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#1
11-19-2004, 05:20 PM
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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 
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#3
11-19-2004, 07:57 PM
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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.
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#4
11-19-2004, 09:27 PM
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Quote:
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#5
11-19-2004, 10:34 PM
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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. Last edited by DslBnz; 02-08-2007 at 10:08 PM.
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#6
11-19-2004, 10:44 PM
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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... 
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#7
11-19-2004, 11:02 PM
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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
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