Adjust AFM spring tension and bypass screw without guess-work.

[salvageland]

The method I used to calibrate spring tension is rudimentary, but it works quite nicely!

The original AFM on my '93 vert had been altered at some point in its life. This caused the engine to run super rich throughout the RPM band and also made for a poor off idle response. I purchased 2 sealed AFMs ('89 sedan '90 coupe) and took the covers off to measure reference values. Then, I calibrated the original AFM to those values and it now performs exactly as the 2 reference ones! I guess I now have 2 AFMs to get rid of or maybe keep as spares...haha!

Reference values were averaged between 2 previously unaltered AFMs but both units were close to each value by .01 volts.

-Start car and get it to operating temperature so that idle is where it should be. 750RPM
-With the cover off the AFM, place one lead of Multi-Meter on the reference point, and measure DC voltage on points 1 and 2 labeled below. (positive/negative isn't important here)

point 1 should be between 3.88-3.92 volts
point 2 should be between 1.12-1.08 volts

Turn spring tension wheel one or two notches, bleep throttle and let idle settle, recheck values.


To re-calibrate the bypass screw, there is a small number stamped next to the screw hole. My AFMs had 17.7mm and 17.4mm which were both between 2.5 to 3 screw-turns out. This is a measurement from the top of the adjustment hole down to the top edge of the adjustment screw.

Use a pair of calipers to measure the depth and adjust accordingly.

[Eskie]

I'm going to try this tonight on my AFM to see if it has been adjusted out of spec. Great information, thanks!

[salvageland]

I'm going to try this tonight on my AFM to see if it has been adjusted out of spec. Great information, thanks!

My pleasure! :)

Curious to see how it works out for you!

[Eskie]

My pleasure! :)



Curious to see how it works out for you!

I just got done adjusting to your specs. I actually had to turn the afm wheel clockwise which I believe is leaning the mixture out. I also had to turn the idle mixture screw out to get it to match the spec on the case (18.6). Interestingly enough it has a lot more power. The engine sound better too..odd.

I'm still having the off idle stumble though.

Definitely an improvement power wise.

Thank you!


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[salvageland]

Good to hear!

I'm starting to think that off idle stumble may be related to throttle cable stretch or possibly normal to the E30s? My vert has a tiny bit of it still.

[Eskie]

I've confirmed my is not for sure because I've been actuating the throttle by hand in the engine bay. Have you tried moving the lever by hand?


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[salvageland]

I've confirmed my is not for sure because I've been actuating the throttle by hand in the engine bay. Have you tried moving the lever by hand?

You're right, manual bleeping of throttle body produces same hesitation.

[15Michaeljoseph]

You're right, manual bleeping of throttle body produces same hesitation.

Yeah, +1, I believe it's normal. It's a biproduct of the fact that these cars HAVE an AFM. Just some lag.

I hear a MAF conversion gets rid of that hesitation, but for the $400-600 I think I can live with it :)

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[Kershaw]

It's a biproduct of the fact that these cars HAVE an AFM. Just some lag.

Yep. After I switched to megasquirt and deleted the AFM that stumble or slow response to throttle input disappeared.

[royemello]

88 528 SETA motronic 1.1- ( Rayz's 1990 325i DME) this is what i did.
ZERO VACUUM LEAKS ( u tube i used a cigar & hand pump - worked well & cheep) - ok stop laughing - but it works.
adjusted valves - new e 3 plugs- newer bosch o2 sensor - new rubber intake boot & other pvc vac lines - ck those elbows at throttle body. idle sw set.
i ckd contact 2 - was hi at 1.15v - 1 click lean on tension wheels clockwise - it dropped to avg 1.06v then ckd contact 2 - settled around 3.90.
so - i got contact 1 to be close then adjusted bypass screw 1/4 turn at a time - kept cking contacts 1 & 2. adjust wheel - adj screw ( fine tune)
if you adj too many clicks the ck eng lamp will come on ( reset it) ( 3 times key on / off)
this simple AFM reset worked well & simple - car now runs much better & i was only 1 wheel click ( tooth) off
THANK YOU SLAVAGELAND

[bimmerdriver49]

Hello salvageland, I'm reading your post from https://www.r3vlimited.com/board/for...out-guess-work about checking the voltages on the AFM. I think I'm doing exactly what you are describing but I'm not getting any voltage readings at all. The meter just reads Zero. I tried millivolts also in case that's what you meant. But, nothing there either. My meter works fine. I checked it against the battery and it reads a clear 12 volts. Any ideas what might be going on? Also, before I removed the top of the AFM, the car Was running "well". I've got a Schrick cam and was told I needed to adjust the AFM a notch or two to allow the door to open under less pressure since the cam, apparently, decreases the vacuum at low rpms, thereby causing sluggish response at low rpms. I haven't changed anything yet but, now, ever since I removed the top, the engine boggs down quickly if I blip it too hard, which it didn't do before. If I ease it up, I can get it to run up to high rpms. Just not quickly.

Thanks for any ideas you might have.

[bimmerdriver49]

Doh! I just forgot to plug the lead back into the AFM. Once I did that, I'm getting good readings and, indeed, it was way off at 3.175/1.20.