I own a 1995 525i E34 thats having some cooling problems. If it's hot outside, i usually refrain from using my air conditioner because the car tends to run a little hotter then normal. Well my mom was forced to drive my car and she ran it in 100 degree heat with the air on full blast. I went to check on my car to make sure everything was ok only to find the radiator cap hissing loudly. i ckecked the coolant temp guage and it was just shy of the 3/4 mark. My dad took a look at it and said that we needed to cool it quickly. The car was not started. after about 10 mins once it cooled of a little, he opened the radiator cap and the coolant in the resovoir came bubbling out- alot of it came bubbling out and it was really freakin hot. we let it cool a little more, then topped it off with 50 percent coolant and 50 percent water as we normally do. Still the car runs hotter than normal, and the liquid in the resovior becomes boiling hot even thought the temp guage is not showing in the red.. it only appears to be elevated slightly from normal(normal is in the middle) needless to say, im not driving the car. its parked until i can find out whats wrong. Im thinkin either the water pump or the thermostat, but ive also heard that there may be air bubbles in the coolant system. any ideas on whats goin on? help would be appreciated! Thanks!

A car that is running a bit hotter than normal when the A/C is on is suffering from a problem with the engine driven fan clutch, the aux fan, thermostat, or all of those. Given the recent history I wonder if the temp gauge reached the red during that drive. BMW engines are not at all tolerant of an overheat into the red and even a single occurance can damage the head gasket. When that happens air can be pumped into the cooling system, which leads to inexplicable loss of coolant and random overheats.

At this point the first priority is to properly bleed the system eliminate air (procedure below). The second priority should be to diagnose and fix the cooling system problems that cause the car to run hot when the A/C is in use. After properly bleeding the system finding air in the system over the next few driving cycles or experiencing overheats would raise the possibility of a damaged head gasket.

1) Using ramps or a jack get the front of the car 1' or more higher than
the rear. That will make the radiator and bleed screw the highest point
on the engine and facilitate removal of air.

2) With the bleed screw open, add coolant until no more air comes out of
the bleed.

3) Leave the filler cap off, or at least loose, and set the heat for max
temp and fan speed. Leaving the cap loose will prevent air that's still
in the system from causing a "coolant fountain" once the engine heats
up. Start the engine and allow it to warm up to operating temp. As it
warms up occasionally crack the bleed screw to release any air and top
up the coolant as necessary.

4) Once the engine is at temp bring it up to 2000-2500 for a few seconds
several times. Then crack the bleed until no more air is released. At
this point the heater should be throwing lots of hot air, which
indicates that the heater core is filled with coolant. You may have to
repeat this a few times to get all the air out.

5) Drive the car a bit, allow it to cool back down, and recheck the
bleed for air. Over the next few days you may get very small amounts (a
few bubbles) of air out of the bleed screw.

thanks. the Bimmer's gonna be looked at on Thursday, I'm taking it to the shop tomorrow morning. I just noticed when i drove it today that theres a hissing noise coming from the radiator. I thought maybe the cap was loose or something but turns out the noise is coming from somewhere else.

can you please update on this post when you get the chance. I'm having the same problem.

The check engine light came on a week ago, but I did not find anything visibly wrong. This was a mistake.

I'm having the same hissing noise today and within a 2 mile drive my temp meter was in the red.

well i took the car to be looked at today. mechanic was tellin me that chances are good that a plastic piece in the radiator busted, resulting in air pockets in the coolant system, thus overheating. On one of the previous posts hinted at possible headgasket damage so im kinda wonderin on reasonable prices for repair here. I hope its not somethin wrong with the headgasket, but it could be because it has been in the red before, only as soon as i noticed it, i pulled it over and turned off the engine. SO if the headgasket is damaged and needs to be replaced/fixed, how much should that cost? Also if the radiator's butsed what should i expect to pay?