ProtoCab

Hi,

I suspect I have a faulty AFM, but cannot verify this without swapping in a known good AFM and seeing if it makes any difference.

If I feather the flapper door slighly open (about 10-20% defelction), the engine idles fine, but if I let go of the flapper door, the engine struggles to idle and dies. Engine runs very erraticly, hesitates, bucks, pops and backfires. Very rich!

I have verified all vacuum lines are OK and even fixed a couple inlet manifold gaskets, which were letting unmetered air into the engine (sprayed carb cleaner and it gets sucked in). Got it fixed now tho.

I still have the idle issue and it seems my mixture is running excessively rich, and possibly flooding the engine, whch is why it's struggling to idle properly and stalls.

I have checked fuel pressure (OK), ignition (OK), plugs (NEW), CHT Sensor (also OK).

The only other thing left is the AFM.

Advice/assistance please?

J. Brinkley

dis check the idle control valve, and closed throttle switch?

ProtoCab

Hi Jerry, yes I removed, cleaned and re-installed the ICV.

The closed throttle switch is fine and I can hear the microswitch engage when the throttle is closed.

J. Brinkley

I'd be nice if it was easy wouldn't it
did you try leaning it out at the AFM, before that though, is o2 sensor good?

ProtoCab

I have not run an O2 sensor for years. It's never caused a problem.

Not sure how I can lean it out at the AFM. Please elaborate.

The fact that the engine idles properly when I feather the flapper door open tends to point to the AFM. Maybe at idle position, the contact is poor or the wrong voltage is getting to the DME/ECU?

If I hold the flapper door at approx 20-30% deflection, the idle is good.

This is bizarre!!! It just won't idle on it's own

J. Brinkley

there's a 944 website with all things related to opening and moving the wiper to a different track. But I'd be scared to open mine.
idle screw is just to the left of the standoff bracket, at the afm body accessed from underneath with 3mm allen. look at this pic at the bottom http://www.911chips.com/dmeconvr.htm
note, idle screw in pic is not what we're talking about.
I'd try to follow steve's instructions and adjust idle, IF it's drivable, I'd get the idle mix measured.
counter clockwise on that screw will let more air in, count your turns so you can return to where you started.
so, I'm assuming with the pedal down slightly it drives fine? flat spots or anything?
I'm getting close to the limit of my know how, Loren has a good checklist somewhere on all this adjusting, Steve's website has a lot to.

Lorenfb

"The fact that the engine idles properly when I feather the flapper door open tends to point to the AFM."

This indicates that the mixture is too lean. By slightly opening the AFM, the mixture
becomes richer. Check this ( www.systemsc.com/tests.htm ) for info on AFM values
if you really need to check it.

Bottom line:

1. intake air leak
2. low fuel pressure
3. O2 sensor input too high in voltage
4. mis-adjusted AFM spring tension
5. the AFM mixture screw is all-the-way "out",
S.B. about 3 turns "out" from fully "in"
6. Test pins 2 & 5 on the DME ECM for
actual idle switch closing & not just "clicking".

ProtoCab

Thanks Lorenfb. I've checked all possible vacuum lines. I did find some leaky inlet manifold gaskets but had then all replaced and re-torqued down the manifold. I then found a small leak where the two intake plenums meet, so I tightened the clamps there and that fixed that problem. All other vacuum lines are OK.

I tested fuel pressure and it's also fine.

I've been running without an O2 sensor for 4 years and have never had this problem

It's unlikely it's the idle switch. If I manually push the throttle down to ensure it's engaging, the engine bogs down and stalls.

The problems I'm having is not just at idle. It seems to be whilst cruising too. Bucks, hesitates, stutters, poor response and drivability. It smells extremely rich too.

As I said, I'm getting good spark and fuel pressure is OK too.

Just wish I had a spare AFM I could use to swap out the damn thing

Thanks guys.

P.S: I find when I try to adjust the idle screw, it makes absolutely NO difference whatsoever. I haven't bridged the two pins at the rubber test socket and tried adjusting. Might give that a try.

Mark Hald

Proto -
Where do you live? I have a spare AFM if you need one.

ProtoCab

Australia

nathanUK '81 930 G50

This thread helped me out

https://rennlist.com/forums/911-forum/219689-checking-air-flow-meter-on-3-2-a.html