Help me kick my off idle miss/hesitation

[fretburnr]

Sounds like the ignition is all in order, save for maybe specific plug gap. I personally hate +2s, but I know they're recommended for the m42.

The fact that it can be made to pull cleanly does rule out most of a fueling issue, but one fueling item that is/could be affected by throttle application is the FPR. It may have an internal vac leak and may not respond completely when vacuum is switched too fast. Try using a vac pump on it and see if it holds or leaks.

Also, use a vac gauge and see if vac levels are where they should be. Might help to narrow down in case of a potential intermittent gasket leak (long shot)

[boom-monkey]

Have you checked the see of the cams are adjusted properly? Just a thought.

Mine did the same thing you are describing, but only once.

I was driving home from a football game and went to accelerate on to the interstate a felt like I wasn't getting any power, like running on 3 cylinders. So i figured I fouled a plug. Eased off the gas like you have described and it went away.

That was about 2 years ago and it has yet to raise it's head again.

[Jordan]

Been done. Made lots of smoke, did nothing else (didn't expect it to)

I've looked inside the cylinders with a borescope and everything is actually really clean. Almost no carbon buildup.

[silversleeper]

Seafom it!

[Jordan]

I've swapped my AFM out with a known good one from a perfectly running M42. No change. I wouldn't think its a fuel issue as it runs great after 2500rpms or so after it leaves the funk zone...

Throttle body is perfectly clean. I did actually check all orifices and make sure they were clear. The one to the FPR I've seen get stuffed up before.

Idle air valve is also clean, seems to move freely and all that.

[StereoInstaller1]

I assume you have already cleaned the IAC (or whatever BMW calls that). It really sounds like you have been thorough as hell (as usual).

I had a motorcycle once that had tiny cracks in the carb castings, causing some issues with the CV diaphragms. Itty bitty vacuum leaks caused all kinds of issues, and these cracks were not visible from outside.

Good luck, dewd!

Luke

[browntown]

Just a wild guess but you might try cleaning your throttle body if it's dirty. Does the m42 have any vacuum lines off the throttle body? Had a volvo 850 where those little holes off the TB to vacuum lines got clogged = havoc.

[KenC]

The AFM is working well? I remember you have an issue with one in your M20.

Possibly have anything to do with the fuel pump? I've only have my M42 for a few months so I haven't had a chance to really dick around with it/diagnose things quite yet.

[Jordan]

All of R3V's members and experience.. and not a one has a suggestion for me?

It hasn't really gotten any better or worse, and I've tested all the sensors I can think of per book specs and everything matches up. I'm just stumped.

[eyagel]

have u talked to thad? he might know what it is..he is a genious(sp?)

--eric

[Jordan]

Car in question is a 2/91 318iS with (duh) a M42.

Here are my symptoms:

The only code that comes up with the stomp test is 1216 (TPS)

It does it worst and most consistently at cold, but often when warm it will do it much "softer" so it is much more tolerable and less prone to bucking.

Off idle if you apply the throttle quickly it will start hesitation, it feels like its only hitting on 2 cylinders and there is a distinct miss sound. If you ease up on the pedal it does seem to do it less. Once the engine reaches the 1500-2000rpm range it completely stops the hesitation and will eagerly rip to redline gear after gear.

If I take off from a rolling start in 1st gear at 800rpm or so (basically idling forward) and just wham WOT it will stutter all the way to redline without letting up. If I ease up just a bit then pound it back down it will stop the BS and pull cleanly. I was able to get it to do this consistently.

It will still stutter to redline if I give it too much throttle (but not WOT) off the roll, and hold it there.

Outside of that RPM range off idle the problem is not present.

The engine does hold a reasonably smooth steady idle, with or without the AC on.




So far I troubleshooted by:

Removed the spark plugs. They looks like fairly new Bosch +2's

Removed the crank & cam position sensors and cleaned the grease cakes off them.

I've already checked all the hoses for vacuum leaks, I'm 110% sure there are no vacuum leaks.

Swapped in a known good ECU
Swapped in a known good AFM
Swapped in a known good coil pack

Installed a new TPS.

Verified continuity of the TPS wiring connector to ECU with a meter, and by eye the whole way... 0.0 OHM and clean wire.

Checked ignition wires in the dark for obvious arching.. none observed.

pulled each plug wire off the plug with the engine running and listened, all produce a nice spark while hovered above the plug. Each when pulled fully away produces the normal dropped cylinder running.

Metered out CPS's
Crank sensor was 610ohms.
Cam sensor was 1480ohms.

Installed new fuel filter
Installed new air filter


This one has me pretty stumped and I really don't want to keep throwing money at what is at best a guess... hoping to get lucky. Open to any and all suggestions!