My wife drives an 86 300SDL with around 165k miles on it. Over the past few weeks, it had been using coolant at the rate of about a pint every 500 - 1000 miles. The low coolant light came on awhile back, so I topped it off with nearly a pint of MB coolant. Since there seems some type of trouble, I have been driving the car. Today, the low coolant came on again (and needed another pint of coolant to top it off).

Strange enough, there are no leaks around the engine compartment and its never left any puddles on the garage floor. There is no excessive smoke and after about 5 or 10 seconds of typical rough idling when cold, it runs fine. The oil I just drained looks to be clean with no coolant traces and the overflow doesn't have any oil spots floating on the top. It does get a little warm when sitting in traffic running air conditioner, but the secondary fan has been fussy. No coolant smells in the cabin either.

Among several other things, we had the head and gasket replaced about 60k miles ago, as there was oil in the coolant. I find it hard to believe the gasket could be gone again.

The radiator is almost brand new, as is the resivoir cap. This is really got me stumped...

Any ideas as to what this could be? If it is the head gasket, how much estimated cost will I be looking at?

MPlatt,
Change brands of antifreeze. If you are using MB coolant, you can look forward to replacing your radiator very soon. Switch to Prestone and save your radiator.
Let's just hope it's not to late.
Bill Lewallen; Lexington,Ky. Home of Valvoline Oil- Maker of MB antifreeze. But we use the green brand...

Also wanted to add, the system holds pressure overnight. I can remove the expansion tank cap and still hear the pressure release.

your loosing coolant and there's no visible leak. This sounds familiar, ....where is the coolant going to ?? I'm afraid to tell you that your head gasket went south again.
Check your oil level, ..get a sample if you can. Kindly let me know what you found out....///////

Checked the oil about 1k miles after changing. It was at the proper level and didn't have a coolant "tinting". The oil also smelled normal.

Well, I took the car in last night to the MB service center. After sitting all night, they said there was so much pressure in the overflow tank, they were surprised the cap didn't break off. Definitely a head gasket problem.

The quote on the phone said 17 hours of labor would be involved. That really seems excessive, does it not? At $80/hr, that adds up rather quickly. Also, that didn't include the machining of the head which is done by an outside shop.

Since the head and gasket were replaced not too terribly long ago, what could have caused this to happen? It didn't ever heat up excessively, at least not to the point of reaching the red area on the gauge (no warning light either)...


Am I being fed a line or does this sound about right?