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Old 03-17-2003, 12:17 PM
Registered User
Join Date: Mar 2003
Posts: 18
300E Overheating What can it be??

Hi Board i'm sure somebody here knows exactly my delimma. She is overheating, it just started for no reason yesterday. I immediatly shut off my A/C opened all my windows and turned on my heater (per emergency checklist) and looked for a safe place to land. Initial inspecion the engine had water although it was slightly low (added water and cooled radiator), belt ok. Restarted car to no avail. Let her sit till she cooled and drove her away. Heres the confusing part........everything seemed ok so i went to take her home and she starts climbing again noting no heat coming out of the heater vents, then this burbling sound and she starts cooling down and the heater inside starts blowing warm air. So thats the pattern for 20 miles home engine heating up with cold air from the heater vents then when she get to about 120C a burbling sound and warm air from the vents and she cools down to 90C. I figure this is thermostat. New thermostat and it still overheats. Is there another component such as a heater diverter that could cause this same symptom or could the blade on the water pump possibly be spinning on the shaft intermittently. New facts, fans work fine and no oil in the water,
no water in oil and water pump changed 80k ago.
New info.. First the thermostat was replaced to fix the problem so its highly unlikely that that is the problem. Yes the cap is suspect and will be replaced next, however the intermittant thing just dosen't seem so simple (I hope so and will replace the cap). Next I assume the consensus is that the heater valve is ruled out as a culprit so Finally what and where is the monovalve and can it cause these sorts of symptoms??
Thanks again in advance

Your help is greatly appreciated, thanks
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Old 03-17-2003, 06:10 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,315
Under what conditions did it overheat? Idling in traffic, or just driving along?

There are a bunch of posts on "burping" the cooling system. You might want to try a search on that.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 03-17-2003, 07:04 PM
Registered User
Join Date: Apr 2002
Location: San Jose, Costa Rica.
Posts: 50
Greetings, I just went through overheating problems myself so I'll put my two cents in, My car is a1987 300 E euro with automatic, I started having problems just like you did and I went through two radiator caps, one H2O reservoir, heater hose, Thermostat, even a radiator which was leaking anyway, and the overheating got better but did not go away for good. Finally what was happening, is that I blew my headgasket, not a lot, but enough to slowly put pressure on the cooling system(exhaust filtering into the cooling lines) like a pressure cooker, so all this exhaust finds it's way to the highest point in the cooling system which is the radiator cap. That is where you hear the boiling sound, the pressure inside the cooling system exceeds the radiator cap rating and you start loosing water until you get vapor lock in the engine.
Do you feel your car a bit harder to start in the morning after the engine is completely cold, because what happened to me was that water would deposit in that piston where the leak was.
So I ended up having to shave off half a mm of the head, machining the camshaft tunnel, replacing guides and seals,reseating valves and so on.... I did take advantage of this to have the engine rebuilt to stantard measurements.
Bottom line is ,It sounds to me like a slightly blown head gasket.
P.S. I didn't find any oil in my water or water in my oil either, the water did smell like exhaust when I opened my radiator cap. MAKE SURE THE ENGINE IS COLD BEFORE YOU DO THIS!!!
Let me know how it turns out.
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