Sorry for posting another overheating question....Ive searched previous posts and can't find what I need...thanks in advance.

Background...car well maintained, never overheats, fluids checked regularly, coolant topped off just a week ago, etc. No white smoke (steam) from exhaust.

This morning, drove 15 minutes from home, and I saw that temp. gauge was pegged to over 120C. I pulled over ASAP and here is what I observed and what happened:

1. Stopped and restarted the car 3 or 4 times to see if the temp. gauge reading was just a fluke...each time, temp. gauge pegged to the max. confirming that engine is actually overheated. Turned engine OFF, then did following with engine off.

2. No indication of any coolant leak under the hood...no wet areas, no broken hoses, no steam rising, etc.

3. No coolant observed in coolant reservoir.

4. I opened the coolant reservoir cap, and a big release of pressure & steam occurred, with bubbling up from hose at bottom of reservoir.

5. I poured a 1-pint water bottle into the coolant reservoir, and I could hear it sizzle and boil as it contacted hot surfaces inside the system.

6. I then added about a gallon of water until fluid level in reservoir was normal.

7. Temp gauge now reads normal (80C). Started car, and ran it for a few minutes...no leaks or related problems under the hood. Belts fine, no squealing, etc.

8. I drove home, and everything worked just fine, engine temp. solid at 83C, no leaks, coolant level normal.... as if it never happened!

I am thinking that maybe the thermostat was stuck closed, causing the coolant in engine to boil, pressurizing the system, resulting in coolant loss through the overflow hose located at the reservoir cap. Does this make sense?

The system works perfectly now, so the best I can do is flush system and replace thermostat (on principal). Anything else I should do?


Thanks for any suggestions.

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

You probably left the cap loose when you last topped off the reservior.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

I agree, you prolly left the cap loose.

Sounds like you're done the obvious. Keep an eye on it for any change.
Keep an eye out for steam in the windshield or water in the front floorboards(heater core).

You could pull spark plugs and see if one or two of them are exceptionally clean. I've seen a crack in the head between a water jacket and the cylinder allow water to escape very slowly. Sometimes a leaking head gasket will manifest itself in a similar manner. The steam in the combustion chamber will clean the plugs so that look like new, it will be obvious if that's the problem.

Hope it's something cheap and simple.

I would do an oil change also if you hit that temp

__________________
1976 Mercedes 240D (Sold)
1985 Mercedes 300D Turbo Diesel: 500,000KM
1986 Mercedes 300E (Sold)
1988 Mercedes 300E (Sold)
2002 Mercedes C240 (Sold)
2008 Mercedes C350 4matic

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