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Old 06-11-2001, 01:29 AM
Mike Murrell's Avatar
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I suspect my problem may be something as simple as a weak cooling system pressure cap(cap on overflow tank).

'91 300-SEL equipped with MB pressure cap 124-500-04-06. Cap is about 2 yrs. old.

At highway speed, temp. gauge reads normal - about 87 degrees centigrade. In stop-n-go, it runs between 90-100C.
Running temps are no different than they've ever been.

After coming to a stop and shutting off engine, I hear all sorts of gurgitation under the hood. Coolant is overflowing rapidly into the expansion tank. I'm seeing some come out from around the cap. It also appears that there may be some seeping out along the seams of the tank.

If it were the T-stat, I would expect to see high temps while driving. - this doesn't happen.

This situation came on out of the blue. Thanks for your time.

Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 06-11-2001, 08:49 AM
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Replace the cap. It is not holding pressure anymore.
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Old 06-11-2001, 03:24 PM
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mike, ..
it looks like that the cap is doing its job. There's a tremendous build up of system pressure and it is being release thru the cap. You are loosing a lot of cooling medium which causes variable temperature on a stop and go situation. I could suspect that the head gasket is going south.....
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Old 06-11-2001, 09:48 PM
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When you figure it out, will you post the result? Mine did the same thing last weekend. When it had cooled off, I popped the cap to add some coolant mix, then popped it back on. Since then, the symptom has not reappeared.

I actually saw your post while searching for posts to find out how to check to see if the auxiliary engine fan is working. My car starts to run hot in slow traffic or stop and go traffic.


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Old 06-12-2001, 12:23 AM
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Do you have a thermostatically controlled fan clutch? I'm not as familiar with the later 126s, but if the bimetallic spring is not operating properly to engage the fan at lower speeds it will definitely allow the heat to build up in stop and go driving, which would cause the rest of the symptoms you describe. You should verify the operation of aux fan(s) as well, before throwing parts at the problem. My sense is that it is not the cap but the fan clutch, but please let us know what you find out.
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Old 06-12-2001, 12:53 AM
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Update - a double whammy

Turns out the coolant cap on the expansion tank was bad, BUT I also have a leak in the neck of the expansion tank. I didn't notice the leak in the neck of the expansion tank last night because of it's proximity to the coolant cap which again was bad. After replacing the expansion tank cap it appeared that I might have licked the problem. Driving a little farther raised temps and then the additional problem with the leak in the expansion tank neck reared it's ugly head.

When a head gasket goes, engine temps run higher, water appears in the oil and you have a cloud of steam at start-up in the morning. In my case none of these have occurred. I simply get a bit of backing up in the coolant expansion tank after shutting off a warm engine - the result of a system that's not holding pressure.

Regarding the fan clutch - I am suspicious of this animal and have been for a LONG time. It seems that you read alot about MBs going from 87C on the highway to 100C in stop-n-go. I haven't heard the ROAR that used to kick in at about 102C in quite awhile. I have noticed that after shutting off the engine when fairly warm that the fan blades continue to spin freely for quite awhile. Then again, once the engine has REALLY warmed up, the fan blades stop immediately after shutting off engine. I'm tempted to replace the fan clucth, but at about $150.00 I think I may wait. I have never seen any temp > 105C and that was in 100F+. I live in south Texas.

Tomorrow I'll order a new expansion tank, etc. As JeffSr says, there will always be something else to fix.

Thanks to one and all for the advise/opinions.

Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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