Dead spot in RPM range

[mrgraphics]

This is the last thing I need to get sorted on this car.

I originally thought it was a slipping auto transmission, but this recent test helped me rule it out.

On highway acceleration in D (drive), once I got to 50, it would bog, not accelerate, etc. But once I finally made it to 65 it would be fine. RPMs in the 2000 mark.

For giggles, I did some acceleration pulls in 1st, and wouldn't you know it, the same dead spot - accelerates great up to 2000 RPM, then stutters and shakes between 2000 and 2500, the back to normal acceleration all the way to redline.

Only thing I can think of is a bad MAF? Ideas?

[jw 325ic]

How are your spark plugs?

[jlevie]

My first suspect in this case would be the AFM.

[Quinthirty]

If you're in the garage, and you rev it to 2k, is the experience the same? If yes, I suppose your suspicion would be right.

[mrgraphics]

I should give some background on repairs / service so far: 1986 325es

New plugs
New cap and rotor
Re-gaped points
Timing and all assorted belts / water pump
Transmission drain, pan drop, new filter - rechanged oil 3 times over two weeks
New differential oil with LSD additive
New CPS
New intake boot
New O2 sensor

Revs are fine in garage, but this happens under load were the fuel/air ratio are more taxed.

Car runs like a champ. I also notice about 50% of the time when starting out there is a slight hesitation and then it goes. So potentially two dead spots - one in the 800 to 1000 RPM, and the other at 2000-2500.

Pick n pull is having a 20% off sale - guess I'm picking up a new AFM or MAF, whichever. I'm terrible with acronyms. That Bosch thing that's at the airbox after the filter ;) with the timing belt change label on top.

[jlevie]

The hesitation off idle could be intake leaks and/or a sticky ICV. Or if could be a worn spot in the resistance trace in the AFM. The dead spot at 2k under load sounds most like the AFM.

[FLG]

Pop the cover off your AFM and see how the resistace strip looks, if its worn through in a spot simply bend the metal piece that runs along the strip so it follows a new path.

http://www.the944.com/afm.htm

[mrgraphics]

FLG - I had already tried that. I think it was more an electrical issue within the AFM.

I went to Pick n Pull, lucked out with a working AFM from an 85. No hesitation, no dead spots now. In fact it actually is running much better throughout all the RPM - and she even has a bit more power. The car has probably been running with the wrong air/fuel mixture for a long time.

[tunethtmkII]

i had a similar problem in my 325i 5spd. when getting on it it would hesitate at about 3500rpm then once i get to 4500rpm it would continue pulling. possible bad afm for me swell?

[mrgraphics]

tunethtmkII - that sounds just like mine. Mine's an ETA, so the RPM range is different, but 3500 is about equal to my 2000 RPM if you think about percent of revs until redline.

[Gregs///M]

i had a similar problem in my 325i 5spd. when getting on it it would hesitate at about 3500rpm then once i get to 4500rpm it would continue pulling. possible bad afm for me swell?

Without a doubt, bad AFM.

[mrgraphics]

Well, the new AFM helped, but it is still doing it. After some playing, I noticed that it really depends on how much gas I'm giving for the symptom to expose itself. Take a look at the video and the description. What could this be? I'm stumped. I did clean the throttle body, so nothing is sticking.

There is a noticeable "click" when I move the throttle body through. It'll go 3/4 of the way smooth, then a hard push and it "clicks" all the way open.

http://youtu.be/MJ7pT8g0yhk

[Gregs///M]

Was this AFM rebuilt or refurbished? Replacing a 20+ year old sensor with another 20+ year old sensor could lead to lots of wasted money on other troubleshooting. I would not rule out the AFM until you try it on another running and rolling e30.

I've refurbished many AFM's and find that is it very common to have wear or "drop out" of electrical signal from the AFM to the ECM in the 75% open range, which can easily correlate to the 2500 rpm range. Most common on the idle speed range.

[mrgraphics]

It was a pick n pull AFM, but on my old one I did crack it open and reset the dial on a non-worn area (making sure not to move it's correct trajectory). The fact that both are doing it in the exact same RPM range seems statistically improbable, although I do understand your logic.

And why would the amount of gas matter? That's what has me confused. After all, isn't the AFM at the same amount of "open" at 2500 RPM regardless of gas? Or is it that on hard acceleration more gas + more air is going into the engine, and therefore the AFM is actually more open than compared to a slow creeping acceleration?

I don't know anybody who has an e30 that I could test my AFM on, so I'm on my own.

[mrgraphics]

Spent the night rebuilding the AFM - moving it both north and south on clean sections of track. It didn't change a thing. I don't think it is the AFM.

The car almost feels like it doesn't know what to do with more gas under load. Just a jerking motion, as though the power isn't transferring to the wheels smoothly. And it won't rise in RPM on a steep hill with the pedal mashed - instead it just starts bucking.