3.2 Air Flow Meter - clock spring

Lesworth · 03-09-2009, 01:06 PM

There have been many threads going back over the years (I have read them

) that detail running issues caused by a rich idle (bogging and stumbling).

I have stumbling when transitioning back to idle and cannot get the idle mixture below around 2.3% even with the AFM screw would all the way out.

I have been through all the suggested checks, injectors, fuel pressures, CHT etc etc. I tried a known good AFM and hey presto, the mixture dropped with only a couple turns. I tried a second hand AFM from a dismantlers yard but this was far worse than mine. Rather then try used AFM after used AFM (I know that the clock spring that provides tension against the barn door weakens over time) and rather than pay a years salary for a new AFM (actually you can only get re-manufactured ones now), I am temped to adjust the clock spring to see if this makes a difference given that its fairly ckear my AFM is shot anyway.

The question is, who out there has fixed this problem by adjustung the clock spring? I have read just as many posts suggesting that this is the way to go as posts that say do not so this under any circumstances. Comments on this welcome....

2.70Racer · 03-09-2009, 02:27 PM

Lesworth,
The clockwork spring doesn't wear out. The resistive track that feeds voltage to the ECU does. Rebuilt AFM's replace the resistive track. Remember the AFM can be over 20 years old.
You can continue to play with your existing AFM and maybe you'll get lucky.
I'm sorry to hear you are so poorly paid that a rebuilt AFM is your annual salary.
It's a Porsche and parts are expensive.

stlrj · 03-09-2009, 10:15 PM

Forget the spring.

If you need to lean it out quickly, move the pointer.

Cheers,

Joe
74 w/86 3.2 conversion

Lesworth · 01:47 AM

Quote:

Originally Posted by 2.70Racer

Lesworth,
The clockwork spring doesn't wear out. The resistive track that feeds voltage to the ECU does. Rebuilt AFM's replace the resistive track. Remember the AFM can be over 20 years old.
You can continue to play with your existing AFM and maybe you'll get lucky.
I'm sorry to hear you are so poorly paid that a rebuilt AFM is your annual salary.
It's a Porsche and parts are expensive.

5 years of owning my car means I'm well aware of the cost of parts thanks (I was trying to inject a little bit of British humour into my post - my bad).

To clarify, what I have heard form several sources (including my mech) is that the clock spring weakens (not wears) like any spring will do after 20 plus years, thus reducing the pressure applied against the barn door, effectively richening the mixture by under reporting the amount of air. I'm just trying to solicit opinion and experience from anyone that has successfully adjusted this out. Opinion seems to be divided over whether this is a good idea or not.

Rot 911 · 03-10-2009, 05:28 AM

All probably due to worn tracks on the potentiometer. This issue has been discussed here several times. A board member, FRWilk, has a link that shows you how to open up the AFM and move the wiper so you are not on the worn track. Easy to do (I did it on my '86) and doesn't cost you anything. His link is to the 944 AFM, but will work on the 911 AFM. Go to his website http://www.the944.com/ and click on the left hand side link "air flow meter."

ZOA NOM · ZOA NOM's Garage

If the spring weakens, the door will open further, and the wiper will report the corresponding airflow. There should be no change in the relative mixture. The wiper is fixed to the door's axis, not hanging off the end of the spring. The wiper resistor is most likely your problem. They get grooves in them over time, and the voltage that can be picked up by the wiper becomes erratic.

Lesworth · 03-10-2009, 07:49 AM

Thanks for the replies so far. I'm going to remove the cover and take a look this evening.

ZOA NOM, wouldn't the fact that the door is opening further (and correspondingly the wiper moving further) send a false reading to the ECU (based on the fact that the voltage rises as the wiper progresses across the substrate material)?

ZOA NOM · ZOA NOM's Garage

I believe the amount of air coming in would also rise correspondingly, but that's a good question. I suppose that would tend to lean the mixture.

Bart_dood · 03-10-2009, 08:40 AM

Quote:

Originally Posted by Lesworth

There have been many threads going back over the years (I have read them

) that detail running issues caused by a rich idle (bogging and stumbling).

I have stumbling when transitioning back to idle and cannot get the idle mixture below around 2.3% even with the AFM screw would all the way out.

I have been through all the suggested checks, injectors, fuel pressures, CHT etc etc. I tried a known good AFM and hey presto, the mixture dropped with only a couple turns. I tried a second hand AFM from a dismantlers yard but this was far worse than mine. Rather then try used AFM after used AFM (I know that the clock spring that provides tension against the barn door weakens over time) and rather than pay a years salary for a new AFM (actually you can only get re-manufactured ones now), I am temped to adjust the clock spring to see if this makes a difference given that its fairly ckear my AFM is shot anyway.

The question is, who out there has fixed this problem by adjustung the clock spring? I have read just as many posts suggesting that this is the way to go as posts that say do not so this under any circumstances. Comments on this welcome....

The AFM looks very simple but what it actually does is quite complex, be careful when you make any changes to it and always document the way it was so you can go back.
Basically the spring changes scalar and the screw position (on the arm) changes offset, it is most likely that like the others have stated your carbon track is worn out. If you are certain noone has opened your unit and tampered with it in the past then it seems unlikely that the spring or offset adjustment has gone that bad, but you never know.

I am working on a replacement for the AFM, a MAF system that simply plugs in to replace it (no chip required), considering you seem to be on a tight budget it might be out of your price range but it would certainly fix the problems you are experiencing.
I am hoping to have it ready for ordering in 3-4 weeks from now.

Good luck!

ZOA NOM · ZOA NOM's Garage

You're going to have a gold mine, Bart_dood. Put me on your list of interested '88 Carrera drivers. I'm local (Livermore), if I can be of any testing assistance, let me know. I happen to have two Carreras, one bone stock daily driver, and one Spec Racer with a Wong chip.

Lesworth · 03-10-2009, 01:37 PM

Ok, I removed the black cover (man those things really dont want to budge) and have posted some photos. I sent the AFM off to be 're-furbished' last year so imagine my surprise when I see two wear tracks on the substrate. What are you thoughts on the amount of wear, particularly at the start point (photo 1). The actual positioning of the wiper looks ok to me compared to the photo on Steve Wongs website so I have left the screw well alone. I took the opportunity to move the wiper up the shaft to reposition it on fresh substrate material (after I took these photos).

Bart dood, I already have your site bookmarked and have been watching with interest....

ZOA NOM · ZOA NOM's Garage

I once had an AFM refurbished, and it came back with the mixture set too rich. They said it they always did that to protect themselves from destroying someone's motor if it was set too lean. I had to have the mixture readjusted.