Flat spot at precisely 2200-2300 RPM on my 3.2L Carrera - need help & advice - AFM ?

[POZE]

Hello,
I had a question that someone might be able to advise on.

I own a 1985 3.2L Carrera, very high mileage of 240,000 miles! the engine was rebuilt a few years ago, but everything else has probably remained original. The problem Im getting is that I have a annoying flat spot at 2200-2300 rpm, constantly. Its not gone away and always occurs at that precise RPM if your slowly acclerating/normal driving though that RPM.

If you accelerate fast past it, its not noticable.

I've been thinking about this for a long time and I think that maybe the AFM is faulty/worn out at that RPM stage. 2200 rpm is a common RPM , as when you go from 2nd to 3rd or 3rd to 4th in my driving style, the engine drops to 2200 odd rpm, so im thinking that if the previous owners also drove generaly the same as me, it may have worn something at that rpm.

Its all i can think off to get by, and I dont know anyone i can borrow a AFM off, so my only choice is to determine the fault, or go out and buy a renovated AFM. My AFM is worn on the track, so i did move the arm up or down ( i can't remember), but didn't do anything different (i only moved it very slightly, so maybe not enough).
Any advice from anyone?

Its starting to really affect my enjoyment of the car as it must be driven fastish to be enjoyable! cheers, Poze

[ianc]

I would have done the 'move the arm' mod as you have already done, but apparently that didn't help you.

There is a procedure here for measuring the output voltage from the AFM as the flapper valve is open involving a 9V battery. If you search a bit, you might find it

If that AFM has seen 240K, it could be time for a refurb!

ianc

[DonMo]

I would try to move it a little more and see. Cheap and easy to do.

[Alan L]

How about an air leak on the intake side - which is worse, proportionately, at low throttle settings. Not so significant at high loads. Or does your fuel susytem compensate for this. My SC had same problem. Still does very slightly. The residual problem I am sure is unequal injector setting. The average fuel setting (CO) is right, but one or two injectors are running a bit lean (from plugs), which I intend to correct as soon as I get time.
Alan

[DanielDudley]

I have had this problem, and I think you found the source. You could also try cleaning the connection to the harness, just for luck.

[POZE]

thanks for all the advice.

Ive ordered 6 new inlet manifold gasgets and ive found a good used AFM from 911Parts in the US.

So im going to order both & whilst they arrive, im going to have another play with my current AFM and retrack the arm some more.

See if I can get an improvement.

ITs just making the car a right pig to drive as thats a real key RPM on it! im finding that im leavign it at home more and more, so hopefully this will sort it!

any help extra or advice is always appreciated!!

Cheers
Paul

[dshepp806]

Do make sure it's not a mechanical issue with the AFM door itself possibly binding. You've probably already checked this (lubed?)

As you stated, I'd try repositioning the potentiometer arm again. Can you see this (possible) bad spot with an ohm meter?

[Alan L]

Yes, my hesitation at the same revs annoyed the hell out of me. It was OK while engine was cold - as it warmed up, got worse. Does yours do this? This I take as a lean sign.
Putting more fuel into it (richen CO) helped, but actually moved the flat spot further up the rev band.
Once you get all the above sorted, hopefully it will improve. If not completely - check your injectors - for pattern and flow. One or two lean or poor injectors could do the same.
With that mileage, if they have not been replaced at some time - they would be good candidates. The springs get lazy with heat, and they clog with residue.
Alan

[scarceller]

See this thread for AFM checking:
3.2L Air Flow Meter calibration research prodject

I have a doc that will be helpfull.