Possible AFM issue??

[BewWoo]

So I recently swapped the blown m20b25 in my 89'325iS with a replacement m20b25 (I am told it is from an 86 or 87) that sat outside on a pallet for years. That said, the engine still runs like absolute shit. I have a very rough idle and it smells like the engine is running rich. I have begun the diagnosing process with a friend of mine who is far more educated on e30's. He unplugged the AFM while the car was running and it sputtered and died. After a few seconds to reset, we plugged the AFM back in and started the car. This seemed to fix the rough idle and the engine ran smoothly but after about 5 - 10 seconds the engine started to bog, the rough idle returned and quickly returned to the same terrible running.

I never get full power out of this engine at any rpm. I should also add that the engine does not have any apparent misfiring or chattering at rpms higher than idle, and at cruising speeds everything sounds fine (barring the disgusting lack of power).

Is this phenomenon of "good idle moments after AFM reset" that turns to "bad idle after engine gets running" the result of a bad AFM or something else?

Thanks!

[Sagaris]

how are the ignition components? Are there any fuel or vacuum leaks?

Was the engine swap kind of a "lets do this as cheap as possible" or have you replaced any sensors, gaskets, and hoses along the way?

[BewWoo]

Ignition:
New spark plugs. When I first got it running I replaced the cap, rotor and wires with another USED set and the issue was unaffected. I know this does not rule out an ignition issue but since there was no change at all, I looked elsewhere for the problem.

In regards to a possible vacuum leak, I did the "spray down with brake cleaner listen for engine rev" and found found nothing. Right after the swap I discovered a crack in the intake boot so I purchased a new one and that also had no effect on the way the engine ran.

I definitely tried to keep the swap as cheap as possible. I have not replaced any hoses or sensors. I did replace the valve cover, exhaust manifold, and oil pan gaskets.

[BewWoo]

bump

[TimR]

Clean the AFM and IVC

[Sagaris]

Clean the AFM and IVC

I dont imagine this will be the solution, but being technical "cleaning the AFM" is only a valid comment if the vane inside the AFM cannot move freely due to the buildup of deposits, which shouldn't happen even under extreme circumstances. The only case where that might apply is if you dog buried the AFM in the backyard and you had to dig it out. Very unlikely.

Cleaning and inspecting the AFM resistor track for worn spots is the correct thing to do (Maybe TimR meant this to begin with).

Also, as far as I know, the AFM does not "reset" or have any sort of memory associated with it. The DME has a long-term fuel trim adjustment which is reset when you disconnect the car batter long enough for all the capacitors to drain (30 minutes should be more than enough time)


As much of a pain in the butt it may be, you need to check fuel pressure to verify that it is correct. If that checks out okay, then the next step is to do a smoke test for vacuum leaks. By then you should have things worked out. If not, there is likely a bad electrical connection somewhere.

[BewWoo]

Cleaning and inspecting the AFM resistor track for worn spots is the correct thing to do (Maybe TimR meant this to begin with).

I will do this and get back to you in a few days.

You're not the first person to suggest a fuel pressure check.. I need a fuel pressure gauge and will try that asap.

Thanks for the advice!

[Sagaris]

In case you dont know what you are getting into, take a good look at this site: http://www.the944.com/afm.htm

It will give you an idea of what to look for as far as wear is concerned and how to clean and possibly get a little more life out of your AFM if there are deep grooves in the resistance track.