1994 BMW 525i Engine Overheating

CHRISTINAG2008

MEMBER
1994 BMW 525I
6 CYL
2WD
AUTOMATIC
134,000 MILES

My car a/c had been working then it started intermittently working on and off, 1 min it's cold then next it's blowing hot then back to cold. I had a small leak and the other day when I came out of the store there was a pool of coolant under my car, I drove a couple of blocks back home and by the time I got there it was almost in the red. I popped the hood and had a stream of coolant shooting 6 ft in the air from a tear in on of the hoses going to the bypass cooling unit. I changed the hoses in that area as they were looking old. I filled the radiator with coolant and found that the leak was fixed but when I went to take it for a test drive I didn't get very far because it overheated right away again. I checked the thermostat and it is fine and the fan is coming on. Does anyone know what else it could possibly be?

6 Replies

CARADIODOC

MECHANIC
33,916 POSTS

Hi christinag2008. Welcome to the forum. Is the question about the AC or the overheating? Since you replaced hoses, coolant drained out and probably left an air pocket behind that has to be bled out. Look near the thermostat housing for a bleeder screw. If there is none, look for a sensor or plug on the engine side of the thermostat that can be unscrewed to burp the air. By "plug" I mean a threaded insert to cover a hole, not an electrical connector. Thermostats open in response to hot liquid. They won't open when hit with hot air.

Caradiodoc

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Monday, July 19th, 2010 AT 1:01 PM

CHRISTINAG2008

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Monday, July 19th, 2010 AT 1:48 PM

CHRISTINAG2008

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First of all thank you so much for answering so quickly, that does kind of make sense but is there a pic you could supply me with to show me what it looks like. Also can you explain to me how to do it? As far as the a/c, right now it seems to work well but when I turn the controls to hot I only get cold air would that happen due to the air bubble as well?

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Monday, July 19th, 2010 AT 2:21 PM

CHRISTINAG2008

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I still have not heard back from anyone can somebody please get back to me asap.

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Tuesday, July 20th, 2010 AT 4:26 PM

CARADIODOC

MECHANIC
33,916 POSTS

Sorry for the delay. Drove into town to try to find a similar car to take pictures. The best I can do is to describe what to look for. How did you check the thermostat? Since you know where the housing is located, look on or around it for anything that is threaded into it. Sensors with electrical connectors or vacuum hoses will do. Often there will be unused holes that are plugged with a threaded insert. Most commonly a six-sided Allen wrench is needed to unscrew them. What you are really hoping to find is a bleeder screw. It is hollow and about a half inch in diameter. The hole in the center is about 1/8 inch, and it typically takes around a 5/16" to 3/8" wrench. A 10 mm is common too. You don't have to remove a bleeder screw, just turn it counter-clockwise a turn or two until coolant starts coming out. It may be necessary to add coolant to the radiator to force the air out through the bleeder screw or whatever item you found to remove.

An air pocket will prevent coolant from circulating so the heater may not work. Don't worry about your heater yet until the overheating issue is resolved. I think you'll find you don't really have a heater problem.

Caradiodoc