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Old 04-20-2000, 06:52 PM
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Has anyone heard of this problem on MB Diesels? I have a 1984 300D with weak compression (200psi) on the #3 cylinder. The other four test above 400psi. The car lopes at idle. It runs smoothly at speed. I hear a ticking noise, like a tappet, at low rpms. Burns no oil, and gets 25-26 MPG.
The mechanic who did the compression test rechecked the #3 cylinder after adding oil. It came up slightly to 250psi.

The mechanic who did the compression test suspects that there is something the matter with the valve train, probably a loose guide.

I spoke to another fellow who informed me of a "common" problem where the piston ring grooves collapse, making a mess of the piston, scoring the cylinder wall and requiring a new motor. I have not yet talked to anyone else who thinks that this is a common or likely problem.

Has anyone had any experience with a motor suffering from piston ring groove collapse? How did such a problem present itself? Are there symptoms that I should look for to tell whether this is the problem, or a loose valve guide? Any advice, comments, etc are greatly appreciated.

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Old 04-20-2000, 07:14 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I vote with the valve problem, maybe even a tight one.

The compression check with oil would have been significantly higher with a ring problem. There are as many as three compression rings on most MB diesel pistons. Broken ring lands are usually from using ether (starting fluid) to start them.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 04-22-2000, 09:40 PM
Posts: n/a

I did have a ring land failure on a 240D, which has the same basic pistons, rods, etc, as the 300D, the 300D just one additional set.

The engine was taken apart because it was tired and the rear main seal was no longer touching the crankshaft anywhere. The engine had 280K miles, but I bought it at 203K, so it very well could have been started with ether like Steve mentioned.

I wouldn't think that broken ring lands are a common problem with this engine family. If someone said that, they may have seen ONE fail and made a determination that it was a weak point. It's impossible to make such determinations without tons of data. People like Steve and Benzmac have probably seen enough of these engines taken apart to have enough data to say whether or not this is a common weakness with these engines. There are several people who frequent this site who have seen a number of these engines taken apart. If you have no other response by now, I think it's safe to say it's not a common occurrence at all.

Good luck,

Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

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