need help w/overheating

[Pat624]

I have a 97 318i and have a overheating problem my mechanic cannot correct. Last year I overheated for the first time , my mechanic replaced the fan motor and thermostat, everything worked fine. Its now one year from that date same thing again, this time he says it looked like there was no circulation so he replaces the water pump, thermostat, heat sensor, expansion tank. He said he tested it actually took it home and eveything is O.K. So I drove the car home on the highway for the first time the gage says I am not over heating but when I stopped I lost coolent.
So I refilled bled it and took it for a 15 minute ride and left it running after about 20minutes, the gage shows its moving into the red I check the fan it is not working, I shut it off and coolent starts coming out of where the expansion tankconnects to the radiator. Does anybody have any suggestions? My mechanic is at his breaking point.

 

[Kirby]

BMW cooling systems are a bit different than other cars. If your mechanic is not a BMW specialist, he may not be aware of a few requirements and tricks.

First off, you said you checked the fan, it is not working. Were you looking at the electric fan? Under normal circumstances, the electric fan only runs when the A/C is on. The electric fan does not run under regular operating temperatures.

If you were looking at the mechanical fan on the front of the engine, and it was not turning, then the fan clutch is bad.

To bleed the system, it can help to park on a hill so the th efront of the car is uphill, this helps by making the bleeder the highest point in the system.

Also, mechanics not familiar with BMWs have been known to put the thermostat in backwards.

 

[Pat624]

help/woverheating

Thanks for the tips, I was reffering to the fan in the front of the motor, as I said in my post, my mechanic replaced the motor last year.
Is the fan motor and clutch one and the same?
I also read on one of the posts that you should be able to manually spin the wheel but it should not continue to spin freely. I did try, it will spin but to what degree I am not sure what is typical.
The mechanic said he by passed the heat sensor to ensure the fan was working, but would you know if there are other factors involved that would make that test ineffective. Thanks Again

 

[Kirby]

I failed to notice that it was a 318. This car has two electric fans, instead of one electric for the A/C and one mechanical on the water pump. Both electric fans will spin freely by hand for a turn or two.

Sounds like your trouble is with the main cooling fan. I looked at my Bentley manual. Oddly, there is plenty of wiring diagram info on the A/C fan, but the main fan is barely documented?!?

From what I can make out, it there are a total of THREE temperature sensors:
1. The most well known one - the temperature switch in the radiator. This is for the A/C fan, not the main fan. If your mechanic replaced/bypassed this, it has no effect on the main fan.
2. Dash board temp sensor - the one for the gage. All it does is display the temperature.
3. ECU temp sensor - From what I can tell, the ECU uses this sensor to determine how hot the engine is, and then turns on the fan when needed. This sensor is also used for the fuel injection. If he bypassed this sensor, it would in theory also mess up the fuel injection....

There are two fan relays in the fuse block, but according to Bentley they are both for the A/C fan, not the main fan. I think there should be a relay for the main fan as well, but there is no specific info in Bentley about such a relay. It's possible that in the 318, one is for the A/C and one is for the main fan. I would probably check these relays, they DO fail sometimes.

Sorry, that's about all I can offer.... If it does get solved, I'd like to know what it was!

 

[Pat624]

Thanks for all your help Kirby I will pass the info along to my mechanic and let you know the outcome.

 

[chief]

Something you might want to have your mechanic check...... the tension pulley bolt (altenator, water pump, power steering pump) may have worked its way loose. Thus, the belt slips. Check the 6 rib belt to make certain it still has 6 ribs. If not then you are looking at the water pump shaft sheering and unless you have a strong battery or road service you will be in deep horse pucky.