Bizzare air flow meter fault

[jonnychesh]

After a body-off resto the big day came to re-start the engine !! This particular fault has taken 2 days to get this far; now I am baffled. My 1995 500 runs and idles like a dog but the AFM output seems fine at 1.5V on Pin 35 at idle, rising as you would expect when you increase the throttle. But its way too lean as shown by the lambda voltages which are near zero. Hence the poor running. (I've got 1.9V on Pin 22 which is supposed to be correct.) If you then stop and re-start with the AFM unplugged she runs a treat with a smooth idle and good pick-up. Although a tad rich at 1.1V from each of the lambs.; I'm guessing this is a limp home setting should the AFM go open circuit. Any ideas would be most appreciated from those more clever than me !!!

[steve-V8s]

If the AFM is working and correctly reporting the air flow I would think either you have un-metered air getting in (an air leak down stream of the meter) or there is insufficient fuel pressure for the injectors to get the right amount of fuel in. As I understand it once the ECU has gone into limp hope mode you need to remove the power to re-set it.

Have a check round the stepper motor idle control, brake servo and various other pipes that come off the plenum and check they are all either connected or blanked off.

[SonicHedgeHog]

Racing Green sorted my idle problem last month by simply cleaning the ECU plug. I'd been changing bits and bobs everywhere without any luck too so it might be something as simple as grib and grot.

[jonnychesh]

Yes, unmetered air was the source of the poor running. I had removed the water heater block on the underside of the choke, (to save weight!!), and failed to spot the threaded holes went thru into the plenum. This was very difficult to spot as they are on the underside and I only located the fault by listening for air leaks with a piece of tube stuck in my ear !
Thanks for your help.

[jonnychesh]

Found another source of un-metered air !! Theres a 5mm brass stub pipe under the stepper motor open to atmosphere. Can anyone tell me if this should be connected or just blocked off ?