Some years ago I had a whole screwed up year chasing engine management problems. My engine would sometimes start missing very badly at high rpm. I tried all sorts of things to attack the problem. I screwed around testing voltages and grounds all over my car. I pulled apart, abraded, and refastened every ground on the car with conductive grease. I did the same for all the important 12V connections. I tested the engine harness over and over again. I cut out the fuel injector connector and hard wired each individual wire. I put in new injectors. I put double grounds on engine and DME. I wired in a bypass to the car’s master relay so once the car was running it had a 2nd route to power and a flakey relay wouldn’t affect it. I put a fuel pressure gauge on my dash and connected it to the data logger. I put an AFR meter on the dash and connected it to the data logger. I painted my naked body blue and danced about the fire with Brian Jones’s GF. Nothing worked. Altho I did get to know Clarissa pretty well.
The problem turned out to be that my AFM spring had been dicked with. The whole damn screwed up year was caused by a common wisdom tweak…loosen the AFM spring for more power.
That got me looking around for how one might get a known cherry AFM. The problem is that they were carefully calibrated by Bosche using some secret process that measured the amount of air moving the AFM door to different positions, and then laser etched the little circuit board in order to add the right amount of resistance for a “bridge” circuit that delivers a voltage ratio relative to a 5V source.
The Bentley has a test where you measure resistance on the AFM pins but the Bentley is wrong. There’s a guy name Wilkes that figured the AFM out for the 944 community and he’s got a great website if you want to know more. My point is that it’s hard to confirm that your AFM is ok because you can’t just put a meter on it and measure resistance. Pretty much all you can do is watch your AFR’s. If your AFR is ok, then your AFM is probably ok. But most of us don’t monitor our AFMs.
There’s a number of outfits that sell refurbished AFMs. The problem tho is that there’s little confidence that any recalibration is occurring. That makes a aftermarket refurbished AFM a crapshoot.
A couple years ago I stumbled across a guy that figured out how to recalibrate our AFMs. Back then he wanted $300 IIRC to refurb an AFM. I tried to get us a deal, but he didn’t go for it. The price has now dropped to $150. I’m going to send him at least 1 AFM. If some more guys get in on this we can get a price break. IMO having a known good AFM is a big deal and this is worth doing.
The link that follows goes into some detail as to the refurb and recalibration process.
http://www.r3vlimited.com/board/showthread.php?p=3152483