1991 350SDL Engine Problem Prevention
 

I am the owner of a 1991 350SDL with approximately 103,000 miles. I have heard that many have had problems with this engine, especially oil leaks from possible piston rod bending in #1 cylinder. Can anyone recommend any preventative measures that can be taken to avoid such problems? Does MB recommend replacement of the troubled rods?

Change the oil and filter every 3,000 miles and watch for
oil consumption. Basically, you should use no oil at
3,000 mile intervals.

If you start to use a little oil, check your air filter and
replace it if it has gone over mileage interval for
replacing. I have found that in my 93 300sd, I start
using oil when the air filter gets dirty.

Although not related to oil usage, etc., I would have the
timing chain checked if you haven't replaced it in 103k
miles.

Regardless of what it costs, doing the major maintenance
is a lot cheaper than rebuilding the engine! You cannot
just replace one connecting rod. The rods and pistons
are sold as sets.

-Ralph Burnette
1993 300SD, 182,000 miles

Check for crud in the crossover pipe between turbo and intake -- coked oil and soot from the EGR dropping off in large, superhard chunks may be the root cause of bent rods -- if yours haven't gone by now, they are good and most likely won't fail in normal service.

You must replace the entire set of six rods -- the old ones are several ounces lighter that the new ones!

If you have crud in the crossover pipe, I'd disconnect the EGR to prevent trouble.

Most of these engines seem to run forever -- but at the cost fix one, news travels fast!

Peter

If you do a search on EGR disable, you should come up with some stuff. The EGR valve recirculates exhaust through the intake, and according to some causes massive gunk buildup in the engines, hense the bent rods. I know that the member oldsouth has sucessfully disconnected the valve, and I'm sure others have as well. Do a search and maybe email oldsouth if you decide to do it. Good luck to you!

Steve17522,

I owned one that failed pretty suddenly at 162,000 miles. So, I would not consider you to be "out of the woods" by any means.

When mine failed I was expecting to have the head rebuilt, assuming the intake valve guides and seals were shot. I was not dismayed by the prospect, but not really impressed with the idea. I have rebuilt the heads on the 220D and my original 240D, but after a few more miles. And on this car I was especially careful to change oil frequently, change the fuel filter, air filter and so on, so I expected a few more miles (the 1982 240D has gone 280,000 miles, no head removal yet) before the inevitable repair. After all the head depends on some pretty severe conditions in and around the valves to work and the fact that they will wear out is inevitable.

I nearly turned inside out when I was informed the engine was shot, and I needed new pistons, rings, connecting rods, bearings, wrist pins, and there was some question at the beginning concerning the condition of the block and crank. I asked repeatedly if there was something I did, or did not do that contributed to the rod failure. I was told no, there was nothing I could have done to prevent the event every time I asked.

I have heard the theory of the gunk in the intake manifold getting into the combustion chamber and causing the failure. I question the actual validity of the theory as I have yet to see a Diesel with an EGR working that does not have an ugly build up of crud in the intake after 100,000 miles. And that would include the 240D, 300D and 300D Turbo's of the 1980's, as well as the 300SD's through 1987. Unless the 350SD/SDL has a unique intake configuration that would make this build up unusually plentiful, I cannot believe this is the only car that would have the problem. In addition, I find the likelihood a large chunk would get through the intake valve pretty unlikely. It would be more likely the chunk would hang up and cause other problems. When I rebuilt the 220D cylinder head the intake valve seals were leaking and had clogged up the area around the valve so badly the valves would not seat and the car would not start. This car had the best chance of ingesting a sizeable chunk of crud (the stuff was being beaten as is "grew" under the valve seat) but never bent a rod.

I am more convinced the rods were just not properly designed for manufacturing. The sporadic nature of the failures, with some happening in truly low mileage cars (under 80,000 miles), some in medium to higher mileage (under 180,000 miles) and some not at all, leads me to believe the issue centers on the normal production variations in forging, heat treating and finish machining. Some rods actually had an infinite fatigue life when they were delivered in an engine, while others did not, by some variable margin that was probably still pretty small. But, in a component like a connecting rod, you rapidly build up cycles and if you did not meet the criteria for infinite fatigue life with one, it will fail sooner or later, depending on how hard you drive and the miles driven. I drove mine pretty hard, typically running it on the highway at speed nearer 80 mph most of the time. High loads, high cycle accumulation, possible contributors to early failures. Until MB offers up the real reasons, we will never really know.

I sincerely doubt MB would recommend replacement of the rods as the official position from MB is there is no problem. I might do a compression check to assess the internal engine condition periodically though. If you find anything suspicous with yours, it has low enough mileage that you might get some consideration from MB if the block needs to be replaced.

I can tell you first hand that the fact that the failure rate seems low in the minds of those who have not experienced a failure offers little comfort when yours fails.

Good luck, and I hope some of this helps you, Jim

Also (and this is critical!)...watch the temperature gauge like a hawk! In addition to the EGR problems there were some engines that failed due to overheating. Something having to do with flaws in the cylinder head. Good luck...I think that with as many miles as you have you have passed the critical point.:o

Boy I sure do hope I have passed the critical point in my S-350. 178,000 miles and I do drive it hard. Uses about a quart of oil every 2000 miles. Only one thing, I think it has excessive blowby. Probably have a compression and leakdown test run soon, but the car runs great.

It is quite a shame that the only diesel engine available in the most desirable Mercedes models ever (later W126, W140) is a flawed engine. There's a perfect 1991 350SDL for sale on eBay by Herbert Philips (a forum member here). The only reason why I wouldn't even consider purchasing it is because of this engine problem.

For you techs out there, would it be possible to swap that engine for a 3.0L I-6 from the 300SDL?

Talk about an old thread! I now have 277,500 miles and no egged out cylinder yet. I did do a valve job when I had to replace the head gasket that was seeping water out and measured the cylinders at that time. They were perfect. I think the engine problems are overblown!

Yes I resurrected the thread after 20 months!

The engine problems may be overblown since we don't have exact statistics, but then when you consider that there is no other MB engine that has ANY such fatal problems associated with it, it would be a wise decision to play it safe and stay away from that engine.

'nuff said.

1991 350sdl
 

my advise for all 3.5 603 engine owners would be to remove the engine oil pan and secure the oil baffle plates so that the bolts or washers dont lock the oil pump and to rebuild the turbo charger. this done as a mininum would have prevented the one i rebuilt from destroying its self.
larry perkins
65 old cars