AFM adjustment - Mercedes-Benz Forum

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post #1 of 1 (permalink) Old 12-18-2008, 01:15 PM Thread Starter
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Date registered: Dec 2008
Vehicle: 1988 260E direct import from Germany
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AFM adjustment


I got to this site from looking for information on a 1988 260E. I got more questions than answers the more I read. This input is a COMPLETELY ANECDOTAL post, and I'm sure one that could stand a lot of correction from people that actually know what's going on with this engine. I freely admit..that's not me! This is an 'on loan' car from friends for my wife's use. I want to do what I can to help the car run as best it can..but that won't include hours of wrenching and/or hundreds of $$ in parts.

What I'm trying to resolve: Engine intermittently dies at idle. Check Engine light comes on sometimes. I've reset that by disconnecting the battery for a few minutes. It might stay off for hundreds of miles. It might not.

Brand new platinum spark plugs, new wires. Based on dash tach, idle is approx. 850 in park, maybe 650-700 in gear. That seems maybe a tad low to me..but not much.

I read of adjustments to this and that acronym..most of the time reading no explanation of what the acronym was. IE: 'Clean the ICV..' is something I read. My first question to that, is...'What's an ICV?'

Finding that to be an Idle Control Valve was pretty straightforward. It DID take awhile to find out even the ACRONYM of 'the thing' I wondered about..finally found it was an 'AFM' in Air Flow Meter. I'd read of an idle adjustment 'under the air cleaner lid'..but didn't know what I was adjusting: Throttle position? Air? Fuel?

This basically describes what has come about to this point:

First off...the adjustment is NOT under the air cleaner lid...meaning you don't HAVE to take the lid off to get to it. You can take the lid off if you want...but subsequent drive tests to find out what you have accomplished with any adjustment is problematic. There IS a plastic cap that plugs a hole in the air cleaner assy lid that, when removed, gets you access to an adjustment. I had heard it was a 5mm allen adjustment. I didn't get a 5mm wrench to accomplish diddle. It didn't seem to 'fit' into anything, much less adjust anything with any twist force applied. Looking into the little hole didn't tell me a whole's 4" or so down and not easy to see with any sensible determination of what's what. It looked to be considerably smaller than 5mm.

A 2.5mm wrench did fit into something...and that something appeared to be spring loaded! I could engage an 'adjustment' when I slightly pushed the wrench into whatever it is I was stuck in. BUT...push too far, and the plate that allows air into the system (throttle body?) moves. That movement doesn't seem to be connected to any additional fuel delivery, 'cuz just a bit of movement of the flap will kill the engine.

I had heard the adjustment would effect idle speed..but wasn't clear on exactly how. For the sake of 'illustration', consider a clock..and the wrench being positioned at 12:00. With the bit of insertion force needed to engage the adjustment (but not enough to disturb the air valve), I turned the adjustment CW to 3:00. The engine idle got rougher. At about 3:30, the engine died. It wouldn't restart! Back to 12:00, engine restarted fine.

Turning the adjustment in the opposite direction (CCW), I moved it to about 6:00 before the idle got rough..and about 7:30 or so...the engine quit.

Back to 12:00, restart the engine. Moving slowly between the two extremes noted above, there was an RPM change..but very little. By my ear, I'd guess it to be 25-50RPM. The dash tach did not significantly move. The problem was I could not always repeat RPM changes: If a 'one hour' CCW wrench movement raised RPM a bit, turning it BACK (CW) 'one hour' didn't always DROP the RPM that amount. Sometimes it wouldn't drop at all.

Ended up with adjusting whatever it is I was adjusting to CCW 9:00. I figure: The car died sometimes. Starting at 12:00, it was a lot closer to dying in the CW direction (3:30) than it was the CCW direction (7:30). I picked a position that was kind of in the middle...a CCW movement of 90º.

Results are yet to be written in stone. As often as the car did exhibit its dying behaviour, it will need to be run for a good week to know if anything has improved.

The main point of this post is the description of the adjustment..required depression to get TO the adjustment, the margins of adjustment (in this case anyway), and what happens when you push a bit too hard (engine dies..and may not restart until you 'normal' the adjustment).

Any correction, comment, or instruction will be gladly received. I've rebuilt an engine or two..wrench on all my own stuff and sometimes other people's stuff, but none of it ('73 340 Duster ('69 motor)? '86 700SC Honda? '00 KDX 200?) as esoteric and complicated as this mechanically injected German engineered rig.

(Never did fuss much with my wife's 282 inch V8 DOHC injected Toyota-mo!)



Oh...Don't forget to put that plug back in the lid!!!!

Last edited by canyncarvr; 12-18-2008 at 01:33 PM.
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