Intermittent Engine Stutter

[saneproductions]

Hi All,

I have an intermittent problem that causes the engine to stutter and not rev smoothly.

The car is an 87 325is with a fresh top end re-build and new rings. It runs perfectly and drives like new 99% of the time. Very occasionally it will give just the faintest hint of a stutter while you are driving. In a super rare event, the engine will not build revs smoothly. When you push on the gas the revs go up and down as you see in the video and it will hardly build any revs. Trying to drive when this is happening is damn near impossible as the car jerks badly. After an attack of around 10 minutes, it went back to normal and I was able to drive perfectly smoothly home.

Advice is appreciated.

We have replaced the 2 main relays and cleaned lots of contacts and grounds, but there may be more. We have also replaced the crank position sensor and got a reading from the tps that is in spec. The battery and alternator are good.

Thanks!

[superj]

the fuel injector harness connector was good?

since you got the crank sensor covered, you knocked out my initial thought. that was the thing that caused it on my 89 325i, back when I had one. the wires into the actual sensor part of the unit had developed a bad connection and when I hit a bump just right, they would wiggle and I would get what you have. then, if I could hit a bump right again, it went back to running normal. otherwise, I had to get out and wiggle the wire leading to the sensor till it ran right and I was able to get home and put another one in place

[TobyB]

I would meter the AFM to try to rule that out, as well.
If you can stick a voltmeter at the ECU, that's the most fool- proof.

t

[CMatos]

Have you done any work on the car recently?

[Kershaw]

Undo all harness connections, spray in electrical cleaner, and reconnect them. Remove all grounds, lightly sand the surfaces, and reinstall them. If you experience this again, have a friend rev the motor and jiggle as many connections as you can. Find the one that's the issue. Make sure your battery connections are tight. Make sure your DME harness is properly 100% seated. It has to be something stupid and small.

I am 99% sure this is an electrical issue.

[saneproductions]

the fuel injector harness connector was good?

since you got the crank sensor covered, you knocked out my initial thought. that was the thing that caused it on my 89 325i, back when I had one. the wires into the actual sensor part of the unit had developed a bad connection and when I hit a bump just right, they would wiggle and I would get what you have. then, if I could hit a bump right again, it went back to running normal. otherwise, I had to get out and wiggle the wire leading to the sensor till it ran right and I was able to get home and put another one in place

We did clean the harness, but now that you mention it and I do more research, it looks like we may need to work on it more. Thank you for your help!

[saneproductions]

I would meter the AFM to try to rule that out, as well.
If you can stick a voltmeter at the ECU, that's the most fool- proof.

t

We will look at this as well, thank you for your help!

[saneproductions]

Undo all harness connections, spray in electrical cleaner, and reconnect them. Remove all grounds, lightly sand the surfaces, and reinstall them. If you experience this again, have a friend rev the motor and jiggle as many connections as you can. Find the one that's the issue. Make sure your battery connections are tight. Make sure your DME harness is properly 100% seated. It has to be something stupid and small.

I am 99% sure this is an electrical issue.

Thank you for your advice, we will work on all of this. I was getting a bit discouraged, but this advice cheers me up.

Thanks

Mike

[Kershaw]

Good luck!

Not to discourage you too much, but electrical gremlins can be the worst. I had an electrical issue at WOT and the only thing that solved it was switching to megasquirt. lol. I swapped absolutely everything in the engine bay, short of the block, head, exhaust, and intake manifold, three times over and in multiple different configurations. So I hope you have better luck.



If you get one of these testers (they sense electrical pulses) you can lay it on the wire from the ignition coil to the distributor and see that the jerkiness/hesitation/stutter is in the electrical pulses to the spark plugs.

[saneproductions]

Hi All,

We tracked down and fixed the issue. It turns out that the ignition harness was severely corroded. We were switching out the ignition lock due to a separate issue with key that would not turn occasionally and noticed all the green corrosion on the back of the switch and harness. Thanks for your help all!