'92 300D 243K head gasket leak, what should I do?

[Wayfarer]

The replacement 300D 2.5 I bought after my last one was KIA'd (had it from 90K to 330K) seems to have just a bit of an issue. I didn't have it inspected by a mechanic since we were talking only a $2-3K car here and I figured I could do my own inspection. This car came from a fellow board member who seemed to know the car very well. He did all his own oil changes w/ Rotella, had done the pressure wastegate conversion, and was a generally knowledgeable DIY'er for these cars. The test drive went great, car drove great with engine, tranny, A/C, heat, all working as it should. Interior was very nice but the exterior was a little sun faded. Suspension was a little tired, so in my mind, I budgeted $500-$1K for a suspension rebuild of the critical components. As long as the engine was good and didn't need major work, I'd be OK. Oil was good and jet black no signs of coolant, coolant showed no signs of oil, and a look around the head gasket showed no leakage. No discernable smoke of any color even on hard acceleration. Checked blowby and there was NONE. I did not have a hand mirror with me on the inspection to check the very back of the head but figured I was OK, especially since the seller told me there were no fluid leaks at all (even said there were no leaks in the ad).

Well, make an already long story longer, knowing thee cars and what to look for, as soon as I got the car home after purchase, I got my hand mirror to check the back of the head to put my mind at ease, and sho'nuff, there was a pretty decent head gasket leak at the back corner on the passenger's side. Evidence of a long-term coolant leak was present running down the side of the tranny under the corner of the head. In fact, there was an inch-long stalagtite off the bottom of the unplugged block heater!

So, next step was to determine how bad it was. It wasn't hissing or actively leaking, so maybe I was lucky and the leak had stopped or maybe even was repaired at some point. So I closely monitored the coolant level in the expansion tank over the next tank of fuel and looked at the leak every time I had the car stopped, and this is what I've found. The car leaks about a quart, maybe a bit less, of coolant every 500 miles. The back corner of the head is wet with obvious coolant where the gasket should be about an inch along either side of the actual corner of the head. There is NO sign of coolant being burned that I can tell, there is no white smoke, I even had my friend start it up and dog it up and down the freeway for me with me closely following in my "other car" (Honda) and I did not see any smoke at all of any color. The car does not burn oil, or at least none that I could discern after 500 miles of driving. There are no other coolant leaks other than the head gasket. It does run a wee bit hot, though, 100 deg C at a steady 70 mph cruise at ambient temp of 90 deg F. Idles all day long at 90 deg C even with the a/c on and 95+ deg F ambient temps. It does climb to 105-110 after hard acceleration from a stop to 90 mph, though, but comes right back down to 100 after I let off the gas. I think the temps may be high just from a semi-clogged coolant system, though, and not due to the head gasket (I learned all about the weak-ish cooling system on these cars from my 13 years of owning the previous one...everything has to be working right or it'll run hot). I think the head gasket leak is confined to just coolant leaking outside. My indy is doing a leakdown test on it tomorrow, but I anticipate that to come out fine since there is no evidence of the cylinder being compromised: no white or blue smoke, car starts right up, no oil in the coolant or visa versa. Oh, and the cooling system does not hold pressure overnight.

So, here's my question: Can I just drive it like it is and put in coolant when it gets low? I don't really care that it leaks as long as it's not too much to keep up with by adding a bit of coolant every 500 miles or so. Or is this situation likely to result in it blowing catastrophicly at some point like when I'm trying to crest the Rocky Mountains on my way driving this car from Dallas to Boise which I will be doing week after next? Are the 105-110 temps I regularly see likely to accelerate the deterioration of the remaining gasket material? Do these kind of leaks generally tend to worsen quickly once they start? My previous '92 300D had a perfect head gasket right up until 330K, by the way, so I am not well versed in addressing this situation. Upon looking at gsxr's photos of the top of his block, I believe there is a coolant return line at this corner and that's it. No sign of oil leaking here, so maybe the gasket is leaking at just this one point.

My goal with this car is to have a reliable daily beater for low $. I'd rather not pour $2K into pulling and rebuilding the head if I can help it. Adding coolant every so often is ok. But at the same time, I need the car to be reliable and don't want to end up on the side of the road in southern Utah at 2:00 a.m.. Other than the head gasket leak, the car looks like it will make a good daily beater.

Bad photos of the leak:

No such things as a cheap mercedes. And reliable and the phrase "headgasket problem" do not belong together.

[sixto]

I wouldn't attempt the Rockies in a car that sits at 100*C while cruising at 70.

How about changing the thermostat and coolant immediately and see what that does to operating temps? Umm... and just for kicks try coolant that isn't green. If that gets it to where you really have to push it to see 100*C, make sure the coolant level sensor works then head for Boise.

Sixto
87 300D

[nhdoc]

I'd do this for kicks. Drain coolant and replace the thermostat. Use 50/50 M-B or G-05 Coolant on the refill then drive the car with the pressure cap loose and see what happens. If you can get away without running it at pressure it will probably stop leaking. You might even try a lower pressure cap or replacing the cap too and see if it might be running at too high a pressure. I'm not saying this is a fix, but it might just save you from having to face doing a head gasket in the near future.

[Wayfarer]

Yep, he ran green coolant , suposedly the stuff that is good "for all makes" "even cars with aluminum heads!" I plan to have my indy do a coolant flush, new thermostat, and then refill with MB coolant. For the regular coolant additions I plan to use Zerex G05 50/50. Then I'll see if the temps cool down. PO said he replaced the radiator a couple of years ago, and indeed, the Behr that's in it has a manufacturing sticker dated 9/06, so it's not too old. Wonder if him running the green stuff could have ruined the radiator in that time, though. In my last car, I always ran the MB stuff so I don't know what damage green coolant does over time.

Interesting suggestion that I try a lower pressure expansion tank cap. The head seems to be just barely leaking, so maybe running with a lower pressure might cause it to stop. I'll try that.

Again, I'd like not to dump $2K into the car if I can help it, but I really don't want to risk blowing the motor if that's a possibility, especially after my experience last month with my former W124 300D. I think I'm spending more time watching the temp gauge than the road, however. If the temps come down with a good flush, new thermostat, and the proper coolant, maybe, just maybe, I can clear the Rockies with that head gasket as is.

[babymog]

I would not worry about the temp, if you're seeing 100C cruising at 70mph/90* ambient, especially if that's with the A/C on. Have you verified the coolant temperature or is it possible that the gauge/sender is/are off slightly? That is not really very hot for a warm day and it sounds like it idles well and recovers quickly from a hot run. A possible indicator that it simply has a "happy gauge".

It is possible that the water pump has rotted away to a point where it isn't moving enough coolant (see cavitation pitting below), but again the quick recovery and reasonable idle performance indicates a healthy system, maybe there's deer fur behind the condensor or a weak fan clutch. Running the A/C automatically runs the electric fans on low so that would mask a bad fan clutch during your idle test, high temperature also runs the fans (high speed) and would bring the temp down quickly. Do you ever hear the roar of the clutch fan?

As far as green coolant, no BFG unless it is not a phosphate-free quality green coolant, it's just the color. Change it to the proper coolant, green does not hurt gaskets and radiators, aluminum-engine and aluminum-radiator/heater (green) coolant has been around for over a quarter-century. The more important part is the cavitation resistance, especially in a diesel engine, that's what the G-05 and other proper HOAT diesel formulations bring to the table, a good quality diesel formulation changed (or balanced) regularly will not hurt the radiator. The bigger problem would be cavitation-pitting from the wrong coolant (per Mercedes-Benz literature et al). Change the coolant to M-B or Zerex G-05 and don't worry about the old coolant color any more.

Running a lower pressure could lead to localized boiling, which can cause problems also. I would say that your better bet is to run the proper system pressure and keep adding coolant if you're going to run it as-is.

Not holding pressure overnight, what does that test again? You've already established that you have a leak so, ...
Personally I think that there are too many ways that test can be wrong. A better method IMO is to start the engine cold, and feel for pressure building quickly; before the engine is warm. Or have it tested for the presence of exhaust in the coolant (many shops can do this).

There is no way that you can guarantee the car will not catastrophically lose the head gasket in its present condition, nor with a new head gasket. If dependability is your goal then you're going to have to ante-up, and what about the vacuum-pump and timing chain, the belt tensioner, the transmission, ... ironically I've been left more times with new cars in the first 2years than any of my older cars.

[Wayfarer]

Thanks Jeff, Marty, Sixto and others for the advice and help. I dropped the car off at my trusted indie today, tomorrow I will be getting the call. I'll post the diagnosis.

The pressure test is warming the car up and then leaving it sitting overnight. If the cooling system holds pressure 12-24 hours, it is likely from a cracked head or internally leaking head gasket. It should slowly release pressure as it cools. My car passes this test, as well as the other quickie tests like looking for coolant in the oil, oil in the coolant, and white smoke. All good there. I'm keeping my fingers crossed it only an external leak.

And the gauge is reading correctly as verified by my IR thermometer, and the water pump was replaced six months ago by PO and still looks new.

[nhdoc]

I'd still try a lower pressure cap. I believe the standard cap is like 1.4bar and you can get one at 1.2 bar or even 1.0. All that will do is lower the boiling point some but even at 1.0 bar using 50/50 dilution it will still be above 260F.

[engatwork]

I would say that your better bet is to run the proper system pressure and keep adding coolant if you're going to run it as-is.

That is how I see it.

Just keep coolant in the reservoir and I feel you can drive for a long, long time like this. I purchased a 95 E300 that had obviously been driving around for a long time like this. When the dude delivered the car he had two 1 gallon jugs of the green Zerex in the trunk (he had driven up from Tampa, Fla). The worst damage I saw was a little cavitation damage in the area right in front of the water pump impeller. The o-ring at the passenger side rear of the head gasket had failed. The new head gasket does not have this same design.

On the other hand I just purchased a 95 E300 that is right at 250k miles that is sitting in the garage waiting for me to change the head gasket out. I have been maintaining this car for a few years and it just started seeping about four or five months ago. The interior of the car looks brand new. The lady I purchased it from bought a brand new 09 E class bluetec. Wonder if the bluetec will be as good a car as the 95 in fifteen years?

[Wayfarer]

Originally Posted by engatwork

That is how I see it.
The o-ring at the passenger side rear of the head gasket had failed. The new head gasket does not have this same design.

That gives me hope that I can drive it like it is and just add coolant when necessary. 1-2 pints per 500 miles isn't so bad, that's about what it seems to loose. Of course once I replace the bad expansion tank cap, that might drop.

Your post made me think to look up a photo of head gasket in these cars. It most certainly appears that the rear passenger corner is outside the normal sealing area and is sealed only with some sort of an o-ring as you point out. I wonder why it is designed that way, since that appears to be an obvious potential weak spot. But on the other hand, maybe they designed it that way so that if the headgasket failed, it would fail there, which seems to be a non-critical area. So it makes me wonder if that is the most common place for the head gasket to fail and if others have had similar failures that they drove with. I hope so, because that's what I plan to do if at all possible. Thanks for the info!

[babymog]

Originally Posted by Wayfarer

The pressure test is warming the car up and then leaving it sitting overnight. If the cooling system holds pressure 12-24 hours, it is likely from a cracked head or internally leaking head gasket. It should slowly release pressure as it cools. My car passes this test,<>

I realize this, but it is far from a conclusive test. My point is that since your coolant system has a leak, it is likely to leak all pressure out through that leak, so it would never hold pressure overnight without replacing the head gasket or whatever is leaking. Your car can not fail this test.

[Wayfarer]

Originally Posted by babymog

I realize this, but it is far from a conclusive test. My point is that since your coolant system has a leak, it is likely to leak all pressure out through that leak, so it would never hold pressure overnight without replacing the head gasket or whatever is leaking. Your car can not fail this test.

With a leak to the outside, yes, I now see what you are saying. So scratch that test. But there are no other indications of any internal leaks. Thanks!

[babymog]

If it were my car, and head-gasket replacement weren't an easy option:

An external leak of coolant will likely continue to get worse, but likely it will get worse externally not to a new point. I would consider that it is unlikely to suddenly start barfing large quantities of coolant into the oil or cylinders (cylinder seal rings are a completely different mechanism) and thus the likely failure mode is either coolant/oil contaminiation, or a significant breach of the coolant to outside of the engine. More of the same.

Check often the coolant and oil to ensure that such a change will be noticed, and carry extra coolant. If the car suddenly starts losing coolant at a high rate, you'll have to run it easy with the cap loose / system not pressurized and stop frequently to fill/check the levels. Not an ideal situation, but if you can't repair it now, it's not as bad as coolant in the cylinders IMO.

[Wayfarer]

Thanks again, Jeff. What you are saying is music to my ears, since it gives me some hope I can drive my car the way it is. No, pulling the head is not an option, since I do not have the time to do it myself (it would take at least 3x the time than it should, going by my track record for such repairs) nor do I have the money to pay my indy to do it. I'd like to wait until I can do it right and have the head rebuilt at the same time. $2K is not in my budget right now.

So it seems the most likely failure mode would be a sudden increase in coolant loss. A second less likely possibility would be coolant getting into the oil or visa versa.

It really seems like this corner of the head was designed to be the first failure point. Below are two shots of the head from gsxr's website (thanks, Dave) that clearly show that corner and how it is sealed outside the normal head gasket by some sort of an o-ring. Also, it looks like there is one oil return passageway that is close but nothing else, and that is on the other side of normal gasket. So I've got my fingers crossed I will be able to drive it...with my eyes glued to the temp gauge...

[Wayfarer]

A quick update: indy says head gasket leak is only minor and not too worry (for now). My coolant leak was actually coming from the center of the disconnected block heater. Apparently, the center element corroded out and started leaking. So my 1-2 pints of coolant loss per tank of fuel leak is being lost through the old block heater. Coincidentally it is right below the corner of the head, so it was easy to confuse the obvious leak stains down the transmission housing as coming from the head gasket when they were from the leaking block heater instead.

Anyway, the car checks out. Looks like I got a good one. I'm having my mechanic do a major service with ALL fluids changed, plus replacing the steering idler arm and damper which he said was bad. So it looks like I rolled the dice and won *this* time.

One last question: my mech said that the passageway at the rear corner of the head is actually an oil return line, not coolant. So why is it leaking coolant at this corner and not oil? Seems strange. Makes me worry there might be a little cross-fluid contamination since the coolant has to be coming from further inside the head somewhere and passing right by that oil line. Maybe I should do an oil analysis to check for glycol just to be sure. Or maybe I should just not worry about it at all.