excessive oil consumption W140 300SD
 

A friend of mine just asked me if I knew what could cause excessive oil consumption on a 1993 300SD. By excessive me means a quart every 150 miles.

He says there are no leaks and he just had the valve seals replaced. There is also no excessive smoke. The engine has 200,000 miles on it, so it may just be at the end of its' life.

I told him the name of a Mercedes diesel expert to take it to, but I thought I would post the question here as well.

I know these cars, and these engines, don't have the best reputation. That's why I was thinking someone here might have been through this before.

He hoping he can get by without a rebuilt engine.

Any suggestions?

Then, you already know the result.

A bent connecting rod and an ovalized cylinder.

No, the block cannot be repaired and he needs a new or rebuilt short block. The problem with this approach is that the engine will cost more than the value of the vehicle.

Plenty of folks have been through it. Search "rod bender" and sit down for all the info that you could possibly want.

Wow. I knew these engines were not the best, but I would not have suspected something like this. What you say, though, makes sense.

This fellow is a Doctor in a rural area and travels about 80,000 miles a year so he was wanting something comfortable and thrifty. He also collects Mercedes (his current project is a 1970's 600) so he may just want to have the engine replaced.

I will do the research you recommend and get back with him.

Thanks so much for the info.

FWIW, a properly running 603 should not be dead at 200k. A member here owned one into the lower 400k area (414k I believe was last I can recall). He passed this spring and I do not know what happened with the vehicle since. The head had been off twice but bottom end was fine. Mine is running well at around 250k but the head needs to come off and I need a new headgasket as I have a ruptured oil gallery port.

A W140 would be one of the last vehicles I would consider thrifty for long term ownership.

The owner of Houston Biodiesel swapped in a new long block from Metric Motors into his W140 Diesel... I am going to say 18 months back or so. May want to give him a buzz and get his input. Name is Chris.

The problem is what to replace it with?

Although M/B supposedly fixed the issue with stronger connecting rods, there is some skepticism as to whether they fully solved the issue on this engine.

If he does replace it, be absolutely sure that the new engine has the updated rods...........although this is still not 100% assurance that the issue won't occur again in that engine.

One alternative, although I don't know if it can easily be accomplished in the W-140, is to use the 603.961 engine from the 1986-1987 SDL. This is an engine that has none of the issues that plague the 603.971 engine in the W-140. Furthermore, he can use the existing head on the 603.961.

If you choose to go this route, contact Sixto who can talk you through the potential pitfalls. If it were me...........this is the approach that I would take if at all possible. This approach works perfectly in the W-126 of 1990-1991. It should also be possible with the W-140.

Thanks, Brian. I will pass this on to him.

I know I looked into buying a W140 and a well respected Mercedes mechanic pointed out that while they are very nice everything on them cost twice as much as the 124 I was driving at the time.

Then he went into the wiring and A/C troubles....

My insurance costs would also have doubled.

The old 124 started looking better all the time!

While the car is still running he should trade it in on another one, thats what I would do.

Seems like a better option to me than dumping $5k into a $4k car.


Let the dealer have fun with it.

pull the om603 and drop in an om606.

Was it using oil before the valve seals were replaced?

Perhaps a compression and especially leak down tests are in order. You don't want to exchange engines without knowing for sure that you need one. You know the oil is missing. You need some indication of where it is going. Test, don't use the SWAG method of diagnosing.

Interestingly, you won't get any confirming results with such tests. The engine runs fine............it just leaks a lot of oil past the oil ring. The culprit cylinder might be slightly low, but it probably won't be the red flag that you seek.

The only sure way to diagnose it is to pull the head and search for a piston that won't quite make the deck height............and the difference is very subtle.

I lived this horror first hand. The unit develops a loping idle, and will puff oil smoke rings out the tailpipe. I am not kidding about the smoke rings.

If no smoke is visible the car is not suffering from a bent rod as that leads directly to serious oil consumption that exits the system via the tailpipe. A quart every 150 miles is about par for the "course" of rod bender dying. Check the coolant. That is the only other place the oil can go, but my money is on the fact that the car is really smoking like a California forest fire in the summer just East of LA....

They will fix the block if the bore is not so ovalized it cannot be repaired. The oil consumption is not an very good indication of how badly ovalized the cylinder is - once it is significantly out of round, all that oil passing by does a fine job of lubricating. Or, you can get a "factory rebuild" which in all likelihood is done by Metric Motors. Expect to pay upwards of $10K.

Jim

I agree with Brian

The engine might be suffering from a bent rod with minimal oil consumption. It takes a few thousand miles to gradually ovalize the cylinder.............usually with the owner completely unknowing............

At 150 miles per quart, the process has been underway for a while. I agree when it starts the oil consumption may be unnoticeable. But when you lose a quart every 150 miles, or 3 hours of operation, the smoke is not really subtle. And the idle speed on a rod bender at that point is a constantly varying, loping kind of idle. You will know what I mean when you hear it.

And, a bent rod that has no oil burning or other symptoms will go undetected as long as there are no symptoms. In this case the originator of the thread described a pretty severe symptom. That much oil in one cylinder is about 25% of the injected fuel load at 60 mph. It interferes with engine operation then, in the one cylinder, even if it is not visually apparent in the rearview mirror. And, it really interferes at idle where it can be more than the injected fuel load. Thus the loping idle and serious smoking at idle.

Jim