I have come up with a possible reason for what may cause, and or may have caused, the early OM603 aluminum heads, (86-87 300SDL and 87 300D and TD) to develop fractures, cracks, in the aluminum head casting that allow compressed combustion to diminish dramatically and also leak into cooling chambers, ultimately rendering the head as useless!

While there are many original #14 OM603 aluminum heads still installed and in daily use today, many others in the past have suffered what is becoming known as the "603 head crack syndrome" since nobody knows for absolute sure why? We know it's mainly heat related and overheating will do it but in some cases overheat, or boiling over, have not been present.

As I understand it, MB had taken early notice to head cracking issues but never published any detailed info other than to suggest owners were not implementing the recommended 30 second cool down before switching off the ignition. Other related and possible causes could be traced to the early trap oxidizer (calif. mandated) which not only generated more heat, but also threatened to come apart internally (ceramic particles) and get sucked up by the turbo and then rammed into the firing chambers thus damaging the turbo and engine. MB redesigned the trap and recalled all OM 603's for refit, but some owners either ignored the recall or were not aware. I've seen one still installed recently!

At some point early on MB must have realized the 603 heads had an inherent weakness and under certain circumstances were vulnerable to fracture and cracking. Design modifications to the OM603 aluminum head were implemented to reduce or eliminate fractures and cracks from occuring. That production run as I understand it began with #17 up from #14 although I've heard a small number of these #17 heads also had cracked too. Further modifications as I understand were implemented. I'm aware of numbers in the production run going as far up as #22 up from #17. I'm not sure if there was a #15 or #16. MB stopped delivering diesel cars to the US from 1987 through 1990. Later heads have a design modification to the precombustion chamber, and some people here on this forum have been successful machining older PCC's to fit the new heads, which are aftermarket as I understand it. So I'm not sure later production numbers such as #22 are MB, or aftermarket numbers?

More to come.........part 2 wow!

Steve...........................

Mercedes redesigned the head, AND head gasket, a total of five times - that's six different part numbers each. The original head was #14. There was a #15 but it's rare, I've heard of *one* in the USA. Next up is the #17, used on USA engines from 1990-95. After that there's a #20 head, followed by a #22. The revision between 20 and 22 is modification of the oil passage in front of the #1 cylinder, between it and the timing chain cavity, to help eliminate gasket failure in that area. The change between #15 and #17 is documented in the factory engine manual... "The reinforcements at the valve tappet guides and exhaust passages have been reinforced." I don't know the difference between the 14 and 15, or the 17 to 20. I also don't know the number of the missing "sixth" head, or if it exists - but the EPC shows six different part numbers (all superceding to the current #22 head.) The #17 heads are almost crack-proof, they require massive abuse (overheating) before they will crack. I've heard of one or two and both, IIRC, were driven for a while in the 120C+ range. Oh - the #22 head is an OE design, you buy a new head from the dealer, it will be #22. There are no aftermarket heads.

The precombustion chambers were changed from 1990-up in USA engines, but for emissions reasons. The new style is inclined (angled 5 degrees) and uses shorter glow plugs as well. The old PC's may fit new heads, they may not. If not, you need to either machine the old PC's (like I did), or machine the bore in the head (much more difficult), to allow them to fully seat. Or, convert to the inclined injection setup at exorbitant cost (new PC's, lock rings, and injectors - $1500+ new.) According to the experts, the problem seems due to sloppy machining tolerances on the newer #22 heads - nobody knows why though.

Last comment: Although Mercedes did not import any 3.0L 603's (603.96x) into the USA except in 1987, they WERE shipped in the rest of the world from 1986 through about 1995. So there are tens of thousands of them running around Europe. Only the USA models got the dreaded trap oxidizer, however! There seems to be fewer failures in Europe, which lends credence to Marshall Booth's theory that the trap oxidizer's additional heat load may have been the cause of the rash of USA failures.

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Dave
Boise, ID
Check out my website photos, documents, and movies!

Minor correction - the 603.961 was available in the US-spec 86 300SDL.

Sixto
95 S420
87 300SDL

Thank you Dave for providing the evolution of the 60X heads in sequense.

After having read several posts here recently from new owners, and perspective owners, looking for info on either 300SDL or 300D and TD, and then become aware and then disillusioned, of cracked heads scare, and know not much else, other than people telling them to beware, and rightfully so! I thought it would be a good time to express a theory I have been tossing around in my head for over the past year and a half, and see what responses and thoughts from others it may generate. I think it is important because as time passes, and these cars are getting older, and more vulnerable, now 18 years operating, if there is anything to my theory, owners of these old relics could possibly take measures to prevent head cracking of the original #14 heads if still installed. I know that for many a 603, when the head cracks, it spells the death-nell! Realistically, for many the cost of repairs versus end-value of the repaired vehicle is for all practicle purposes, prohibitive! Exception: those who have found a hobby keeping an old Benz alive and loving every moment of it. But for those less fortunate to be able to perform their own repairs, these are not practicle automobiles for most people.

It is important to note for anyone reading this post that I am expressing a theory only, and encourage others thoughts pro or con, and is subjective ONLY to the OM603 diesel engine with the original #14 cast aluminum head.

I bought my 87 300D almost two years ago. I bought it from a used car wholesaler who in turn bought the car from an auto auction. It had a bad head gasket he told me! He had also purchased a good running 86 300E with light front damage and with plans to use the 300D for it's sheet metal etc. but he lost interest and offered each MB for $675 After reading his ad, I was the first one in the door.

I took a good look over the 300D, and because it had obviously been garaged, and with no dents or rust and an immaculate interior, I paid the man and had it put on a flat bed that day and delivered home.

I had no prior knowledge of 603 head crack issues, and thought for the little money I paid for the car, I should be able to repair it at a resonable cost. Later, I found I was wrong! Over the next year I spent quite a bit of time locating a good replacement engine with good head. Buying a used head seemed foolish based on what I had learned about the cracks, a new head was just out of the question $3K !!! I paid $1600 for the replacement 603.

Like a few others who have had a cracked 603 head on their hands, I asked a couple of independent MB shops about welding the cracks closed and one guy said he'd do it, another told me, not possible to do! and said "don't believe anyone who says they can"

The cracks that are visible to the eye on the aluminum head are located on the flat bottom side at cylinders #6 #5 #4 #2

At cylinder #6 and between precombustion chamber port and exhaust port metal valve seat, distance between approximately 3/8" is one crack approx. 3/16th inch in width.

At cylinder #5 and between PC and exhaust is one crack and intake valve to exhuast valve seats is one crack with same dimensions as above.

At cylinder #4 and between PC and exhaust valve seat, one crack, same size as above.

At #2 a repeat of #4

Also present was heavy pit corrosion generally located at and around cooling ports throughout the flat surface.

Tomorrow, I will conclude with my theory, I promise!

Steve..............

Then the logical solution would be to completely eliminate the trap-door oxidizer for the sake of the vehicle and hope that emissions and mileage standards of fuelish USA gasoline V-8 engines clean up their act.

I think the last time MB had head-cracking problems was with the 230.4cy gasoline engine of the 115 back in the early/mid 1970's.... possibly for the same reasons, i dunno, USA 4 cylinder 115's are now practically extinct.

Benz wasn't the only company with head cracking problems, the Volvo/VW/Audi turbo diesel of similar vintage does the same thing. Usually not so bad, but I've heard of them with the valve guides free in the head. Usually repairable because of the location of the water jacket on that head, unlike the one on the 603.

Design evolvution, things don't always work properly the first time, after all. I believe the head on my 300D is a #14, now had 205,000 miles and going strong. Others don't last as well.

Overheating will kill any of them, by the way, they cannot withstand the thermal stress. Probably never will, either. It's the cost of using aluminum (easier to produce, much lighter, better cooling).

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

When I thought about expressing a theory about the 603 head cracks, I didn't think writing it would be so long but I felt I should explain a few details for those who are uninformed, or not aware. While I do not claim to be a professional mechanic, or have vast amounts of technical knowledge based from some data base compiled from experts, my qualifications are limited at best. However, I have researched, gathered info, and have had hands-on with the 603, and some of what brains I still have in my head, of which I wonder sometimes!

Amongst other ventures in the professional world, I worked 9 years as a marine surveyor and it was my job to inspect, scrutinize, and report my findings. So it almost is by nature that I pick-apart issues with my MB 300D and report! In this case, theory.

I'll be short and explain quickly my theory. Most of us are aware of the thermo dynamics of most all aluminum alloys. There are a variety of alloys formulated for a number of tasks. To mention one of the alloy's that are used for casting engine heads is aluminum alloy 356* and may be what MB used for their #14 heads, later a more fatigue resistant alloy W319-T7 aluminum was born.

One thing aluminum does not do well with is repeated cycling, another is high amounts of heat, and yet another is a corrosive atmosphere and as a non-ferrous, wont stand long if in direct contact with a ferrous metal.

While we are aware and agree that overheating, even once, has proven to be fatal for the 603 head, I disagree that just the one time overheating event was when all the cracks formed that killed the head. I believe that the "one time overheating event" was only the straw that broke the camel's back!

It is my belief that the cracks that proved fatal had been started long before and were waiting for the one time only needed to breach completely open allowing coolant flowing were it's not supposed to be and compression to diminish below needed combustion to effect detination.

How many of you guys have a lead foot? Imagine heading out onto the freeway going up a short and steep on-ramp up grade and on into very fast traffic also ascending a steep up grade after the on-ramp for about a half mile before you crested the hilltop and then begin very rappidly desending for another half mile or so.

Going up the on-ramp, foot really into the pedal, turbo engaged in full boost, black smoke pouring out the tail pipe, after you've murged into the traffic you're still into the pedal because you need to get up to 75 or 80 to flow and your still on an incline, you still have your foot into it but not as much and now you can take your foot off the pedal completely because your now going down hill, and rapidly!

Where does the majority of the rapidly gained high amounts of heat generated go in the above scenario? It is transfered in two directions, #1. dissipated into the metal core of the engine and engine head and absorbed into the passing liquid coolant for a trip through the radiator. #2. Through the valved exhaust port and on to the turbo where it gets bogged down a bit adding a little back pressure before it is expelled out the tail pipe.

Lets go back up to the exhaust valve at the time you crested the hilltop, I'd bet the area just around that exhaust valve was extreme with heat, about as hot as it can ever get there! and right next to it is the intake valve and with cold air being forced in. The confined area within 3 inches or so, are two extremes in temperature. Very hot, and relitively speaking, very cold by comparison. Hot and cold! Remember, your going down hill fast and the engine is rapidly cooling!

In my opinion, I think it is no coincidence that the cracks that formed in the aluminum head casting of my 603 were emanating from the exhaust ports in all but two of the cylinders where there were no visual cracks at all but if one could dissect those 2 cylinders one could detect underlaying cracks forming in the same location as the other 4 appear, and poised ready for cracking too!

If my theory has any weight, repeted high-end turbo boosted acceleration runs with frequency will gradually fatigue the area around the exhaust port and when at the time of a cooling system failure the already stressed aluminum completes the progression of relief by cracking open, generated from repeated cycling of hot and cold extremes confined closely together!

So my advise is, loose the lead shoes, or buy a new 17 or 22 head. That's why Mercedes redesigned them!

Please express your thoughts and tell me I'm not crazy! I can elaborate more if you'd like.

Steve.............

Interesting theory, but I wonder if the metal around the intake port is really that cold. Since aluminum is such a good conductor, I would expect the heat to be relatively even throughout the metal. Also, how cool is the intake air anyway? After being compressed by the turbo, it is going to be warmer than ambient, plus the turbo itself is really warm so it is going to pick up some heat from that. I am not saying your theory is invalid, but I am not sure it is so simple. Usually failures of this nature are hard to trace back to a single factor. Rather it is usually a combination of factors that lead to problems. In this case, I would hazard a guess that thermal characteristics such as you describe play an important role, but I don't know that the higher compression of the diesel engine can be completely disregarded. Does anyone know if this was the first use of an aluminum head on a diesel engine? If so, it may have presented issues that aren't prevalent in a gasser engine. I am not an engineer, so that is just conjecture on my part, though.

__________________
Keep everything as simple as possible-but no simpler--Albert Einstein

HABANERO

I used the word "cold" to demonstrate better the extremes of temperature difference in a relitively small area between what is coming into and going out of the firing chamber.

To venture a guess, at the point of diesel ignition 15 ATDC, the escaping gases have generated a heat well beyond 500f (a guess) while upon the next stroke, air coming into the chamber, originating from the turbo, difference by measure is quite extreme. I have had the crossover from the turbo off and have put my hand into the air being boosted out by reving, an up-to temperature, engine and it was not hot at all, but rather luke warm. Still however hundreds of degrees cooler than that of what is entering the exhaust port.

Steve............

Side note - injection still starts at 24 BTDC, not 15 ATDC. The 15 number is simply a reference value, where a tooth inside the IP will connect with the RIV tool, to make setting timing easier. But the start of injection is still 24 BTDC, same as the old engines. (Clear as mud, right? )

I understood you didn't mean "cold" as in cold, but rather cooler than exhaust. I forgot to include that in my original post. Actually under loads you are speaking of the outflowing gases are probably more in the 800-1000 degree range. I still don't completely buy in to your theory, though. I still maintain there is no one magic answer to the problem. I doubt Mercedes engineers completely understand what the problem is, they probably just redesigned the head to better stand the symptoms without knowing the disease.

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Keep everything as simple as possible-but no simpler--Albert Einstein

Good theory. I don't know if the "lead foot" theory can be proved, some of the longer lasting 14 heads seem to be owned by people with lead feet, Turbodiesel and me for example, I don't know how plantman drives. I know Brian drives slower but his car has a newer head.

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1969 280SE
2023 Ram 1500
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Thanks for the reply. Yes, it is just a theory and there is an infinite number of different ways how people drive, and there isn't a practicle way to test my theory.

However, if for those who drive heavy on the accelerator never have a major cooling system failure that causes an overheating, or boilover, may never experience cracking. But what I'm suggesting is that underlaying in the area I've described is already fatigued from years of cycling from one extreme to the other, waiting for the time when the heat go's higher than ever before and at that point the stage is set for the weak area to crack, versus if you were to take a brand new #14 head and then overheat it and see if it will crack. If it does not, than it still remains possible for my theory. If it does crack, than my theory is likely full of holes! I'd bet a new 14 overheated wouldn't crack!



Anyway, something happens that causes the cracks and I have just expressed one possibility of many I'm sure.

Does anyone have their own theory?

VW/Audi/Volvo and Renault both produced aluminum head diesels before MB, if I remember correctly (late 70's). There are some head cracking problems with those engines, too, made much worse by overheating.

I suspect the issue is complex, as gasoline engines, particularly turbo boosted ones, run MUCH hotter than diesels most of the time.

The exhaust valve stems are sodium filled to remove the heat from valve and seat better ( and have been since the 50s at least, probably the 30s) on the diesel engines, so heat buildup will be less of a problem.

I would personally suspect an expansion problem, not directly related to differential temp between intake and exhaust. This head has pretty large valves very close together and is a fairly thin casting. I would imagine the casting was beefed up as the design progressed, but cannot say so with any certainty.

Cylinder head design is sorta complex -- if there was a simple answer like "don't drive with a lead foot", MB would have found it -- Autobahn driving has to be experienced to be believed. If cyclical flat out acceleration followed by off throttle conditions crackes those heads, every single one in Germany would have cracked! This in NORMAL driving on the autobahn.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

If what you say is correct (and my 3yrs of engineering school cant argue w/ you - no offense intended but there is not a lot of facts to go on here...) then maybe it'd be worth an owner's time to hunt out and install a thermostat with a lower temperature setting- give yourself a bit more margin.

-John

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2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

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