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Old 03-22-2008, 04:23 PM
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Rod Bender True Stories - How it Broke and Why

Recently, an old friend had a OM603.971 engine failure. I am trying to understand it so I can help him decide what to do. I have read everything I can find on the Diesel Forum. If you, like Jack, have also experienced a 3.5 liter OM603 failure and repair or know about them, please tell us how it happens.

Jack's car is actually a 1992 300SD with 82K miles. It is from Oregon but Jack moved it to California. It as an excellent all original garaged car. Suddenly it started smoking huge amounts of blue smoke and clattering. He took it to Mercedes-Benz dealer. They compression checked each cylinder via the combustion chambers - all good. They suspected a turbo oil leak. They took off the turbo and saw wet oil in the exhaust side and ordered a remanufactured turbo from Germany. When they installed it - no change. Their conclusion is that the car has a defect inside the engine, so it needs a remanufactured engine for $13,000. Jack is has plenty of money which he has not accumulated by letting the MB dealer's service department have their way with him every time he comes to the shop.

I am not a mechanic, but if one wanted to see if the engine was drinking oil from the turbo, would you not simply remove the air pipe between the intake manifold and the compressor and let the turbo run while the intake aspirates normally ? If oil is only leaking out the drive side, the cold engine wont make blue smoke. I have not seen any threads on exploding turbos, so I dont think it would over rev.

When I asked the service advisor(who is paid by commision rather than salary) if they ever removed heads from any engines at their shop, he said no. I suppose that labor charges being equal that the profit margin on an engine is probably a bit more than on a head gasket.

What actually happens with these engines?
Is the bending always caused by hydraulic lock from fluid in the top of the cyclinder?
Which is the offending fluid - fuel, oil, water?
Where does it come from?

There is actually what appears to be a known defect in the head through 3-31-94 shown in the offical MB repair manual, I will try to attach the pdf article. It shows a "known to happen" oil leak into the #1 cyclinder. In fact, in the repair manual it says if the engine is smoking blue and clattering, that is the likely cause.

Has anyone actually ever heard of a 3.5 liter OM603 with only a bad head gasket?

With only a bent rod, can the engine get enough oil up from the bottom to burn blue smoke?

Does this sound like a "typical" 3.5 liter OM603 failure?

I will appreciate your input and I will also try to get someone to open the engine to see what we can see and get back with the results.

Thanks

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Last edited by jmfitzger; 03-22-2008 at 04:29 PM.
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Old 03-22-2008, 05:29 PM
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The cause is not hydraulic lock.There is no known reason why it happens.
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Old 03-22-2008, 05:45 PM
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I am glad to hear that. I think that this means that there is a good chance that Jack's engine actually does have only a leak at or above the head gasket. Not the dreaded bent rod(s) ---yet.
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Old 03-22-2008, 06:14 PM
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The only way to tell is to get the head off. You will see if the head has a defect or if the head gasket is blown, or, more likely, one of the rods is bent. Basically these are wonderful cars with crummy engines. If the rod is bent the afflicted cylinder goes oval and then your oil consumption increases drastically and your idle becomes kind of loping - uneven speed, with a regular increase and decrease in rpm. Taking the head off answers all the questions. Jim
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Old 03-22-2008, 06:43 PM
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Having NO experience with this engine my thinking is that if this came on suddenly you probably have a better than 50/50 chance that is a cylinder head or gasket problem. The reason i say this is having read a number of posts about the "rod bender" my understanding is that the bent rod causes the cylinder to wear to an oval shape and ultimately the worn cylinder and piston rings allow lube oil to pass to the combustion chamber and cause smoking.Logically this would be a gradual process with first a little smoke then increasing to eventually alot of smoke. I would think the bad cylinder would show up on a compression test,but then so would a bad gasket! Boils down to what has already been said ,get the head off and see whats going on. Don
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Old 03-22-2008, 07:14 PM
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If you back the injector fitting off #1 and the smoking reduces it is likely the head gasket. As mentioned the onset of the bent rod problem is gradual in all the cases I have read as well.
The only exception would be if the cylinder was doctored up for resale. I just am not sure if this is possible. Car from dealer or original owner?
If a person paid the 13k for a new 3.5 engine it could suffer the bent rod symptom at some point as well. So if ever this engine were to be replaced a 3 litre block that does not exhibit the problem is a better replacement in my opinion. In most cases you only need the short block.
Expensive car new to be plagued with this type of problem and never really resolved by mercedes. They also should have sent the turbo out for a seal and bearing change rather than stiff your friend for a totally new unit. Oh yes it would not hurt to quickly eliminate the replacement turbo from any possibility it is bad as well. Reading the archives covers it well.
If head is removed the deck height of all the pistons must be critically checked. This is the best way to find any bending in the rods presence.
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Old 03-22-2008, 07:26 PM
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I don't know how a bad head gasket would cause oil burning.
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Old 03-22-2008, 07:32 PM
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Quote:
Originally Posted by ForcedInduction View Post
The cause is not hydraulic lock.There is no known reason why it happens.
I suspect there are a few engineers in Germany that have a good idea why it happens.
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Old 03-22-2008, 08:46 PM
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I own one of those rod benders with a "newer" MB engine,installed under recall about 3 years ago...before I laid eyes on it,my point in this is the following:there is guy close to my workplace who runs a tow truck business/dealer,something like that,who has in his back yard a complete 93'300sd with a new/newer factory short block,last time I looked at it(tried to buy it for parts),still had the label from MB on top of the engine...the story behind the car was that it was sold about 2 years ago,buyer paid,never picked it up,it has been sitting ever since,started once in a while.Looks good,complete,most likely a parts car by now,after sitting so long...however I belive it will easy roll on a transport truck .If your friend is interested those guys will most likely sell the whole thing to him,or just the engine,they have a shop and diesel mechanics there for the tow trucks...send me mail if your buddy is in the market,insted of selling his car for parts,because,you cannot find those engines anywhere used,in good shape.
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Old 03-22-2008, 09:01 PM
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I don't know how a bad head gasket would cause oil burning.
On the front of the engine there is a passage. Obviously a pressure oil head feed. Apparently the head gasket can fail in such a way as to allow oil from that area into the combustion chamber of #1.
Since his problem developed almost instantainiously rather than gradual over many miles it is a possibility. Thats the content of the mercedes tech sheet he is mentioning I believe.
What also suprised me is the continious ongoing efforts to refine that head were still going on at that late a date. Or perhaps since the engine was discontinued even is a possibility.
Another thought I have been entertaining reciently is this engine type may actually benifit from the constant use of vegatable oil. Since the flame front or pressure applied on each stroke would be spread over a longer degree frame the rod loading at any instantainious point would be much less. Maybe the block could stay together that way for many hundreds of thousand miles May in fact be the only way to keep these engines together till normally worn out otherwise.
As someone above mentioned. It is reasonable to expect mercedes engineers would know enough to pass an opinion on this. For probable liability issues it is doubtful if we will ever be told though. Just common sense in another way though in my opinion.
He could also pull #s 1&2 injectors and see if number one is a lot dirtier than #2. From their description though the head has to come off. I would still do as much diagnosis with the engine together first as possible. Especially a leak down test.
One more last point. If it was an ebay transaction there is a small chance if recent they can backclaim on this. I would not hold my breath though.
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Old 03-22-2008, 09:20 PM
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main reason for this happening is the head cracks and let oil from the front oil gallery enter the combustion chamber. A few things you can try. getting the head fixxed (metal added and then milled to surface level. This can only be done on alluminum..) And then have the head retempered.
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Old 03-22-2008, 09:31 PM
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FYI: WIS service operation numbers

AR03.10-P-6111AW repairing connecting rod.

AR01.30-P-5800HX Remove/install cylinder head.

AR01.40-P-9291AW Boring and honing cylinder bores.

AR01.40-P-9273AW Installing cylinder liner.

AR01.40-P-9202AW Measure cylinder bores.

AR01.40-P-9272AW Remove cylinder liner

AR01.40-P-9271AW Widening cylinder bore in crankcase.

RA05.00-6-0103-41X Removal and installation of the guiding rails.
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Old 03-22-2008, 09:59 PM
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Quote:
Originally Posted by oldiesel View Post
Having NO experience with this engine my thinking is that if this came on suddenly you probably have a better than 50/50 chance that is a cylinder head or gasket problem. The reason i say this is having read a number of posts about the "rod bender" my understanding is that the bent rod causes the cylinder to wear to an oval shape and ultimately the worn cylinder and piston rings allow lube oil to pass to the combustion chamber and cause smoking.Logically this would be a gradual process with first a little smoke then increasing to eventually alot of smoke. I would think the bad cylinder would show up on a compression test,but then so would a bad gasket! Boils down to what has already been said ,get the head off and see whats going on. Don
I agree. The typical rod bearing failure is very gradual and the oil consumption gradually increases with no sign of oil smoke in the exhaust until it's consuming one quart every 250 miles.

The current scenario is a sudden onset and severe oil smoke which points to a head gasket issue.
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Old 03-22-2008, 10:19 PM
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I really appreciate the input from all of you very much. It is unfortunate that Mercedes-Benz has taken the dishonorable decision not to admit to an obvious design and/or fabrication error. Designing the connecting rod would be a challenge because you want it to be light and tough and it is fatigue loaded. But to pretend there is no problem seems wrong.

Now one thing I forgot to mention is that Jack's wife is rather hefty and he takes her everywhere. She weighs about the difference between a W126 SDL and W140 SD. Some people think the extra weight is a factor. I hate to be the one to suggest it but...........could Jack's wife be the rod bender here?

I am a hair's breadth from buying the car myself just to see what is up with it? But please, if you have any more insight, I will aprpeciate the benefit of your experience. I promise I will report back when Jack's car is properly sorted out.
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Old 03-22-2008, 10:58 PM
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His wifes additional weight percentagewise overall is of little consequence in my opinion. What may have aggrivated this problem could be pushing the car harder than the last owner did or just luck of the draw.
Good old coincidence in action. The 3.5 can really accelerate if you come onto it. Something to see in a good example. Just get it moving a little first. The head cracking with car not showing any signs of overheating is a possibility. If no overheating observed the head gasket gains in favour. These are just my opinions though. Others will and should vary. Please post whatever it is you find out. Again most the 3.5s give a clear signal the rod is bending or has bent over time.

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