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1992 500SEL
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188 Posts
Discussion Starter #1
My 1992 500SEL is giving me fits. A few months ago, I lost an expansion plug in the middle of one of the heads. Coolant immediately spilled out and I had it towed into the shop for repairs. When replacing the coolant, the mechanic had one heck of a time getting the reservoir to take liquid in the quantity he was expecting. It's like it had an air lock and it would not fill. He told me to let it temp cycle a few times and keep topping it off.

I have now driven it about 1000 miles and find that I'm adding fluids to the cold engine. The check coolant light comes on giving me an indication that it needs another drink. I can observe no leaks. It seems as though each heat/cool cycle it will take more fluid. When I pop the radiator cap on a cold engine, I hear massive amounts of bubbles girgling up inside the block of the engine. Very weird. I have added a cumulative of about 1 gallon of coolant and the light just came on again tonight asking for more fluids...

The car is a horrible starter. On a cold engine, it seems to start on a couple of cylinders. After barely running for about 5 seconds it snaps to life and runs normally. And it runs beautifully until it is allowed to sit overnight, at which point it starts hard again. I drove it about 400 miles last weekend and was pleased to find that I got 20.8 Miles Per Gallon on good fuel. WooHoo! Once it has been started, it will restart normally without a worry for the rest of the day.

On two out of dozens of cold start mornings, I pulled out into the road on a still cold engine and noticed white smoke for about 2 miles. No observable misfires, rough idle, loss of power, or any other indication of a problem. I can observe no overheating. The temp gauge is routinely about 85C. On every other cold start I can observe no fumes out the back--white, black, gray, or otherwise.

The coolant loss is bugging me as I have put in a cumulative amount of about one gallon since that expansion plug repair. And the poor starting is driving me nuts. I'm looking for ideas on what to look at and what to try next.

Thanks in advance for the feedback.
 

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1993 300SEL (Sold) 2007 X5
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7,218 Posts
I don't know if the two are related at all. To find the coolant loss you really need to pressure test the whole system as what can happen is that the coolant leaks and runs out over different parts of the car if you can see drips it might not even be leaking from where you think it is. Perhaps check the seal on the cap of the expansion tank just incase it's boiling out that way hence no puddle under the car.
 

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3phase-4pole
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4,881 Posts
have you isolated that your headgasket isnt an issue? (water mixing with you oil?)

on one of my cars, I found out the culprit was a leaky heater hose that dripped onto part of the engine block so it steamed a bit when engine was hot and would not drip. do this by switching off the engine at its peak and listen for dripping
 

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1992 500SEL
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188 Posts
Discussion Starter #4
have you isolated that your headgasket isnt an issue? (water mixing with you oil?)

on one of my cars, I found out the culprit was a leaky heater hose that dripped onto part of the engine block so it steamed a bit when engine was hot and would not drip. do this by switching off the engine at its peak and listen for dripping
I do not observe any water in the crankcase. I do not smell any anti-freeze when around the car or under the hood. I have not ruled out a headgasket, but it seems illogical to me that coolant would be leaking into a cylinder and not the other way around. The cylinder is under much higher pressure than the water jacket.

I checked the fluids this morning on a cold engine. It had about 1" in the bottom of the reservoir and the dash light was on last night when I parked it. When I popped the radiator cap, it was under some pressure but not excessive. I noted the fluid level jumped from 1" off the bottom of the reservoir to about 1/2 of the reservoir when I released the pressure. Water does not compress. So there must be air in the water jacket--is this normal?
 

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1992 500SEL
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188 Posts
Discussion Starter #5
Somebody suggested a can of "SeaFoam" may solve my problems with starting. They are a genius...

I added 1 can on 3/4 a tank and added another can on my next fill. After approximately 300-400 miles of driving, the hard starting issue is virtually gone. It seems to start just fine now after about 3-5 seconds of cranking.

Who knows for sure what the problem might be. But the solvents in the SeaFoam appear to have helped. Thanks to whoever suggested trying that.
 

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SL55, 300SE, GL450, Daytona 955i
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what color is your oil?
 

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1996 S500 coupe
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782 Posts
well the coolant capacity is 16L, roughly 4.5 gallons,.. chances are when the expansion plug came off, you lost most all your coolant, and still have not filled up 4.5 gallons. Get 2-3 gallons of MB coolant, premixed with 50% distilled water handy and keep adding it.. once you add more than 16L you can safely say there is a leak somewhere .. you can also pressure test the cooling system.. any garage can do it for you with the proper tester
 
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