MAF Sensor + Surprise

[L.Tigre]

Last weekend I had disassembled the engine intake and filter; and probably was not so careful about keeping dirt/debris out of the clean-side of the air filter. When re-starting, the engine ran rough and would not run over around 4krpm. Thinking it's a fouled MAF sensor; I blew out the MAF with clean air and then with MAF cleaner. The engine runs much smoother now; but still can't exceed 4k rpm and struggles to get there. It's fine at lower revs but just won't rev high.

About 10 years ago I accidentally fouled the MAF sensor on another engine, a Maserati Sypder V8 circa 2002. It led to a sick/stuttering behavior very similar to this BMW behavior. In that case 10 years back, MAF replacement did the trick. So that's what I am planning now.

But then: yesterday it seemed the battery and/or alternator also started a death spiral. Electrical systems started shutting down from lack of power; it wouldn't start without a jump-start, etc. So I'm doing some tests tonight then shopping for batteries or alternators, or both.

Question: Is there any reason the one issue would somehow cause the other, the MAF and the 12V system? I am viewing these as isolated events; do you think that's the case?

Separately, any advice on MAF or alternator replacement?... both of which seem fairly straightforward to me. But I've not done either one, on this vehicle....

[kronus]

our motors have flappy door AFMs, not MAFs. They don't have a resistive element, so MAF cleaner cannot really affect them in any way.

what could be happening is your electrical issue could be causing your motor to run poorly above 4k due to reasons not associated with the AFM, like something else getting jostled or disconnected. Check that your pulse and reference sensor wires are correctly plugged in, and verify that you are getting +12V to your alternator exciter wire.

[L.Tigre]

aha! - thanks for that, I had some idea it was an odd sensor compared to what I expected, but I didn't "get it" til your post. With a bit of search I found this guide, which I think is the same sensor or pretty similar: http://clarks-garage.com/shop-manual/elect-22.htm

The barn door swings freely. I guess the only thing that could have been fouled or damaged by debris would be the temperature sensor. That seems easy enough to test. I am not sure if it would cause my engine issue or not.

I did unplug and replug the pulse and reference sensors (a few times) as I was working on the bracket that holds them. I'll re-evaluate those as well, and the 12V on the exciter wire...

[L.Tigre]

Update: voltage was low and all signs pointed to the alternator. I replaced it and voltage is now fine. (BTw changing the whole alternator was much harder than it should have been…)

Engine however is not fine. Idles ok but just will not pull to higher revs. I tested the AFM, it seems fine.

I made an alarming discovery. While idling I pulled out a spark plug wire. And it made absolutely no difference at all to the running of the engine! I gulped and went to shut it off immediately.

On investigation the last owner or mechanic had used plug-end terminals…. But wires meant for terminal-less threaded ends, that do not fit over said terminals. Multiple of the wires are barely sitting loose in their holes, with tenuous connection (or no connection) to the juice that creates the spark. The three easiest to reach all had this issue.

So my new theory is major engine misfire or lack of firing on multiple cylinders. (And my garage smells like gasoline for the last three weeks, I’ve looked for leaks and found none. Seems consistent?)

So now: all new plugs asap. But then what? Is there more you’d recommend? Any chance that catalytic converter is a molten mess?? This is uncharted water for me.

[kronus]

glad you found and fixed a variety of issues!

I’ve run a car before that was misfiring so bad that the cat turned red hot from afterburning all the in burned fuel. It was 100% fine after I fixed the misfiring issue. I wouldn’t worry about a plugged cat until the other issues were sorted.

[L.Tigre]

Some more detective work and the car is ‘fixed.’ It was indeed a melted catalyst!

Changing plugs last night made it idle very smooth. But paradoxically it was even less capable to rev. Previously, w old plugs and wrong plug wire ends, sitting still I could get it to 2500rpm.After new plugs w tight connection it couldn’t even reach 2000rpm. I had done more checks on various stuff…. Crank sensor, ignition coil, etc.

So I added a hole to the catalyst inlet and took a peek inside. Front brick was broken; molten ceramic was visible in the broken piece. I chopped it as finely as I could via a heavy screwdriver thru that inlet. It ran noticeably better. Repeated the same with the rear brick through a 2nd hole, and now it drives like new, can take it up to 5krpm+ under full load.

It sounds like hell of course w two half-inch holes in the cat. But- it needed new exhaust anyway…