Author Topic: Engine hesitation. (Read 21067 times)

St�ck · « **on:** August 19, 2007, 09:54:44 AM »

I have an interesting hesitation that is giving me a hard time on my recently purchased 91 318iS.

It does it worst and most consistently at cold, but often when warm it will do it much "softer" so much so its almost not at all.

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.

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

I've already checked all the hoses I can see for vacuum leaks, I'm reasonably sure there are not any. I've worked on quite a few m42s and am familiar with the rats nest under the manifold.

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

Any suggestions on where to start?

So far I have:

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.

ak96ss · « **Reply #1 on:** August 19, 2007, 04:52:04 PM »

The hot vs. cold has me wondering a bit; my first impulse would be to say to check the AFM or TPS, but those shouldn't really be any different hot or cold - I would expect them to exhibit the same symptoms all the time.

Don't have a spare DME there do you?

I wish mine would idle smoothly. I get a lovely hunt, but only when the engine is hot. Cold idle is steady.

St�ck · « **Reply #2 on:** August 19, 2007, 06:56:38 PM »

I dont have a spare ECU no.

I'm kinda leaning towards it being the cam position sensor. I know on my e36 M3 when it went bad on me it displayed similar symptoms. Same busted ass missing through some RPM ranges, and others it was fine... until it stopped working altogether.

dude8383 · « **Reply #3 on:** August 19, 2007, 11:18:34 PM »

Have you checked the condition of the spark plug wires?

I had a similar issue with my car about a year ago.

St�ck · « **Reply #4 on:** August 19, 2007, 11:53:08 PM »

I have not removed them all to verify condition, but from what I can see they look ok? That is a good idea though. Next time I have a chance I will pull off the cover plate, start the engine, turn out the lights... and look for arching.

I wouldn't *think* this would be the problem though, as it runs fine past a certain RPM.

St�ck · « **Reply #5 on:** August 20, 2007, 12:14:04 AM »

Just checked for arching in the dark... nothing.

Also pulled easy 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.

Don't think its a spark issue, at least not on the side of the coils/wires/plugs

kowalski · « **Reply #6 on:** August 20, 2007, 12:15:12 AM »

I would say cam pos. sensor. sounds like the problem i had as well and fixed it with that. as well i would check the MAF, the engine is mostly in those RPM's and the carbon strip could be worn out on the path for the lower rpms throwing off the signal.

nuvolarossa · « **Reply #7 on:** August 20, 2007, 04:56:44 AM »

Quote from: kowalski;31983

I would say cam pos. sensor. sounds like the problem i had as well and fixed it with that. as well i would check the MAF, the engine is mostly in those RPM's and the carbon strip could be worn out on the path for the lower rpms throwing off the signal.

+1 on camshaft position sensor... mine whas like it misfired when it was bad, it had a bad idle and car was losing a bit of torque too..

but I changed it at the same time of lambda sensor, that was bad too.. now the car is like new:)

e9nine · « **Reply #8 on:** August 20, 2007, 10:46:55 AM »

Quote from: ak96ss;31950

The hot vs. cold has me wondering a bit; my first impulse would be to say to check the AFM or TPS, but those shouldn't really be any different hot or cold - I would expect them to exhibit the same symptoms all the time.

Well the basic air fuel mixture equation shows the coolant temp sensor as well as the AFM and the ECU as other players in the party.

These engines are sensitive with the air intake path as we all know and since his spark seems to be 100% I'd start off with cleaning/testing the AFM. It's annoying to open up an AFM but they can be sealed back easily as we all know.

I had a tempermental issue that was related to the AFM at one point. Do you have one to swap with?

The other I had which was related to throttle application was the TPS wires being frayed in the sheath/sleeve of the connector.

One time my cam sensor came loose on the track and the car had a odd stuttering issue.

G. Luck

bmwman91 · « **Reply #9 on:** August 20, 2007, 02:22:07 PM »

I had a similar issue on mine and it turned out to be the AFM. It was pretty intermittent (though consistent enough to really piss me off), but after installing my MAF conversion over a year ago I have not had one problem/misfire. I tried testing the AFM with an Ohmmeter and there were no resistance jumps. I still cannot say what it was in the AFM that was messing up for sure, but I an 99% sure it was that. *knocks on wood*

My guess about it was that the volume of air flowing around the range you are describing is where the AFM parts spend most of their time when cruising, and the carbon track just gets a little worn there. Maybe the thermal expansion in some of the parts is just enough to put the follower in a badly worn groove or something. Good luck, keep us posted!

St�ck · « **Reply #10 on:** August 20, 2007, 03:04:49 PM »

I will pop the top on the AFM this afternoon. I've had to bent the arms before to account for a worn track back when I had a AFM on my 328iS. Fixed it then, would be nice if it was the same deal since that is free

St�ck · « **Reply #11 on:** August 20, 2007, 04:11:58 PM »

Opened the AFM, cover has never been off before. The carbon tracks look very nice and the resistance range is steady.

While I was there I decided to OHM out the cam and crank position sensors. Car has been parked about 30mins or so from fully heated up.

Crank sensor was 610ohms.
Cam sensor was 1480ohms.

Seems to be the cam sensor is WAY outside of range..... thoughts?

bmwman91 · « **Reply #12 on:** August 20, 2007, 04:43:06 PM »

According to the factory manual, the cam sensor should be 1150 - 1410 Ohms. The crank sensor should be 540 +/- 60 Ohms.

Taken from:
http://ee1394.com/bmw/docs/factory/repair/en/index1.htm which can be downloaded from a link in the Reference Forum.

St�ck · « **Reply #13 on:** August 20, 2007, 04:55:24 PM »

Eh.. so maybe its not that then.

bmwman91 · « **Reply #14 on:** August 20, 2007, 05:07:11 PM »

Haha well, if this is your daily driver then I probably know how you feel. I tested everything repeatedly, tried disconnecting various sensors to see if it would at least run better in "limp" mode, you name it (though I did not try it without the AFM connected...that never goes too well). It seemed extremely unlikely that it was the AFM causing the cut-out, but ended up being that. A few other guys on here have had the same issues, and remedied them by replacing the AFM.

If you want to try a "good" one out let me know. The one from my car gave me no troubles when I smogged it a month ago. It seemed to get bad only in cold weather. For the cost of shipping you can borrow mine if you like. It'd probably be around $12. Then again, if you still get parts hookups or have a Worldpac account then you can probably get a new one cheap.