Why does my 1986 300E overheat at stop lights. But, if I turn on the heat it cools down. This is very strange to me.

How's your coolant level? If the coolant level is OK, I would suspect a worn fan clutch. With the engine off, there should be resistance when you try to turn the fan blades. DO NOT under any circumstances attempt to stop a moving fan! If you can move the fan easily, it needs to be replaced. You really only need the fan to cool until about 30 MPH, after that the flow of air through the radiator does the job

Turning the heat on when the engine is running hot is an old trick to limp a car home when over heating. That starts dumping heat from the cooling system into the passenger compartment, sort of a heat transfer operation, which buys you a little time. Not something I would suggest doing on today's cars, with aluminum heads and/or blocks.

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Mike Tangas
73 280 SEL 4.5

Thanks, I will check that today


Cardude

i would relley like to know what happens so that i can try it to.

Cardude:

I would also advise checking to see if your Aux. fan is kicking in at high temps to prevent over-heating. If no, this could be a variety of things, as well...

- "212" (100C) switch
- Aux. fan relay
- AC Pressure Switch
- AC Pre-Resistors

Search the posts here...I found a ton of info already posted about coolant problems before my own flush/sensor replacement this spring.

Cheers,

John

Fan clutch seems fine. Coolant level also fine. This is a mystery. How can I check the other things listed in the previous post.
The Aux fan does not come on at all. Where can I trouble shoot this. But, I thought this was only activated during AC operation.

Cardude

[This message has been edited by Cardude (edited 06-16-2000).]

When you say overheat, what is the temp you are talking about?

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

Mein Mekaniker, Herr Fuchs, who also owns a 300TE, recommends switching to a 75 degree thermostat on the 300E's.

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Chuck Taylor
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,
4 250C's

Check the fuse for the aux fan first. There are two other inputs which will activate the fan. There is a temp sensor on the head and a switch on the A/C receiver dryer. Fan should be coming on either by increased temp or activation of AC. Common for fuse to be corroded or faulty.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

Almost to the point of boiling over. The aux fan fuse. Is it located in the normal fuse box?

Cardude

The fuse is located in the fuse box. Don't remember the position number. Check the diagram on the cover. Replace the fuse and clean the contacts. Check for 12 volts with key on.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

You may have a coolant flow problem with your vehicle,Because engine fan clutch need radiator heat to turn on its full speed.I will recommend you check your radiator. First when engine temp hot,STOP your engine by turn ignition key off,then put your hand to touch & fell inner side of radiator see if the temp different from center to side of radiator that will tell you coolant flow are normal or not.(temp should even hot and no temp different from center to conner),next to recommend to replace thermostat to try out .That is cheap to do anyway. good lock

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MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner

It might be time for a citric acid flush too.