I recently purchased a really nice but overheating 500sel. I've swapped out the thermostat, also ran it without the thermostat, put in a new waterpump, visco fan is working, radiator has good flow, plenty of coolant, rad hoses are in good shape. Car still overheats. When the car is cold, I can start it and it takes quite a bit of time to get to 110*C, if I start it and drive, it will overheat faster, suggesting a coolant flow issue. Top hose can get pretty warm, bottom hose stays cool. Radiator doesn't get particulary hot either. The heater puts out plenty of heat and doesn't seem to have any effect on the temp gauge. I've heard something about an auxillary water pump, but I can't seem to find a paper manual for this car so I can research further. Any help would be appreciated

How did you determine that the radiator has good flow and isn't clogged? The fact that the engine overheats without the thermostat implies a blockage somewhere. The auxiliary water pump circulates water through the heater core so it provides heat faster than cars without one. It's not meant to increase engine cooling per se.

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Fred Hoelzle

My 300 sd was like that the old rad was plugged, I can use my hand to feel around inside of the core, is stone cold on certain area, soon as I bought a good used one, all the night mare was over.
u need to backward flush the rad.

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thanks for the help, purchased a new rad and waiting for it to get here.

My SD had the same problem, put in a new Radiator, problem solved.

It might not even be clogged, but it may be that the fins are so plugged up that hardly any air can get through it.

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New radiator arrived. It is in and car still overheats with same symptoms as before. Feels like I'm throwing parts at it now. One more part to go? Another thing I forgot to mention, I run the heat on high and sometimes it blows cool and the car is headed toward redline on temp. Maybe with this last tidbit somebody might help identify what is wrong.

Well, for one thing, MB engines are not designed to operate without a t-stat.

All you end up doing is short circuiting the radiator.
Look at the manual, there is a flow description.

The aux pump is irrelevant wrt cooling. It's only there to boost flow into the heater core at idle. Not all MBs are equipped with aux pumps.

Perhaps you have a head gasket problem? Your description of the heater output suggest that this may be a possibility.

Jim

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85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

If your overflow canister purges when overheating then it could be a coolant vain to cylinder headgasket leak . Check the exhaust for white water vapor and the overflow canister for drops of oil and the oil for antifreeze residue.

Ben

I haven't noticed anything going on with the overflow tank, nothing unusual with the exhaust either. So far I don't see signs of any oil/water contamination. One other thing I forgot to mention... as the gauge approaches 100*C, I can rev the engine up and the gauge lmany times will bump down suddenly. Electrical problem? I did a pressure test today (auto zone will let you take theirs home free with a deposit) and it came out fine. Also just put on a new radiator cap. I am not sure what you mean by "A/C heat and blower on max should get rid of any air pockets" I have done this procedure with the heat on and blower on high. Also I mentioned earlier that the heater will many times start to blow cold? I am not sure if you have considered this in possibly making a diagnosis, but I thought it might be significant. Thank you for any help and any insight you can provide.

new symptom: while idling with heater on and radiator cap off a burnt rubber smell started to come into the cabin (temp was at about 90*C) WTF?

ok, I finally pulled the monovalve. There is a slight tear (enough to replace it I guess). I have found ********az has best price on the repair kit (under $40). Upon reinstalling the same monovalve, I've discovered that the heater is working fine and the car NO LONGER OVERHEATS? what the ****? I can leave it in the driveway with the heater off and it will stay at or just below 100*C, hit the throttle and temp will go down another 5-10*. Next step is a citric acid flush (I think the local homebrew supply will give me food-grade citric acid at about $7/lb). Anybody have any insight on why the car stopped overheating?

Perhaps you burped out an air pocket when you replaced it. Could be that your heater core was full of air, and you weren't getting full circulation.


-tp

Ran 2.5# citric acid through the cooling system yesterday--no discernable flakes or deposits came out. Headgasket? Oil is free of water and coolant is not discolored. is there a smilie for pissed?

I had a similar heating problem with my '84 500SEC when I first got it.
Did not boil out any water, but temp. guage ran higher than normal.
Not sure of your location, but here in California, a smog shop indicated I may have an ailing cat. So as a cheaper fix, I first replaced the O2 sensor.
Car heat problem solved. Cat ok. Just FYI

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Well it sounds like you have no problem any more. Why are you still concerned?

Everything you have posted indicates to me that you had an airbound cooling system. These cooling systems are notoriously difficult to purge air from. Sounds like when you replaced the monovalve you fortuitously also bled the air from the system. Is everything working ok now?

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