Mercedes-Benz Forum - View Single Post - M103 Rough idle and vacuum leak
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post #116 of (permalink) Old 01-11-2018, 02:53 PM
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H.D.'s Avatar
Date registered: May 2014
Vehicle: 1988 300CE
Location: Germany
Posts: 1,349
Mentioned: 35 Post(s)
Quoted: 511 Post(s)
Originally Posted by KJZ78701 View Post
... H.D., ... I wonder if you have spent much time on the bottom end.
From ‘83 to ‘84 I spent 8 hours a day on almost nothing but the M103‘s bottom & top end … LOL

Originally Posted by KJZ78701 View Post
... My M103 and the one I bought at the junkyard both had enough bottom end wear to cause my oil consumption, which can slowly foul the O2 sensor leading to problems x,y,z..... With your systematic approach, I wonder where you place the bottom end in your list of things to check. ...
Oil consumption can, of course, foul the o2 sensor … (see what I said in post 30).
According to my experiences with the M103 (after it came off the production line ) oil consumption is much more often caused by hardened valve stem seals than by bottom end wear. Since they can relatively easily be replaced (with the head installed) I suggest to replace the valve stem seals first … certainly if they have never been replaced.

As for oil loss, the M103 is equipped with something that here in Germany is often called “Sollsiffstelle“ (“predetermined drooling point“) … LOL … It‘s often wrongly diagnosed as “head gasket failure“, but it‘s only a failure of the much easier to fix upper front cover sealing.

Originally Posted by KJZ78701 View Post
... All, H.D. mentions checking the play of the AFM in an earlier post. The focus seemed to go to adjusting the height of the plate and its exact position in the aluminum body, but I didn't see enough stress placed on making sure the plunger in the middle of the FD was NOT depressed while the motor idles. ...
Did you read posts 39, 41, 45, 49, 52, 82, 84, 86 and 91 ?

Originally Posted by KJZ78701 View Post
... The rich idle could have been the result of the flapper spring fatiguing over time as well. ...
On the M103 the AFM does not have a flapper spring, it has a counterweight. … See what I said about the movability / resistance of the AFM up from post 39 !

In order to not confuse the OP, I think it‘s best to do one step at a time. A few problems on this car have been fixed during this thread. The remaining problem (at this point) is:

“a lack of power before the car heats up. The engine seems very bogged down, and if I fully depress the gas pedal this gets worse.“

It‘s evident (since post 108) that the CIS-ECU does not receive a proper signal that is necessary for acceleration enrichment. So, IMHO, that should definitely be taken care of first. … And if that does not completely fix this remaining problem, I haven‘t cancelled the EHA on my list of suspects yet. …

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