hotter for a bit after your car is shut down because some of it stops at hotter areas of the engine and absorbs some heat.

Good luck.

--
96 855R, 95 855,854, 90 744 Ti - 310,000 miles driven

The fan relay on my '96 is under the battery tray on the bracket with the airpump.

there is a relay for the fan under the top of the radiator shroud i believe.

I'd look for a blockage of some sort, perhaps a bad upper or lower hose that is losing a flap of rubber on the inside. I just saw a post like yours a few weeks ago.

But I would think a blockage would cause it to spill out the resevoir when running, not when shut off, as the water pump isn't being driven anymore. Besides, I have been checking the temperature of the rad inlet and outlet hoses intently, and there naver seems to be much of a difference.

bubbling out of the resevoir sounds like a broken over-flow tank cap. Mine was replaced 3 times in 6 years. These damned plastic caps get hot and crack (sometimes underneath where it is hard to see from above). Well, my 2 cents worth. The whole system is under pressure and that's normal (don't know about the fan running after...seems mine didn't) so if the pressure is released by a leak in the system (like a cracked cap or bad seal) then of course it is going to sizzle out. Is your engine temp up more than normal?

Cap is in perfect condition as far as i can tell, no cracks on the inside. Of course, i think when 14 psi is reached, the pressure is relieved in the cap and it will boil out no matter what. Temp gauge never read high, I have never even seen it go past 2/3 maybe 3/4 way on my car. Of course, there's a possibility of something being wrong with the collant temp sensor.