AFM troubleshooting

[wizard124]

I just resolved a bad running condition which I want to share. My FI car would start and idle well but would rev to about 3500 rpm where it would stall and miss. Poor performance and power to that rpm as well.

The AFM (Air Flow Meter) door was free without any binding. My Fiat FI Diagnosis manual calls for some resistance checks at the connecter (can be at the ECU or at the AFM). My manual specs the following:
Terminal 6 and 8.....about 180 Ohms
Terminal 7 and 8 .....about 150 Ohms
Terminal 8 and 9.......about 100 Ohms
Terminal 6 and 27 .......2280 to 2720 at 68F

My AFM readings were 473, 393, 302 and 1913 at the AFM connector. Temperature in my garage was probably 75F. The 6/27 resistance is to the air temp sensor and decreases with temperture ( 760-970 at 122F).

I also have 2 AFM spares of unknown operating condition. I grabbed these to check their readings. In the same order they read
(357, 357); (354, 278); (199, 198); and (2230,2200).


I took off the cover to my AFM. I noticed that if I manually opened the metering flap by twisting the spindle, I could obtain a smooth running condition. There is a "W" shaped retaining wire that holds the spring tension. I weakened the spring tension by 10 notches but there was not a significant improvement. So, I didn't go any firther with this "fix".

I substituted my spare AFM (357,354,199,2230). I started the engine and the miss/stalling was totally eliminated.

[ORFORD2004]

AFM

[narfire]

The AFM has a strip inside that two pins ride on as the afm opens. Over time this area wears on the strip giving a stumble until higher rpms.
Not a difficult fix but getting the black top off the meter can be a pain. Have a box of band-aids handy...
One can loosen the screw/bold that allows the pin mechanism to slide up and down on the shaft and with perhaps a slight bend, the pins will sit on a new untouched part of the strip. The pins only have to move 1mm or less to find a clean part of the strip.

[RRoller123]

The arrows show the tracks and the bolt that is loosened to move the arm up or down. Down is easier, as it moves the arm further out onto the TFM and you don't have to put a bend in the arm like you would if you raise it on the shaft.



Disassemble to this level and clean everything.



LIGHTLY abrade the contact point on the arm cap where it meets the contact point on the center shaft.



The cover has this channel that fits over the casting lip. In putting it all back together, I filled it with RTV caulking and it seems to be fine and seals it well. Prying the black cap off the first time will likely cause some gouges in the channel edges, I didn't find any way around this, but it is ok.



The intermittent stumble at about 3000 rpm instantly disappeared and has never come back.

Make absolutely sure that the door flap moves freely through its entire range of motion. It must do this while held in its operating orientation. You can see some rub marks on mine. I sanded the inside of the housing with 400 grit, very slowly, and after a couple of hours, had it free and clear. Clean the grit and aluminum debris out of there well afterwards.

[dutchgeordie]

I've opened my AFM up. I thought my problems down to a sticky flap but I'm now thinking it's to do with the arm going of the circuit board. It is at that point that the flap sticks but lifting the contactpoints up does not immediately release it.
Should I adjust/turn it so the contact arm stays on the track? If so, how do I do that?

[RRoller123]

in that first picture, is that the position when at rest (idle)? or full throttle (flap wide open)?

[dutchgeordie]

first and second pictures are with the flap open, full throttle
last picture is flap closed as far ar it will go with the contact arm not yet at the beginning of the "track"

[4uall]

curious to know, when you say clean it what/how do you recommend?

[RRoller123]

My guess is that some PO has fooled around with the registration of the arm on the TFM. It should not be showing track wear marks off the left end of the TFM. It can be adjusted by moving the notch that the clip sits in, so that it sits at the start of the track when at idle. BUT we are in uncharted territory of my knowledge here. It is clear that someone has also bent the silver contact arm at the right so that it doesn't contact the switch that turns off the Fuel Pump, thus your fuel pump runs all the time that the ignition is on. I bet that they also moved the orientation of the arm on the track when they did this, not realizing how it effects the car's running! I am no expert, but you could make note of the current notch, maybe with a dot of paint or white out, and then move the arm orientation to the start of the track, CHECK WHEN THE CAR IS ACTUALLY IDLING , i.e. leave cover off for a while and watch what it does. But I am no expert and make no guarantees. Others please chime in who know much more than I.

[Kevin1]

The contacts should remain on the resistor surface throughout the entire range of motion.

If that AFM were mine I would start by loosening the clamping bolt that holds the contacts in place on the shaft to rotate the contacts (clockwise relative to the shaft, as it appears from the top) until the contacts are just inside the resistor area at the idle position, and still within the strip with the flap opened at the "full throttle" position.

If you are not sure whether or not the spring tension was changed, leave it for now - it can be adjusted later if needed.

Carefully bend the wire bail back into position so that it just opens the fuel pump contacts when the flap is all the way closed.That should get you close to having the moving parts aligned correctly.

From there you will have to make any further adjustments - such as spring tension - either by trial and error or with a meter attached to the oxygen sensor. At full throttle (while driving) above 2500 rpm, with the engine fully warmed, the oxygen sensor should read .9v. Lower is lean, higher is rich. Adjusting the tension on the spring changes adjusts how far the flap opens for a given flow of air, which changes the mixture. Less tension = more fuel/richer mixture, more tension = less fuel/leaner mixture.

[Kevin1]

One more thought…

Consider why an PO might have done that to your air meter. Adjusted as it is would greatly richen the mixture - especially at idle. My guess (worth a grain of salt) is an air leak somewhere between the AFM and intake valves, and more fuel was required to get the car to idle. Any leaks must be found and repaired before you can accurately adjust anything to do with the FI.

Most of the mis-adjustments I have seen on L-jet FI have been compensating for another problem, air leaks, low fuel pressure, etc., which went undiagnosed and unrepaired. In any event getting your AFM straightened out should help.

[spider2081]

If you look at the flap from the air box side you will see the flap Stop held in place with 2 screws. I have seen the wiper off the resistor to the right side (engine not running position). Looking at the flap with respect to the Stop I found the flap passed beyond closed and opened again because the stop was bent. Straightening the stop so the flap stops moving in the closed position realigned the resistor wiper to the end of the resistor. I think maybe the car backfired in the intake and the flap bent the stop.
At any rate inspect the range of motion of your flap. If you look at the shape of the flap housing at the air box end. You will see a point or position where the flap closes more than any other point. I believe that is where the flap stop should limit the flap travel. Then the wiper can be set to the right most travel point on the resistor. Rotating the flap full open the wiper should remain on the resistor.
I have not seen anything in writing to back up my theory. Maybe someone more knowledgeable can comment on this thought.

[dutchgeordie]

Update:

For some reason the previous owner or some other idiot filled the screw head holding the contact arm in place with some goo so I could not loosen it with a screwdriver. I haven't quite managed to get the goo out but have managed to adjust the rotor so the contacts stay on the tracks. I've also managed to adjust the silvery arm switching off the fuel pump (btw I had no idea that's what it did).
I haven't done anything else with spring tension or whatever. I closed it all up, reinstalled it and my problems are over.
I'll get someone to have a proper look at adjusting springtensions and such to get it perfect but for now I'm very happy with the result. The engine ticks over nicely when idle, picks up fine at half or full throttle and does not splutter and stall.

Thanks for all the input.

[RRoller123]

BINGO Baby!