Stumbles at Idle

[JMI]

The Car: 1989 535i Auto 125,000 miles

The symptoms: Happens only at idle (about 700 rpm). The engine will start to stall, rpm dropping off, and then will surge momentarily to about 1100 rpm and then settle back to 700 rpm. Sometimes it will completely stall. I can usually prevent this surge/stall by slightly stepping on the accelerator.
There appears to be no regular pattern when this happens except maybe when slowing quickly.
Stomp test gives 1444 - no code.
Can't find any vacuum leaks, but have not ruled that out altogether.
Have cleaned ICV throughly. It buzzes when engine runs. Resistance is in range -approximately 40/20/20.
Throttle Position Switch checks out OK.
Tempted to change out ICV but don't want to spend $145 bucks for nothing.
Been wondering if it could be a fuel delivery problem but wouldn't that show up at any speed?
Any ideas what to try next?

TIA

Jim

[scabzzzz]

Fuel filter? Sounds like some sort of fuel evap issue.

[JMI]

Fuel filter? Sounds like some sort of fuel evap issue.

Fuel filter was one of the first things I changed - no go.

Thanks

[thejlevie]

A fuel filter or pump problem most often affects higher rpm operation before it affects idle.

On this engine valve adjustment, intake leaks, bad plug wires, etc., are more likely to cause issues at idle.

[JMI]

On this engine valve adjustment, intake leaks, bad plug wires, etc., are more likely to cause issues at idle.

The valves were just adjusted (.012 - cold). Have sprayed throttle cleaner all around the vacuum hoses while running looking for a change in idle speed - nothing .
Can the ICV test out OK but still have an intermittant problem?

Jim

[bloxsom]

how do you test the icv? just try cleaning it and the HFM
good luck

[JMI]

how do you test the icv? just try cleaning it and the HFM
good luck

Check the resistance across the 3 pins. Sould be approximatly 20/20/40 ohms. I has been cleaned twice. When closed it is air tight.
What is the HFM?

Thanks

Jim

[thejlevie]

I think he/she meant AFM (Air Flow Meter). If the ICV is free to operate and isn't open or shorted it is probably okay. That would suggest that the problem is elsewhere.

[JMI]

The Car: 1989 535i Auto 125,000 miles

The symptoms: Happens only at idle (about 700 rpm). The engine will start to stall, rpm dropping off, and then will surge momentarily to about 1100 rpm and then settle back to 700 rpm. Sometimes it will completely stall. I can usually prevent this surge/stall by slightly stepping on the accelerator.
There appears to be no regular pattern when this happens except maybe when slowing quickly.
Stomp test gives 1444 - no code.
Can't find any vacuum leaks, but have not ruled that out altogether.
Have cleaned ICV throughly. It buzzes when engine runs. Resistance is in range -approximately 40/20/20.
Throttle Position Switch checks out OK.
Tempted to change out ICV but don't want to spend $145 bucks for nothing.

Unbelievable!
Posting, reading, playing parts jockey. Sensors, sensors, sensors!!! Checked them all or so I thought. Checked the resistance/condition of spark plug wires twice. Saw no need to replace. Not once anywhere does the pulse generator/cylinder ID sensor - #9 come up.
Maybe because the pulse generator is generally included with spark plug wires it does not get mentioned. When someone replaces the wires in an attempt to fix an idle problem what really is doing the fix is the new pulse generator.
Bentleys only shows it in a picture where it plugs into a wiring connection but no discussion of it or how to test it. So much for the Bentley BMW bible
Arrrggghhhhhh
Kudos to Eastside Bavarian, Issaquah, WA (424 392-4450) for pinpointing the problem. 1 hour charge for the diagnostic. Replaced the spark plug wires/cylinder ID sensor myself, saved a few bucks.

The car is now again a joy to drive

Thanks to all who offered their help!

Jim<!-- / message -->

[Robert Venable]

WTF does this pulse generator do?

[jensklemp]

WTF does this pulse generator do?

Generates a pulse duh! Haha i'm sorry plz dont kill me...

Simple pulse generators usually allow control of the pulse repetition rate (frequency, pulse width, and the high- and low-voltage levels of the pulses. More-sophisticated pulse generators may allow control over the rise time and fall time of the pulses. Pulse generators may use digital techniques, analog techniques, or a combination of both techniques to form the output pulses. For example, the pulse repetition rate and duration may be digitally controlled but the pulse amplitude and rise and fall times may be determined by analog circuitry in the output stage of the pulse generator. With correct adjustment, pulse generators can also produce a 50% duty cycle square wave.

Taken from Wikipedia... not sure if it helps or if its even the right thing. But hey, I tried!

[tonymasone]

try seafoam it's an extremely cheap shot, like $6 bucks I think, I've heard nothing but good things, and I actually just used it tonight on my M Roadster which had a slightly rough idle and a low RPM laggy throttle, man did it clean that up almost immediately. look at the instructions, it's worth a shot for how cheap it is, this could do absolutely nothing for you but for $6 bucks who cares

[JMI]

WTF does this pulse generator do?

A pulse generator is also known as a magnetic pick-up. Often used to determine rpm or a position. In this case when #6 sparkplug fires an electrical current is induced in the induction coil surrounding the spark plug wire and this is used as a signal for the electronics that controls the fuel injection. Basically lets the brains know where in the firing cycle the engine is.

[Robert Venable]

AHHHHH, I know exactly what it is . I believe BMW calls it the 4a line or someting like this in there books.