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Old 10-07-1999, 03:36 PM
Registered User
Join Date: Oct 1999
Location: Culver City, CA, USA
Posts: 62
I have a '78 300D. The car has started to overheat gradually. On Monday it started going into the 200-250 F zone. On Tuesday it was just as bad.
The symptoms: Car starts to gradually overheat while I am standing at a stop light. When I accelarate and start cruising at 30mph or more it cools back down.
Things I have done and checked:
I have checked the coolant level. It was OK but I added some water to top it off.
I have changed the fan clutch. I did that because the used one I installed was leaking fluid for the longest time. I figured it finally went and so I replaced it.

The car was O.K. yesterday, but today it is starting to overheat again when I am stopped at a light for more then half a minute.

NOTE: The only dealer open at that time had a leaking fan clutch. The clutch had a fluid spilled over the bottom of the part. Like a thick wax or fluid like silicon. I thought the clutch was bad so I asked for another one. The second clutch also had the fluid spilled. I thought it might be some sort of packing grease.
Am I wrong? Is the clutch faulty? Is there anything else that can be wrong?
Thanks in advance for any help and info!
Sorry about the long post.

Old 10-07-1999, 03:53 PM
Senior Member
Join Date: Apr 1999
Location: Walnut,CA,USA
Posts: 554
Have you check your engine "thermostat"
that can or may cause your engine overheating
Old 10-07-1999, 03:57 PM
Registered User
Join Date: Oct 1999
Location: Culver City, CA, USA
Posts: 62
I have not checked my thermostat.
But if the thermostat was bad wouldn't the car overheat at all speeds?
How do you recomed I check the thermostat?
Old 10-07-1999, 05:16 PM
Chris Ecklund
Posts: n/a
My guess is a worn water pump.
Old 10-07-1999, 08:43 PM
Aaron's Avatar
Join Date: Aug 1999
Location: Cincinnati, Ohio
Posts: 1,937

The reason those fan clutches were leaking is because they must be stored upright or else they will leak fluid. However, in your overheating case, I'd suspect a bad water pump or else a bad radiator. Start by replacing the thermostat first. If that doesn't help, try the water pump next. If it still overheats, then you might be looking at a radiator. If you do have to put a radiator in, then make sure you go with the original metal MB/Behr radiator, not the plastic ones. The metal ones are much more expensive, but well worth it.

Aaron Greenberg
MB technician
'67 250SE Cabriolet
'77 450SL
'79 6.9
'79 6.9
'80 300SD
'85 380SE
'89 420SEL
'93 300E 2.8

Old 10-07-1999, 09:03 PM
Larry Delor's Avatar
What, Me Worry?
Join Date: Jun 1999
Location: Sarasota, Fl.
Posts: 3,114

Just by looking at your location, and the climate there, I am wondering if you had the A/C on during the overheating episodes.
If you did, one of the first things that I would check, would be the elec. fan in front of the radiator.
Good luck! (stay cool!)

'83 300D
Old 10-07-1999, 09:32 PM
Registered User
Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
The thermostat can fail in many ways, one of which will cause a failure at low speeds only. That is because it is only partially open. At high speeds the water pump is forcing the coolant by and therefore cooling OK. Replace the thermostat first. Then the radiator if not fixed.
Don't overlook the cap.

1981 280GE SWB
1987 16V

Old 10-07-1999, 09:37 PM
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Join Date: Oct 1999
Location: Culver City, CA, USA
Posts: 62
Unfortunatlly I do not have my AC working. Or is it fortunatly? =8-)

Does it mean that if the clutch leaked it is bad? Should I take it back and get a non leaking one?

I just got to school and had a chance to observe the car in stop and go and highway speeds again. Here is what it did:
When I was waiting to get on the freeway it heated up to about 195F or so. Once I got on the freeway and started going 60mph the temp dropped imediatlly to about 150-160F. YIKES! ( I hope my head will not crack! )
Then it gradually got up to its normall 175F.
Once I hit traffic it stayed at 175-180F, that is sort of normal.

I am begining to think that the coolant is not circulating when I am at idle. I do not know whether its one of the following:
1. air in system. ( how would it get there )
2. bad pump.
3. bad thermostat.
But which one and what order of possibility?
Old 10-07-1999, 09:40 PM
Registered User
Join Date: Oct 1999
Location: Culver City, CA, USA
Posts: 62
I was typing my reply when you posted (Benzmac). I will replace the thermostat and keep all of you posted. I have a spare one somewhere.
Thanks to all for replys!
p.s. this is the best site for MB related info! I would say even better then the site for my other 300 (the nissan 300zx that is).
Old 10-12-1999, 09:51 PM
Registered User
Join Date: Oct 1999
Location: Culver City, CA, USA
Posts: 62
The thermostat was it! I replaced the thermostat with the spare I had, cleaned the system with a cleaner and fulshed the the system twice. Now my temperature goes just above the 175 mark when I am at 70-75 mph and drops maybee to 160-165 when I am at idle.
I like IT!

BTW. The thermo I had in there was 75C and the one I put in was 79C. That should not hurt anything, should it? ( The engine is running cooler then before. )

Thanks everyone for your help.


[This message has been edited by mark300d (edited 10-12-1999).]
Old 10-13-1999, 01:23 AM
Registered User
Join Date: Sep 1999
Location: Redwood City, CA, USA
Posts: 18
I had a similiar situation with my 91 300SEL. Unfortunatly the car self destructed in the dealer parking lot. When I noticed the car running warmer than normal, I drove to the dealer to buy a new thermostate & radiator cap. When I parked the car, the gage was about 3/4 high. When I got back to the car, It just made a high pitch whine when I turned the key(no compression). $4500 later, it is running great. The dealer said what started the whole chain of events was a broken radiator neck -Go figure....
P.S. Exhaust bubbles from the coolant resevoir is a bad thing
P.S.S. I agree with the rest of you, this is an extrodinary site!

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