Spark Plugs: 3 Clean - 3 Jet Black in M129 - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 11-12-2006, 08:44 AM Thread Starter
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Spark Plugs: 3 Clean - 3 Jet Black in M129

Ok. M129 in my 67 W111. Plugs 1,2 &3 clean as a whistle. Plugs 3,4 &5 black as coal. All are bone dry. With brand new NGK’s, the 3 rear plugs foul immediately. A respected vintage MB mechanic suggests leaking intake manifold, having only been appraised of the issue by me over the phone. Car also suffers a distinct lack of power. Haven’t done a compression test yet.

Picture of the plugs here: http://advanced-events.com/Plugs/plugs.jpg
http://advanced-events.com/Plugs/plugs2.jpg

Any suggestions?

Thanks in advance.

Last edited by Shab; 11-12-2006 at 09:09 AM.
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post #2 of 6 (permalink) Old 11-14-2006, 07:46 PM Thread Starter
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Any takers on this one?
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post #3 of 6 (permalink) Old 11-15-2006, 08:02 AM
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Hi Shab, I have the same eng. and wwould like to suggest checking the injection pump pressure valves (the outlet ports).these can get dirty and cause missing and stumble. Just at start up or otherwise. Changing there positions and re-checking may determine the culprits. Good Luck
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post #4 of 6 (permalink) Old 12-08-2006, 09:58 PM
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You can rule out intake air leaks by spraying carb cleaner on the area where the intake manifold bolts to the head, with the engine running. Spray one area at a time and wait to see if the engine rpm changes. If it does, you've just located the leak.

I would sure do a compression test though....and probably follow that up with a leak down test. Depending on the results I got, I'd then adjust the valves. Following that with another compression and leak down test.
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post #5 of 6 (permalink) Old 12-10-2006, 06:34 AM Thread Starter
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Thanks for the tip on the intake leak. I'll give it a try!
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post #6 of 6 (permalink) Old 12-10-2006, 11:27 AM Thread Starter
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Ok, did a compression test this morning. 1967 M129 250SE W111. Do these results look encouraging?
1- 150
2- 145
3- 155
4- 150
5- 145 +
6- 145 +

Plugs 1,2 & 3 again clean as a whistle, 4,5 & 6 dry fouled simply black as coal.

I am next going to check the oil level in the MFI pump. I can see that the line feeding #5 cyl is leaking fuel at the pump. I thinking valve adjustment as well?

Also FYI the engine does not smoke at all, no blue, no white, nothing. Revs eagerly standing still, can barley get out of it's own way when put in gear. PO said the car sat for about a year before he sold it. Got moved around the yard on it's own power as often as weekly but that's about it. No real driving.

FYI new cap, rotor, OEM plug wires, NGK plugs, condenser. Voltage is right all around the engine bay, did CSV test as well with good result.

Did the carb cleaner spray routine around the intake manifold and detected nothing, no change in idle.
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