In the Summer I would go to the bank and while waiting in line i'd notice the temp gauge rise almost into the RED, but not quite. However when I got out on the road and was moving the temp moved back down to around 80-90 range. I haven't noticed any need for coolant. It worried me that this it would rise that high on while idleing. I do watch it all the time but here lately it's been fine with the cooler weather.

Mine never really showed any erratic temperature issues, although since replacement the temp runs maybe a couple degrees cooler than before. The cold pressure and ever increasing coolant usage were my signs.

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Warning that is not normal at all! I could never get mine over 100C no matter what I did last summer. I would say 105C is about as warm as a 603 ever should run.

Did the electric fan come on? How old is your radiator?

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Amen, Hattie. Sounds to me like a possibly bad radiator. and the fan clutch is suspect too... also need to verify that the electric fan is coming on at 105C, or just replace the switch as preventive maintenance (every original switch I've seen has been bad - those aren't good odds.) A 603 shouldn't get over 105C idling in summer, even in 110F ambients. Something is wrong - fix it ASAP...!

Q: Will all #14 heads fail?
A: No, probably not. But they are known to be weak and must be treated with respect.

Q: If I buy a car with no "cold pressure" or other symptoms, and I never see the temp go over 100C, does that mean the head will never crack?
A: No - sorry guys. The overheating incident can happen months or years prior to actual failure. This happened on my car - trust me - I know.

Q: Does the high cold pressure mean the head is definitely absolutely for sure cracked?
A:Well, no, but it's about 99% likely.

Q: Does *no* cold pressure mean the head is OK?
A: Yes, probably, but ONLY if the cooling system is tight and there is *zero* coolant loss.

Q: How do you know the head is cracked? I know a lot about other engines and I bet it's just a bad head gasket.
A: That's great you know a lot about other engines. Now welcome to the world of the OM603. A failed head gasket is not uncommon on a 60x but the symptoms are typically MUCH different - either mixed liquids (oil in coolant or vice-versa), or massive oil consumption & smoking when the gasket blows out into the timing chain cavity. The "high cold pressure" symptom will NOT be from a bad gasket.

Q: Rumor is the trap oxidizers contributed to the head cracking. Is that true?
A: Nobody knows for sure, but there is a lot of circumstantial evidence pointing to the trap adding excessive heat load in the area of the exhaust ports, probably exacerbating the problem. This would be worse if the car was driven with the trap plugged, and/or the car came from a hot climate (central CA, Texas, Florida, etc.)

Q: Are the #17 and newer heads crack-proof?
A: No, but they'll take a *lot* more abuse before cracking. I think I've heard of one or two that cracked after being severely overheated.

Q: How many cracked 603 heads do you know of?
A: Well, this past year (2004), I think I heard of at least 6-8 total. Prior to that, probably 3-6 per year from 2000-2003? That's just from this forum and the MBZ.org email lists - who knows how many others are out there.

Q: What is the percentage of heads that have cracked and/or will crack?
A: Unknown. My guess? Probably 20-40%. Maybe more. There are a LOT of people driving around, blissfully ignorant of the "high cold pressure" syndrome, until something bad happens like hydrolock or extreme erratic operating temps & overheating.

Q: What causes the cracked heads?
A: You mean besides a flawed casting design? The main culprit seems to be overheating. That means a trip into the red zone (120C+) with a full cooling system. If you pop a radiator hose like I did, you can lose some coolant and have the head get steam pockets near the top of the waer jacket, resulting very high localized temps, while the dash gauge temp sensor (which is lower) can be in liquid and only read ~115C. That's not a likely scenario but it happened to me.

Q: When did Mercedes figure this out, and how many variations are there?
A: Apparently Mercedes changed the head design around 1988, right after the last 603.96x came to the USA. There are six different head casting numbers... 14, 15, 17, 20, 22, and the sixth is a mystery as nobody has ever reported a head with another number. The head gasket design was also changed 5 times (six total part numbers). The #17 and newer heads are the crack-resistant heads. We're not sure about the #15, and that particular casting is as rare as a quiet night at home for Paris Hilton.

Q: So what temperature should a 603 run at?
A: 99% of the time, it should be in the 80-100C range. End of story. Most common range is 85-95C. If it's in the 100-105C area (that's the remaining 1%!), it better be HOT outside, either idling in traffic or pulling up a grade, or otherwise under heavy load. When the load is removed, the temp should drop back under 100C quickly. If not, something is wrong. If it gets to
110C or higher, something is wrong. Period. Fix it! (Search other threads for the cooling system woes - that's a whole separate topic unto itself.)

Q: My head is OK for now, I think. What can I do to prevent cracking?
A: Keep the cooling system in top shape and watch the temp gauge like a hawk, looking for the 80-100C range noted above. Replace the temp sender and the electric fan switch as a preventive measure if they are not recent. Keep the radiator & condenser fins cleaned out with compressed air or water. Use ONLY (!) Mercedes anti-freeze (or Zerex G-05) and replace it every 2-3 years. Replace the plastic coolant reservoir tank also, the new ones have a silica pack to control corrosion, and the old ones have a habit of cracking in the rear where you can't see the crack. A bottle of Water Wetter doesn't hurt either.

Q: My head is cracked. What will it cost (approximately) to replace it?
A: As a DIY project with a used head, $1200-$1500 and 15-30 hours of your time for a novice. To pay a shop to do it with a used head, $2000-$2500 and 10-20 hours for a pro. For a new head, add $750-$1000 to those figures.

Q: How much does a new or used head cost?
A: If you can find a used #17 head, they're typically $800-$1000, sometimes more. That usually includes valves & springs, but may not include prechambers, lifters or cam. A brand new (#22) head comes bare with no valves or springs, and current price is about $2000 list, $1600 wholesale.

Q: Are any modifications needed to bolt on the newer head?
A: In general, no. However the newer heads all came with different (angled/inclined) prechambers and injectors, and our 1986/87 engines have vertical injection. That means your vertical prechambers may not seat properly, requiring modifications to the old prechambers for them to seat properly. This only affects some newer heads, probably less than half - no way to tell until you get your replacement head, and do some measuring with the prechamber inserted. Yes, you can convert to the inclined/angled setup and elminate this problem. But that requires the newer prechambers, lock rings, injectors, AND glow plugs - then you need to bend the metal injection lines as well! Forget this option unless the used head includes all those parts.

Q: Should we just stay away from these cars (if they have the original #14 head)?
A: Well, that depends how much you like them. I like them a LOT and I'm willing to spend the money on the head replacement. When it's done, you get one of the most incredible diesel cars Mercedes ever built. If these numbers are out of your budget, stick with the old/cheap OM617 models until you can afford to move up... you can get a whole 617 engine for less than a used 603 head. (And there's a reason for that, IMO! )

Q: Is this at all related to the rod-bending 3.5L?
A: No. The 3.5L (OM603.97x) engine, while sharing many of the 3.0L (OM603.96x) parts, had a design flaw in the connecting rods. Since this engine came out in the very late 80's and was imported to the USA in the S-class for 1990-95 models, it came with the improved #17 head. These engines are good donors for used heads.


OK - if there are any errors above, let me know and I'll correct them, or add to the list, etc...

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Very nice, Dave. I'm very happy with my 617, but always thinking ahead....Before long, my kids will be coming of driving age. Maybe an SDL with air bag and ABS is in my future. Of course, Dad would need to test drive it on a regular basis and to make sure it got its regularly scheduled Italian tuneups!

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"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Perhaps should not mention on this thread. Where do these heads like to crack? There is an automotive machine shop in montreal, canada that services all canada and perhaps some of the states as well. It seems to have a reputation of doing the impossible. Know a lot of mercedes dealerships use their facility as years ago sent them cams that had the first lobe worn off on the 130 type gas engine. Was surprised by a few things. First they quoted the repair would last, second that they had all mercedes cam profile guides in stock, that they do them everyday, and turned out way below expected price, they do sub work for automotive machine places at least a thousand miles away, across country for all I know. When my friends started having their bmw heads crack, a two thousand dollar proposition, suggested they try this firm and they did. Results were perfect on every repair and the heads lasted in service. You cannot get a perfect metal match when repairing so often wondered how exactly they did it. Bmw became famous for cracking around the fifth cylinder. With the american dollar differential maybe someone should call and talk to them. Will try to locate name and number again if anyone is interested. The guys I talked to seem to have had really good character back then. (six hundred miles away)Told it like it was with with no snow job. Just maybe they repaired these heads every day when these cars were in their prime. Kind of would not suprise me as everyone was buying new or used cams years ago when I was using this shop. They were repairing mine and paying the return shipping for 50.00. This also included reprofiling the whole camshaft. That was cheap even back then.

Mine was cracked at cyls 1, 5, and 6.

I will have to later check the cylinder numbers for sure, but mine had cracks between the valves on two cylinders and one cylinder had the prechamber to valve crack. Looked like another between-valve crack had been fixed previously.

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GSXR you nailed it, wow those pics are nasty $$$. Hopefully I will never have to deal with that. If I had to I would probably buy a new one and put all of my old parts on it myself. If I could get the new head to my door for $1,800 I could probably do the whole job for $2,200ish. Ouch

btw Brian C, I think has a 15 head on his SDL.

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1969 280SE
2023 Ram 1500
2007 Tiara 3200

He does.

Does not make much sense, however, because you fellows have a #14 on 1987 vehicles. I wonder if it was replaced sometime when it was fairly new?

That's my deduction as well. No way it came from the factory with a #15 head. I'm pretty sure yours is the only one I've ever heard of in the USA. Most are #14, #17, or #22. Brian Toscano has the only #20 head I know of (there may be a second one that I'm forgetting though). Both the 15 and 20 are pretty rare!

Thanks, Dave

Maybe I can donate mine to the smelter, to be made into Humvee armor! No, not the head, the whole damn car!

I am seriously planning to melt my old head down and cast it into something. Dad has a coal-fired forge that should work for melting the aluminum easy enough, just need to read up on casting techniques. I don't know what scrap aluminum goes for, but I figure for the three grand it cost me to replace it, it isn't worth it to sell for scrap. Better to make something cool from it.

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

I'd love to see somone cut it open in cross-sections so we can see what's in the middle, where the cracks are!