Hello again guys,
The patient is an '84 euro 300D (non-turbo). It is my brother's car that I've commandeered for the week whilst my '82 is down with a sick water pump.

The coolant level was low and the car would start to overheat every time I came to a stop; the temp was okay (if slightly higher than I like) when moving. There's no telling how long he's been driving it like that (mostly around town stop-and-go stuff).

I went to add some water this morning, after the car had been sitting overnight, and the rad cap (new 1.4 bar) opened with a gunshot-esque POP and shot out of my hand. That much pressure after sitting for 14 hours makes me worry about exhaust gasses in the cooling system, the head gasket, and generally more wrench turning than I care to contemplate. It doesn't have a coolant reservoir so I can't see if there's oil in the coolant, but the oil doesn't appear to be contaminated. It runs like a top with no smoke and oil consumption is very low.

I'd appreciate it if someone could reassure me that I'm just being an alarmist and that this is somehow normal and/or easily remedied.

Thanks,
Tom

The new cap should release at about 20 psi and that can pop the cap out of your had. Mercedes engines have been known to hold pressure for days (I have a 190D 2.5 that will pop after sitting aas long as three days - never looses fluid - never any cooling problems, but has occasionally exhibited maintained pressure duruing the 5-6 years I've owned it). Unless you have consistant symptoms of cooling loss or overheating (after the system is completely burped) I wouldn't worry. Occasionally a brand new cap can be defective.

Marshall

Thanks Marshall. I've never had a car with such a "tight" cooling system, and was pretty freaked out about it given the overheating.

Good thing this didn't happen on one of those dumb OM603s. It would have been hosed a dozen times over by now...

Marshall,

I'm a bit confused here. The 617 would never exhibit any pressure in the cooling system after sitting overnight. In fact, there was a bit of vacuum that would occur after the fluid and remaining air contracted. Over time this has changed over to pressure when cold. You can feel the pressure in the top radiator hose and you can feel it release when the cap is open.

An oil analysis has confirmed that anti-freeze is getting into the oil and the symptoms seem to agree with this. The symptoms include loss of coolant (about 32 oz. per 1000 miles). Everything points to a bad head gasket.

It would appear that significant pressure in the cooling system should not be so quickly dismissed as normal.

Brian, Despite your pronouncement, pressure in the cooling system accompanied by NO coolant/oil loss or cross contamination happens on some (not most) OM61x and OM60x engines and is of NO consequence. I have owned both 61x and 60x engines that intermittantly did this and neither ever exibited any additional symptoms or caused any problem. Some of the Mercedes gasoline engines do it too (I've owned several that did as well) and I suspect it's a radiator cap failure.

If there is coolant loss, then there is a problem. The most usual cause IS a failed head gasket or cracked head. A blown head gasket is a rare problem on OM61x engines (much more common on 60x engines), but it does happen. Cracked heads are even less common on 61x engines.

Marshall

An update, good news first:
There doesn't appear to be any coolant loss yet, but I haven't had much time to track it very well. It does run nice and cool as it should since I added coolant. Oil and coolant still don't appear contaminated with each other.

The bad signs:
The exhaust smells sweet. There is very little smoke, but that which I could see was light in color and not the usual black sooty stuff. I ran the engine up to temperature with the cap off to look for bubbles, and found them when revving the engine (see pic below). I guess it is possible that these are innocuous air bubbles escaping as the system burps itself from being run low?

I know that head gasket problems are really rare on this engine, and I'll continue to watch the temp and coolant level and hope that Marshall is correct (you haven't steered me wrong yet). Guess I'll order an oil analysis for definite confirmation yea or nay.

Thanks for all the help,
Tom

I would monitor the problem carefully. The bubble MAY be from refilling the system. If they are still present in a week or two, then I'd be more concerned.

In the early stages, a blown head gasket may only exhibit symptoms after the engine has run for a half hour or more and is heated to above the lower end of normal operating temperature. A compression check will often NOT pick this up unless run when the engine is well above the minimum operating temperature (say with the engine at 95-100 deg. C.). I had OM621 and 61x engines that despite being driven with a light load a half hour to 45 minutes at a time, remained in that state for years, only requiring that perhaps a half pint or so of coolant be added every 3-6 months. They could not be counted on to endure several hours of driving under heavy loads or in really hot weather. A new head gasket solved the problem. The "track" on one of the head gaskets (I didn't examine the other one) was barely visible when examined after the fact. It was little more than 0.005-0.010 mm in width.

Marshall

Marshall,

I believe that I misspoke. What I meant to say is that my 617 never exhibited any pressure in the cooling system after sitting overnight.

We are in agreement that pressure in the system overnight, accompanied by coolant loss probably warrants an oil analysis for confirmation. In the case of my 617, the oil tested positive for antifreeze. A definite head gasket.

I would monitor the problem carefully. The bubble MAY be from refilling the system. If they are still present in a week or two, then I'd be more concerned.

I will do that. I drove the car 600 miles yesterday (1600 in the past six days) and it seemed to do just fine. Stable temp and no signs of contamination.

This morning I removed the rad cap to replace it with the cap from my '82 on the off chance that it was bad as you suggested. It popped and some coolant bubbled out as usual. Several hours later after changing the water pump in the '82 I removed the cap again (forget why) and it hissed as it released yet more pressure that it had built up while sittng stone cold. Eek.

Thanks again,
Tom

One more thing I forgot to ask:
Can the head bolts be retorqued? They are the 12-point heads and I think that means "no" but thought I'd check anyway...

Tom,

I don't like the sound of it. I hate to disagree with Marshall on this, but, my 617 does the same thing as yours, to a lesser degree. It shows no sign of oil in the coolant and no sign of coolant in the oil. It loses more than a pint of coolant every 1000 miles. It never runs above 90 degrees except when the a/c is on or if it is under an extended load.

My best advice, and the final determination of whether the head gasket is history: Get an oil analysis for $15. Then you will know.


With regard to the retorquing of the head bolts, I was thinking about doing the same thing myself. However, I was going to purchase a new set of bolts to do it. But, after thinking about the work necessary to get to all the bolts (cam must be removed) and the risk of failure (no improvement), my thoughts have shifted to removing the head.

If you decide to do the bolts, please let me know the details and the result!!!

The 12 point heads are no issue. You just need a special "double hex" socket bit for them.