I qualify my question by saying that I’ve reviewed the tons of great information about the infamous OM603 head cracking. However, after reviewing the material, and material about the A/C compressor clutch, I’m stumped as to the cause of what is happening with my car.

The starting event was that the rubber for the A/C compressor clutch tore. Fortunately, this was not a clutch seizure. My A/C guy suggested it was likely due to high engine temperatures, caused by a failing viscous fan clutch and weak electric fan. The fan clutch did have the tell-tale dirt and grease and the blower was weak, so I replaced both with Behr and Bosch parts respectively.

About a month later, after replacing the parts, the clutch tore again. My A/C guy was able to get a replacement, with no charge to me. He also verified the compressor (which is a 2 year old rebuilt Nippodenso) worked correctly and there was no obvious problem with the system. If this happens again, he wants to put in a new compressor, which is logical. However, it doesn’t compute to either of us and before I go that route, I want to make sure I’ve covered other bases before going to the expense of buying a new compressor, just to have the problem repeat.

I suspect a cracked head may be causing higher engine temperatures, melting the rubber in the compressor clutch. Here are my symptoms.

• High engine temperature, especially at idle. 100-105 plus degrees (prior to this summer, it never exceeded 95 at idle). It does go to the expected 85 at speed.

• After running the engine, I open the hood (with it running) and feel a blast of VERY hot air coming from the right side of the engine fan, between the block and the radiator. Coincidentally, it’s near the compressor. Of course, the engine puts out heat, but I don’t remember it being hot enough to nearly singe my eyebrows!

• Original (#14) head. Trap Oxidizer replaced at some point prior to my purchase in 2003.

• Hard upper radiator hose after overnight cool-down. Opening the overflow releases pressure and the hose becomes pliant. Textbook, I know, BUT …

• NO coolant loss noted in two years. The Coolant Idiot Light comes on sporadically and the level does go down at higher RPM. I checked the sensor for resistance – it checked OK.

• NO white smoke from acceleration.

• NO oil mixed with coolant visible. No foam on top that I can see.

• The electric fan works, at least at idle with elevated engine temps. The viscous fan clutch appears to be working correctly.

• The Behr radiator is of recent vintage since it has the reinforced neck.

Cutting to the chase. Do the mixed symptoms indicate a cracked head? If so, how long do I have to save $$ to replace it? Unfortunately, none of the threads I reviewed give a timeline, or the severity of symptoms. BTW, I am very clear on what must be done in the case of the head replacement and I will not cut corners, should that be required. I just need to know if my suspicions about the cracked head are correct, or if I should investigate elsewhere (i.e. a new radiator, etc).

Thank you for you help

__________________
Casey Cooper
"From a long line of Mercedes ownership"

'86 300SDL 250K miles (Gone, but not forgotten); best diesel I have ever driven, too bad about the Achilles heel.

'81 240D 370K miles (Sold to my brother after 9 years and 150K miles of reliable driving!)

[Five other Mercedes in family clipped for less length.]

Firstly, I strongly disagree with the "excess heat" from the engine causing damage to the compressor clutch. The engine could run at 110°C. all day long and the compressor would not be worse off for the heat. I'm saying the two failures of the compressor clutch are coincidence and have nothing to do with the temperature of the engine.

If no coolant is lost, and no oil appears in the coolant, you have insufficient evidence of a cracked head or a blown head gasket. I would not perform any additional work to remove the head at this point. I would, however, get an oil analysis performed at the next oil change and verify that no coolant is appearing in the oil.

With regard to the elevated engine temperatures, I would immediately change the thermostat. If this does not lower the temperature, the final culprit is the radiator. Although it might be of the newer design, you can't rule it out if it has not been maintained properly. See if you can borrow, or purchase, one of the infrared temperature sensors so that you can scan the radiator for cold spots. You might find that the radiator needs to go. But, due to the cost, I would not change it without confirmation.

No doubt you'll get some responses here, but as far as your A/C comp, seems I remember reading a post from Larry Bible about rebuilt compressors, and only remember something said that they were not very good, and didn't last.

You didn't mention if you have ever changed out your thermostat, and we know that even some new radiators have blockage according to GSXR. However, temps you indicate at idle, 100c, 105c, are not normal!

If you can only get temps down by driving the car, maybe the water pump is faulty as I have read where the internal pump vain can actually disintegrate via galvanic corrosion, or something similar. If you are not getting oil in your reservoir, and the hose is not hard after running from cold 5 minutes or so, that it remains squeezable before getting fully warmed up to temp, it sounds to me like only your cooling system such as a blocked radiator, a failed clutch fan, bad thermo sender for aux fan or bad aux fan as well as maybe a bad water pump or failed thermostat, one or two or maybe all of the cooling system has problems?

Brian nailed it. While changing the t stat, I'd flush the system and change the coolant.

Also take the fan clutch and shrould out. Then get a garden house and spray though the radiator backwards. This should clean out a bunch of crap and may help a bit.

__________________
2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I agree 100% with Brian's analysis. No way can Engine Temperature cause the rubber in the A/C clutch to fail. I would think this is more likely to be due to a problem with the compressor or the drive belt.

The Temperatures that you quote are not really abnormal assuming that the 105 Deg is the worst you see at idle just after a high speed run. The 85 Deg at speed is perfectly normal. Otherwise 95-100 is the max. you want to ever see.

It sounds to me that you have some Air trapped in the cooling system. That could give the "Hard Hose" symptom that you describe and will also cause pressure to remain in the cooling system. It also causes the level to drop significantly as engine rev's rise to the point at which your low coolant sensor can be activated. I had exactly the same thing happen with my SDL.

I would try bleeding air out of the system before you try anything else.

__________________
1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

I haven't changed out the thermostat, and that's on my list of things to do. Do the simple things first, I know. That will give me a chance to drain coolant out and find out if there is oil in it.

What still throws me off is the hard upper hose in the morning. Other than a cracked head, what would cause that? Again, I checked them the morning after running the car normally. I will also try to run the engine from cold for five minutes and check the hose this afternoon. [I just saw the previous post about air bubbles. How do I bleed the system?]

My concern is to ensure I'm going down the right path. No sense in replacing the cooling system if the head is cracked and vice-versa. I'm thankful it's looking like the vice-versa, though. I bought the car with the green stuff, and while I was able to purge most of it and put in the good stuff, there is no telling what other damage it may have done.

I certainly have no trouble with replacing cooling system components. I'm going to look into the info about the rebuilt compressors not being good. If I need another, I want to ensure I'm getting my money's worth.

__________________
Casey Cooper
"From a long line of Mercedes ownership"

'86 300SDL 250K miles (Gone, but not forgotten); best diesel I have ever driven, too bad about the Achilles heel.

'81 240D 370K miles (Sold to my brother after 9 years and 150K miles of reliable driving!)

[Five other Mercedes in family clipped for less length.]

The recommended oil analysis will be money well spent, to be really sure one way or another about the condition of your oil and possible presence of coolant.

Have you had a good look at your water pump? If it's weeping a small amount, this is a sign that it's on its last legs and shoudl be replaced. Presumably your ac guy would have caught such a thing, but maybe not... doesn't hurt to look in there carefully for yourself with a flashlight. It may be a very small, almost imperceptible amount of fluid coming out, and difficult to see or notice.

Just for the hell of it, I'd like to see you replace the cap on the expansion tank and see if the pressure in the upper hose goes away.

If the cap has pressure issues it can create havoc for the system.

One other item:

Have you had the cooling system opened up for any reason in the last few weeks? Is it possible that there is excess air in the cylinder head? On mine, when changing coolant, the temperature went right over 100°C. upon first start. Had to fill the head via the upper hose to get the temperature down. There is a lot of places to trap air in this system. If you did have the system open, the air in the heater core won't be released until the system is run with the temperature dial on full hot for 10 minutes or so.

Some water pumps use a small drip tube to carry any weeping away from the engine, other 603 engines I have seen do not have this tube possibly because it was left off when the pump was replaced. I don;t however think that alone is going to give the problems you described, it would eventually result in low coolant however (very bad) or cause the system pressure to be too low (also bad!) you have seen pressure at the upper rad hose so the system sounds like its "tight" as they say.
I ditto the suggestion for oil analysis. This is a very sensitive test and is highly recommended even if you don't have a overheating problem, its a good reference point for the future (esp if you have a #14 head!) - do a search for Blackstone Labs for a good place for analysis. I had an analysis at 5000 miles and found my oil was still good, but they saw silicon in my oil and thought my air filter was plugged but I found air was being pulled thru a warped rubber hose bypassing the filter.

Also, I would like to add that the fan clutch can be a big factor, someone here learned that the hard way (after replacing other component$)
I think the fan should spin no more than than something like 2 or 3 seconds after the engine is shut off. (someone please correct me if that figure is wrong). And hot the fan should offer some resistance, if it feels really "loose" on the shaft it might just have lost its oil. Someone used STP in their fan clutch in place of the special thermo-viscous oil and that alone made an otherwise hot running engine run cool.

I haven't checked my fan for how long it spins but my engine runs cool (everything except the fan clutch and the AC condensor has been replaced, including a shaved head after a head gasket failed).
I only see temps approaching 110 in my '87 300D if I am running long and hard up a steep hill on a hot day O/W it stays around 95 and occasionally hits 100 after a hard run in hot weather and the car comes to an abrupt stop. Just running down the road mine runs between something like 87 and 90 depending on the outside temp and AC on or off, that can make a difference.

The fact is that the OM603 is prone to running hot unless everything in the cooling systems is pretty much perfect. Checking the fan clutch is the easiest to do, but could be better done if we just had some scientific numbers for how long a good one will let the fan spin after shut off.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

You might try removing the radiator and cleaning it. Here is a picture of what came out of the radiator on my 93 explorer. All this stuff was in the radiator between the radiator and the condensor after I cleaned it out it ran so much cooler.

Attached Thumbnails

 

I would like to take a moment and express what my understanding is about what causes the radiator hoses to get hard and the proper way to check.

When we start are OM603's up after not having run in many hours, and engine is at ambient temperature, if we open the hood and squeeze the upper radiator hose, or if you like, the lower radiator hose, the hoses would normally be limp, or depressurized, cracked head, or not, depressurized!

If we start a cold 603 that is known to have no cracked head, cooling system in proper and good operating condition, and we get out of the car, open the hood and squeeze the upper radiator hose, it would be depressurized so that we could pinch the hose with our fingers together. After say 15 minutes of running, temperature now at 85c, or full operating temp, the upper radiator hose would be building pressure, or already pressurized, and this would be normal indicating a properly operating cooling system.

If we start a cold 603 that is theorized to have a cracked head, we squeeze the radiator hose and we begin to feel the hose pressurizing, all within a few seconds after start up............Why?

If there are cracks in the head, and we then start the engine, compression generated within the firing chamber escapes into the cracks that are in the head and on into the cooling system, thus pressurizing the radiator hoses very soon after start up. This depends only that the cracks are in the region of the cooling passages within the head. I believe it is possible for there to be cracks in a head and not pressurize the cooling system, for a while at least.

This is my understanding of an early, or premature cooling system pressurization caused by a cracked head. Is this it, or have I missed something?

First, thank all of you for the replies. My heart palpitations over getting the new head have subsided!

I was also thinking the fan clutch could be a culprit. I used a Behr rather than the more expensive Sachs. I just read the following thread, which describes some of what I've been seeing. I'm leaning toward the t-stat at this point, though.

Fan clutch issue 603????

I haven't followed the prescribed tests, but will tonight. One thing I'm not 100% about is whether I put the fan in the right direction. There are markings on one side, which I directed to the front. This means the "clipped" side of the fan faces the engine. It's how it was in before, but the FSM does not speak very loudly to whether this is correct.

Does anyone have a web site that has this knowledge about cracked heads, or an FAQ? If not, I volunteer to compile one. There is a lot of interest in this area and I would like to help.

__________________
Casey Cooper
"From a long line of Mercedes ownership"

'86 300SDL 250K miles (Gone, but not forgotten); best diesel I have ever driven, too bad about the Achilles heel.

'81 240D 370K miles (Sold to my brother after 9 years and 150K miles of reliable driving!)

[Five other Mercedes in family clipped for less length.]

There is a lot of info in the archives here including theories as to why they crack. I doubt you will find nearly as much info elsewhere!

Oh, forgot to mention, the aux fan, or fans, should pull air into the grill, not out of the grill.

This sounds like a perfectly reasonable scenario. However, it would only be applicable if the cracks in the head were sufficiently large so that the combustion pressure is immediately discharged to the cooling system in sufficient quantity to pressurize the system within five minutes or less. This would be a significant failure of the head and it would probably be drinking coolant in such a scenario.

However, the other scenario is where the engine builds pressure normally and does not pressurize the hose sooner than expected. But, while the engine is running, some of the combustion gases manage to squeeze through the cracks and pressurize the cooling system greater than normal during the operation of the vehicle. This might even cause the tank to let some of the pressure off and discharge onto the pavement. When shutdown, the system cools down and the pressure drops, but, it does not drop all the way to zero. Therefore, the next morning, you feel firmness in the upper hose and pressure remains in the system. The crack has now closed due to the colder temperatures and the coolant can't escape back throught the crack into the combustion chambers.

It is my belief that the second scenario will occur first.

If this is ignored for sufficient time, then the first scenario will eventually occur, in combination with the second scenario.