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  #1  
Old 07-06-2018, 10:33 PM
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1991 350SD advice

Hi, I'm looking at a 1991 350SD W126 with 230K miles in total and 150K miles on engine and transmission. According to the owner the original engine/tranny had been replaced under warranty.

I don't know much about these diesels but I've read about early 6 cyl. OM603 engines not being very reliable. I want a daily driver that will take 20-25K miles a year under regular maintenance.

Someone told me that Mercedes solved the problems so late 603.97x are supposed to be solid. Any input? What should I be looking for?

Thanks!

Last edited by Don Andres; 07-06-2018 at 10:47 PM.
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  #2  
Old 07-06-2018, 10:42 PM
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The 3.0L 603 had head issues, solved with a #17 or later head casting and is an extremely durable engine. The 3.5L 603's had quite a few problems with bent rods, ovalled cylinders, headgasket breaches dumping oil in the cylinders, etc. Supposedly later revisions solved the bent rods/ovalled cylinder issue, but it is still a relatively fragile engine compared to the earlier 3.0L version. Some say that if the engine has >100K miles on it, it's probably good to go.

With any 603, keep an eye on the temperature gauge like a hawk and make sure the cooling system is kept full. If you have a leak, fix it. They don't tolerate being overheated well. Kept maintained, they're pretty bulletproof.
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  #3  
Old 07-06-2018, 10:45 PM
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Quote:
Originally Posted by Diseasel300 View Post
The 3.0L 603 had head issues, solved with a #17 or later head casting and is an extremely durable engine. The 3.5L 603's had quite a few problems with bent rods, ovalled cylinders, headgasket breaches dumping oil in the cylinders, etc. Supposedly later revisions solved the bent rods/ovalled cylinder issue, but it is still a relatively fragile engine compared to the earlier 3.0L version. Some say that if the engine has >100K miles on it, it's probably good to go.

With any 603, keep an eye on the temperature gauge like a hawk and make sure the cooling system is kept full. If you have a leak, fix it. They don't tolerate being overheated well. Kept maintained, they're pretty bulletproof.
Thanks for the fast response! I have yet to see the car up close, it's 3 hours away. In the pictures I have it does look very clean and well maintained. What is a reasonable price range in your opinion?
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  #4  
Old 07-07-2018, 12:22 AM
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Well, it looks like I bought a car....

Spoke to the owner and after he told me all about the car I decided to go for it. He agreed to hold it for me so I will pick it up next Saturday. I'm excited about this car and will go thru it carefully to make sure it will be a reliable daily driver.
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  #5  
Old 07-07-2018, 12:40 AM
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If it's a solid car to start with and you keep it that way, it should be a reliable driver. The worst thing you can do is put off maintenance. It'll save you now, but cost you dearly later! if you don't have history of maintenance, start with fluids and run an eye over things to find anything else that may need attention.

My SDL is pretty rough cosmetically and had a rough start mechanically, but since putting it back on the road in 2016, I've been daily-driving it with remarkably few issues. I wouldn't trade it for anything!
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'83 500SL Euro - "The Money Pit" 116K
'86 300SDL - "The Diseasel" 186K
The Diseasel Thread - Everything You Didn't Know You Wanted To Know
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  #6  
Old 07-07-2018, 12:51 AM
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Quote:
Originally Posted by Diseasel300 View Post
If it's a solid car to start with and you keep it that way, it should be a reliable driver. The worst thing you can do is put off maintenance. It'll save you now, but cost you dearly later! if you don't have history of maintenance, start with fluids and run an eye over things to find anything else that may need attention.

My SDL is pretty rough cosmetically and had a rough start mechanically, but since putting it back on the road in 2016, I've been daily-driving it with remarkably few issues. I wouldn't trade it for anything!
That's good advice, thanks! Maintenance is cheap insurance against things you don't want to have happen to you. I'm almost religious about maintenance, annoys the f@#$ out of the wife when I catch her going past scheduled oil changes. :-)

The current owner is a former Mercedes technician from Germany. Lots of expensive maintenance items have been taken care of already. So I'll just change out fluids and filters to have a clean start.

I think I need to sell one of my other cars now. I have a feeling it will be my 2000 CLK430 convertible.
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  #7  
Old 07-08-2018, 09:09 AM
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I had a 91 350 SD. Loved the car but the engine was never completely right. It had serious oiley gunk build up issues in the intake and may have had a slight case of bent rod syndrome.

I would not buy one unless it was very clean and the price allowed me to replace the 3.5 engine with a sound 3.0 engine. There is at least one very extensive thread here regarding the "Rod Bender" (any 3.5 diesel).

I discussed this at length numerous times with my most trusted engine builder who did a valve job for me on it. We believe when they expanded the bore on the 3.0 engine to create the 3.5 engine there was not enough material left in the block to provide the structural soundness to support a good seal at the head gasket.

The leakage might then build up over time enough to deposit enough liquid into the combustion chamber to create a partial hydrolock, bending the rod just a bit. The bent rod then twists a bit too causing a wallowing action when the piston goes up and down eventually compromising the compression on that cylinder.

The most obvious sign of it is an uneven idle as the weak cylinder fires it is a weak firing or possibly a dead miss if its bad enough.

Mercedes never officially admitted anything and supposedly provided stronger rods on later engines but in my humble opinion they never solved the problem and eventually stopped offering that engine.

It is a truly lovely car to drive though and if you found a clean one and was up for a bit of a project it would be worth the effort to change it to a 3.0 engine when the 3.5 started giving too much trouble. The trick is to not get in so deep financially as to be buried.
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  #8  
Old 07-08-2018, 05:33 PM
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Originally Posted by t walgamuth View Post
Mercedes never officially admitted anything and supposedly provided stronger rods on later engines but in my humble opinion they never solved the problem and eventually stopped offering that engine.
Is the point that MB stopped offering the 3.5 because of rod problems? I think they stopped offering the 603.97 because the 606.96 came along. The 606.910 had been available for a couple of years.

Sixto
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02 C320 wagon
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  #9  
Old 07-08-2018, 05:42 PM
t walgamuth's Avatar
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Was the 606 a three and a half liter motor....ever?

I guess we can all draw our own conclustions but to me it is clear the 3.5 liter block was too flexible. They chose to make up the power in other ways.....more boost? More valves? I'm not sure what all but they did not continue with that block.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual. It still needs upholstery redone...I also have a 427 Cobra replica with an aluminum chassis.
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  #10  
Old 07-08-2018, 05:53 PM
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As much as I love and trust the older MB's, I am not sure I would ask any 27 year old vehicle to deliver me 24,000 miles annually year by year.

Good luck; I hope it works for you. My '81 300SD still runs like a top (original owner) but I only drive it a few thousand miles a year. And I love it. Hope you have the same experience.
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  #11  
Old 07-08-2018, 05:58 PM
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The 3.5 didn’t survive but the bore and stroke dimensions continued in the 2.9 liter 602 and 605. Maybe there was an unanticipated harmonic in 6-cyl configuration.

You think it was the block? I think the crank was flimsier. Maybe there was insufficient overlap between main and rod journals.

Sixto
98 E320s sedan and wagon
02 C320 wagon
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  #12  
Old 07-08-2018, 06:01 PM
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Quote:
Originally Posted by sixto View Post
The 3.5 didnít survive but the bore and stroke dimensions continued in the 2.9 liter 602 and 605. Maybe there was an unanticipated harmonic in 6-cyl configuration.

Sixto
98 E320s sedan and wagon
02 C320 wagon
Is the bore and stroke the same? Wall thickness of block? A six cylinder vs a five or four with the same bore spacing and block thickness would create more flexing than the others.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual. It still needs upholstery redone...I also have a 427 Cobra replica with an aluminum chassis.
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  #13  
Old 07-08-2018, 06:02 PM
t walgamuth's Avatar
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Originally Posted by tyl604 View Post
As much as I love and trust the older MB's, I am not sure I would ask any 27 year old vehicle to deliver me 24,000 miles annually year by year.

Good luck; I hope it works for you. My '81 300SD still runs like a top (original owner) but I only drive it a few thousand miles a year. And I love it. Hope you have the same experience.
Oh they'll do it as long as you keep ahead by fixing things when they break or wear out.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual. It still needs upholstery redone...I also have a 427 Cobra replica with an aluminum chassis.
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  #14  
Old 07-08-2018, 06:45 PM
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Originally Posted by t walgamuth View Post
Oh they'll do it as long as you keep ahead by fixing things when they break or wear out.
The current owner is a Mercedes tech and maintenance has been kept up well.
I'll pick it up on Saturday. I'll know more then.

I'm ok with spending money on maintenance and repairs to keep my cars going. I have 40 years of experience with used cars, never bought a new car in my life. Buying used cars I know going in they'll cost more in maintenance. On the other hand depreciation is negligible. Comes down to the least expensive option. Until I started buying Mercedes models 2006 and up. Perhaps I'm just the unluckiest guy but the newer generations are stuffed full of electronics, servos, air struts and other things that do not hold up very well and cause enormous repair bills. And for what?

There is no advantage that I can see to having a shifter lever talk to a computer that talks to another computer and a bunch of servos to put the transmission from P to D. That really isn't a very complicated thing. Am I missing something? The mechanical shifters have worked perfectly fine for decades. They don't have computers that make all 4 wheels of an R350 lock up when they fail (making towing a f@#$ing nightmare) and then cost $1,400 to replace on top of that. Of course, repairs only done at the dealership $160/hour because the replacement computer needs to get matched to the ECM. My usual import car go-to workshop cannot touch it.

There are plenty of other issues I've had with my newer Mercedes vehicles. So my strategy is to go back a decade or two and buy a car that has just the minimum (and beneficial) amount of electronics built into it. I'd rather spend my money on preventative maintenance and repairs than spending it on things that only make dealerships and towing companies happy.

I'm very much looking forward to having a solid Mercedes once again that I expect to cause less trouble if I do my share to keeping it going.

Sorry for the rant! Ask me how I really feel about new-ish cars.....
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  #15  
Old 07-08-2018, 07:59 PM
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Here are a few ways to tell if the engine was replaced with a factory crate engine. The engine number will be blank after 603 on the flat surface by the IP. It will have torx bolts for valve cover and other fasteners instead of head heads. It will have a part number plate on the passenger side bell housing. I have a 92 300SD that I picked up for $300 that has a factory crate motor and is the smoothest running diesel I have ever seen.
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