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Old 04-17-2000, 06:01 PM
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I need some advice. I bought my first Mercedes-Benz last fall. It is a 1984 300D with 204,000 miles on it presently. It was meticulously maintained at a reputable local shop by the little old lady that had it before me. Shortly after I started driving it, I noticed that it loped at idle. I have replaced the fuel injectors, the fuel filters, and the injector pump idle dampener screw. I had a professional replace the timing chain and tensioner, adjust the start of delivery on the injector pump, and adjust the valves. The problem persists.

Frusterated, I took it to another reputable local shop today hoping to get to the bottom of the problem. A compression test was performed which indicated that four of the five cylinders tested at over 400psi. Cylinder #3 only showed 200psi. The mechanic put oil in cyl. #3 and checked it again, which showed slight improvement at 250psi.

The mechanic that worked on my car believes that this is a valve train problem, probably a valve guide and recommends pulling the head.

Big repairs make me antzy, and I contacted an outfit that sells cylinder heads for MB diesels to check out worst case. I spoke to the owner who says that quite often the ring grooves on the piston collapse, making a mess of the piston, scoring the cylinder wall, and requiring a full engine transplant or rebuild.

Are there any clues that would give me an idea about how big of a problem this is? Do I need a valve job or a new motor? Thanks.

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Old 04-17-2000, 06:18 PM
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Join Date: Jan 2000
Location: New Bedford, MA USA
Posts: 1,583
Generally, putting some oil in the cylinder will boost the compression reading more that 50 psi, if, you have worn compression rings. However, in your case, your tech may be correct in that it may be a valve problem. I don't think that all the damage that was described to you would get you to 200-250 psi. To me that would be a cylinder that was flat, i.e. no compression. There are only a couple of ways to get a firm diagnosis. One is with a fiber optic scope fed down the intake runners to look at the valves and thru the glow plug openings to view the inside of the cylinder. Some shops have this kind of equipment, but I don't know who. The other is just simply to remove the head and eyeball the landscape. Good luck..

Jeff L
1987 300e
1989 300e
1987 BMW 325

[This message has been edited by jeffsr (edited 04-17-2000).]
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Old 04-17-2000, 09:31 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
You are safe in most cases with the valve job on this engine. It is likely that it did burn an exhaust valve on that cylinder if a valve adjustment wasn't done on time at one time or another. I'd do the valve job and hope for the best.

Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB

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