AFM spring tension vs output voltage
 

Recently, I cleaned the AFM and bent the little wiper as per various guides. Previously, the AFM tested fine on a scope (no drop outs) but I did see the tracks in the carbon and figured might as well bend the wiper. This service did seem to fix an odd cold-start hesitation issue and perhaps a *little* bit better response.

However, I have noticed a massive drop in fuel economy! That is, it's burning fuel more like it did before my headgasket-carbon cleaning-emissions tune-up work this past spring. After that work, I started getting 380-390 miles to the tank (fill at light turn on, about 60 litres fill).

Now, after servicing the AFM, I'm back to 300-320 miles range at the same fill (60 litres). This has been pretty consistent over a number of tanks now. I'm missing the extra miles!

The change happened immediately after the AFM work.

Could I have bent the wiper incorrectly? I opened it up again today and saw faint new lines in the carbon that correlate properly with the older ones above. But, I can't help but wonder if the wiper's start and end positions are off a bit. To keep it bent, I had to put sharp creases in the wiper and frankly there seems to me to be room for error that surprisingly no one talks about (?).

A recent DME refresh (re-solder of joints that solved a sudden stalling issue) hasn't changed the consumption.

Does anyone have the spec for spring tension vs. voltage output of the AFM? I'd like to confirm the wiper position and general accuracy of the unit

I'd focus on confirming that the car is still able to get to closed loop at idle and part throttle while driving at a steady rate. Bending the wiper is a last resort in my view because of the risk that you'll alter the output curve of the AFM. The O2 sensor should be able to account for minor changes and get you back to 14.7, but it can only adjust so much if the AFM is too far out of whack. If your mileage problem is a result of the AFM, it should show up as something richer than 14.7 while cruising down the highway. That said, you worked on a lot of things a while back, so while I agree the AFM is a likely culprit, I wouldn't rule out other causes.

If you have a good cat I would try find a bad one to hollow out. If the cat internally is good it is probably worth some money to sell to someone that needs a replacement.

Unbolt the tail pipe move it out of your way... Get a piece of round stock or long prybay and smash.....after the bar goes all they way through start it and blow all the pieces out ....I did mine about 15 years ago ....made a mess in the garage. Lol

Reading this reminded me of a something I had read a long time ago from FR Wilk.

I was able to find it, but not sure if it is much use to you at this point. I will post it here though in case it might help someone else looking for general knowledge on the topic.

http://e28-535i.com/technical/AFM%20Repair.pdf

I'm fairly certain that you can see in with an flashlight. No issues. Dig it out ...did it on all my cars... My racecar has straight thru pipe and you saw how good that ran at 944 fest this summer...

I would skip straight to installing a wideband AFR sensor/controller. That way you can adjust your particular AFM to suit your particular engine instead of relying on some arbitrary specs for a 30+ year old component.

Plus the wideband will always be useful going forward!

Not to mention, that would also help you tell if the AFM is even the real cause of the low MPG. Your tire pressure could be low, just say'n... Trying to map AFM voltages to spring pressures and angles seems like a deep dark rabbit hole to me, best avoided by more traditional diagnostics unless you really just can't get enough of that sort of thing (BTDT). At this point you're not even sure you're after a rabbit. :)

Ok ok.. I know you guys are right ;)

Started looking at WB02s. Um... seems you can buy separate sensors and gauges (at $200+ USD EACH) or kits which appear significantly cheaper (ex $250 USD for both gauge and sensor: https://www.aemelectronics.com/produ...uge#buy-direct )

I guess I'm asking what is a good yet relatively inexpensive setup? I'm looking for an analog gauge for a stock-like look. Odd: I can't seem to find a VDO stock-style afr gauge. Don't they make them?