results of compression test, 1991 350 SD
 

I previously discussed my 1991 350SD last month. Had compression test done at dealership. Bad news is...
#1=15.51
#2=19.31
#3=19.17
#4=18.62
#5=18.27
#6=18.96
Dealer says Minimum spec is 18, bar difference must not be greater than 3 bar from Cyl. to Cyl.
Owned the car for 2.5 years, first two years oil consumption was 1 QT every 1200 miles. Over the last 6 months I watch the oil consumption go up to present 1 QT every 500 miles. Warranty records show in 1993 at 38K miles a rod broke and I believe short block was installed. I can't imagine that the one rod would have been repaired in 1993. Car has 149,000 miles now. You can imagine my disappointment. Rest of car is in great shape.
Regardless of rod issue, lower engine needs rebuild or replacement at some point. Of course dealer will drop new engine in. I will get them to give me a firm price. Is it foolish to consider another short block? Any advice is appreciated.
Jim

If you like the car and can afford it, keep it and enjoy it.

Be sure that they address the rods, like be sure that the updated rods that don't bend have been installed.

Best Regards,
Jim

I'm assuming you know about the design flaw in the 3.5 liter version of your engine, if not do a search and settle in for some reading.

If your car has a replacement shorblock engine installed in 93 I don't think you have the new version of the connecting rods that solved the issue with this engine.

A new shortblock, with the updated rods, should fix your problems forever. I'm not sure I'd be willing to drop 6-7K on a car that only worth maybe 7-8K but if you really like it and want to drive it long enough to recoup the investment a new engine would be a reasonable investment.

Tim's right, just depends on how much you really like your car. However, dropping $7k is cheaper than buying a New car, that's for sure and at least you know the car. If your willing to stick with a MB. A older 123 or 126 wouldn't be a bad choice depending on its condition. Even those models can be expensive. Good Luck :sun_smile

Really? Mine burns 1 Qt per 400 miles, crank seal leaks really bad, 30K miles of hard driving since I noticed a problem. Started 1 qt. at 1500 miles, now 1 qt. every 400 miles. Car runs best when fuel quality is perfect(PS Diesel Kleene seems like white-magic to this car). Getting 24 - 26 mpg.

Afraid of a compression test. Cylinder 1 is definitely the culprit. Other than the loud knocking and throbbing and oil smells with each tick-over, it runs nearly perfect otherwise. She starts cold pretty well, too.

Still, the miles keep on rolling by.

Yes, the engine in question is a longblock replacement with the upgrades.

Question: Was that compression test done on a HOT engine or a cold engine? If those are cold numbers, they are useless. But, chances are they are accurate for the infamous 3.5L, especially with high oil consumption.

The good news is you can get an updated long block from Mercedes with factory warranty for around $6k. Constantine put one in his 350SDL a few months ago, and sent me some photos. Ignore the top few pix of his injection pump, the photos of the crate motor are below that at this link:

http://www.meimann.com/images/mercedes/OM603.970_crate_engine/


The new long block will have the updated rods and head, and should last the usual 500kmi that you'd expect from a Mercedes diesel. If the rest of the car is in good shape and you like the car, fix it. Otherwise you won't get squat selling it as-is, and you'd have to go spend as much or more money on another used vehicle with who-knows-what might be wrong with it...

:o

There's also a problem with the early head design which leads to a failure in the head gasket between the #1 cylinder and an oil journal between the #1 cylinder and the timing chain area. Could very well be that all you need is a new head gasket. The real fix is a new head but a new gasket will hold for a while.

This pic from the DaveM/gsxr collection shows the change in oil passage design -

http://www.meimann.com/images/mercedes/head_gasket/heads-both-front.jpg

Sixto
95 S420
87 300SDL

The 350 engine, in all shapes and forms, had a durable cylinder head. Not the crack prone heads that plagued the earlier models.

'87 was the last year of the "bad head".

Only quoting from the book of Dave :D -

"Mercedes redesigned the head, AND head gasket, a total of five times - that's six different part numbers each. The original head was #14. There was a #15 but it's rare, I've heard of *one* in the USA. Next up is the #17, used on USA engines from 1990-95. After that there's a #20 head, followed by a #22. The revision between 20 and 22 is modification of the oil passage in front of the #1 cylinder, between it and the timing chain cavity, to help eliminate gasket failure in that area."

http://www.peachparts.com/shopforum/showthread.php?t=103444

AFAIK, the -22- head was not standard issue on any W126.

Sixto
95 S420
87 300SDL

This advice is courtesy of Dave:

The most inexpensive solution is to obtain a 3.0 litre 603.96 engine from a wrecked '86-'87 SDL (or '87 300D) and install this engine in the '91.

We recently have seen this engine for sale on e-bay for $1700.

I believe you could easily accomplish this change for a total of not more than $3K. This is far more cost effective than $7K. The 603.96 has a bit less power and torque than the 603.97, however, it is a fine engine, especially if it has a newer cylinder head.

JT,

Can you do an engine R&R yourself? That would save you a lot of labor cost.

You might get a rebuilt engine from a quality rebuilder for less than from MB.

P E H

Thank you all for responding. Great information for me to consider. Unfortunately I can't do the rebuild so its sounds like I'm looking at 8-9K for new engine. Not sure if engine compression was done with hot or cold engine, I just trusted the dealership to do it right. I plan to get estimate from dealer and talk with indy shop a friend recommended.
How would I know if an engine from mercedes has all the updates, particularily if they don't admit to the rod problem? With my luck I would get the last "new" engine with bad rods.
JT

Any engine you get from Mercedes today will have all the updates. The last 'bad' engines stopped coming from the factory in the mid 90's. The only other option for a rebuild is Metric Motors, but you may find the Mercedes long block to be very price competitive and have a better warranty. Get a price from the dealer on an engine, if you have them do the work, insist on wholesale cost (they're making a killing on labor anyway). Otherwise you can get the engine at wholesale (about $5800 for the long block) from Rusty or Caliber, or another dealer that actually wants your business. ;)


Factory dealer engine pricing (PDF file) :
http://www.meimann.com/docs/mercedes/OM603_rebuilt_engines.pdf
(price is still the same on spring 2004 price list...)


Metric Motors website:
http://www.mercedesengines.net/


:sun_smile

Also, if you don't belong. Join Mercedes Benz Club of America. Dealers usually give 10% discount on labor. check with them for this discount. And shop dealers. I check the three dealers here in the DFW area when buying my 99 E300. No dealing. I went to Temple Texas and the dealer there negotiated.

Consider a leakdown test. By pumping air into the cylinder you can determine where the leak is coming from. Not an exact science but as the air is pumped into the cylinder with the piston at top dead center. You listen for leaks coming form either the exhaust area, exhaust valve problem, the intake, intake valve problem, or the valve cover oil fill port, the rings are bad allowing the air to flow past the rings and up to the valve cover area to escape. In your case you could determine if it is the head gasket by removing the valve cover and hearing the air coming from the timing chain area.

Dave

Theres a guy in Fla. that specializes in rebuilding MB engines www. Noels.com
Many dealers use him.

buy a good used 3 liter and have an Indy put it in. should be half of what a new engine is. Unless the car is exceptional there is no way its worth it. RT

A used 3.0L is a definite option to consider, a decent one should cost $2k-$3k. The drawbacks are less torque, probably comes with the "bad" head, and lower resale value. If you could get one with a good head and don't care about resale, this would save a few thousand dollars. (A future head replacement will be $2-$4k, which could offset any savings... d'oh.)

:eek:

Blow head gasket at the front of #1 cylinder WILL BEND THAT ROD from sucking oil in from the chain case and pressure passage! Eventually it will get so much oil in there it will hydrolock. Instant bent rod.

Low compression on #1 with oil consumption and "puffing" blowby, I'd pull the head and check #1 piston height and gasket -- if the piston is all the way up, the rod is OK, and the gasket (or head) just needs to be replaced. New head is solid up there with single hole instead of a slot.

Oil consumption on this engine should be negligable -- my 603 (87 300D) uses less than a quart between 6000 mile oil changes. Look for a leaking oil filler cap gasket!

Peter

All of you are very helpful. Thank you. I am really going to have to think this through. The car is in great shape, it would be an easier decision it other things were causing problems. Except for new tires 1.5 years ago, I have just kept good fuel in her and the oil/filter and air filter changed. No repair expenses over my 2.5 year, 43,000 mile romance. She still starts right up and runs well, although in last two week the "chang-chang" sound seems louder.
Also there are no oil leaks. Since I noticed the oil consumption last summer I bet I have been under the car every other weekend looking for leaks. Would a bad head gasket start to weep oil to the outside of the engine?
JT

A bad head gasket *can* leak oil externally but it doesn't always. The blowout into the timing chain cavity is not uncommon, but is often accompanied by massive smoke while driving and poor power. The equally pooc compression on all cylinders leads me to believe either the test was done cold (shame on the techs), or you've got a bunch of ovaled cylinder walls and a short block that's toast. A good 603 uses very little oil, mine is also about 6000 miles per quart. I wish I could say that your problem was a bad head gasket but I very seriously doubt it...

:(

Can you replace it with one of equal quality for the same or lower cost? If not, IMHO, put the right engine into it and drive it for ten or more years.