1988 924S AFM testing question
 

Hi all,

I've been having an intermittent problem with my 924S. Most of the time it runs very well, but occasionally, usually about 5 minutes into a drive it suddenly starts to run very poorly and vibrate. The problem is present at every throttle position from idle to full throttle and acceleration is very poor. Usually after another few minutes it just "snaps" back into running well.

I am only just beginning to diagnose it so there are many things which I have not yet tested/replaced (i.e. basic ignition tune-up stuff still being shipped) but I pulled the AFM and am a bit confused by my results. I am testing according to the Clark's Garage procedure for testing AFM with a 9V battery.

With the flap fully closed, I am reading a voltage of 7.9 V, and as I open it it smoothly increases to 8.3 V. I have cleaned it and adjusted the wiper arm and I get the same results. According to Clark's flap closed is supposed to be 0.5 V increasing to somewhere just below 9 V.

Does this mean my car always thinks the flap is open? This doesn't seem like something that would cause intermittent rough running, but should I replace the AFM?

Thanks for the help!

Jovan

I never tried clarks test procedure, but you can also test it on the car, which I think is easier. Plug it in with the black cover removed and turn the key on. Between pin 9 and the engine block should be 5v and between pin 7 and the engine block should vary smoothly with flap angle. This way, you also test the DME voltage feed wire.

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk

That sounds like a fuel pressure problem to me.

or failing plug wires, bad spark plug or electrical problem. i.e. injector or firing

I totally agree with you guys, it seemed more like the AFM is not the culprit.

That being said, after I did the Clark's Garage repair and reinstalled it I haven't had the problem again, a couple of times it seemed to be running rougher for a couple of seconds but didn't continue. I have now also replaced plugs, plug wires, distributor and rotor so maybe that's helping too.

Thanks for the help!