The Autonerdz Community Forums - Misfires on BMW M20 Motor (Oscilloscope Pics)

el101bob

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Misfires on BMW M20 Motor (Oscilloscope Pics)
Jul 17^th, 2008 at 11:29am

1989 BMW 325i with Turbo M20
MS-II Extra January 5th, 08 revision
MS-II v3.0 PCB
Single VB921 with stock coil and distributor ignition system
2.5ms Dwell
85lb-hr Low-Z Injectors
Scosche Injector FET Mod

I'll take it for a drive and try and get a datalog and msq.

Primary Ignition Current Ramp (50mV/division,2ms/division,1A/10mV using R43 voltage drop)

Primary Ignition Voltage Waveform (0.5V/division,2ms/division)

I tried to capture the sound of the misfires on video, but all you can really here is the valves. Here's a link anyway.

http://www.youtube.com/watch?v=EalN3lqpleg

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Todd Huggard Senior Member Offline One day I'll get it Posts: 129 Edmonton, Alberta Alberta Canada Gender:	Re: Misfires on BMW M20 Motor (Oscilloscope Pics) Reply #1 - Jul 18^th, 2008 at 7:41pm Hi Bob, I cant see that very well.
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Todd Huggard Senior Member Offline One day I'll get it Posts: 129 Edmonton, Alberta Alberta Canada Gender:	Re: Misfires on BMW M20 Motor (Oscilloscope Pics) Reply #2 - Jul 18^th, 2008 at 7:43pm When I double clicked on it it was bigger, but still. I hope some one else has a comment.
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Shannon Lindsey

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Posts: 91

Re: Misfires on BMW M20 Motor (Oscilloscope Pics)
Reply #3 - Jul 23^rd, 2008 at 10:47am

el101bob wrote on Jul 17^th, 2008 at 11:29am:

1989 BMW 325i with Turbo M20
MS-II Extra January 5th, 08 revision
MS-II v3.0 PCB
Single VB921 with stock coil and distributor ignition system
2.5ms Dwell
85lb-hr Low-Z Injectors
Scosche Injector FET Mod

I'll take it for a drive and try and get a datalog and msq.

I would say for the kind of vehicle you've got set up there and the kind of diagnosis you are looking to do that you may need to make use of a DSO (digital storage oscilloscope).

I would recommend making and sharing the following captures:
- Primary ignition coil current, primary ignition coil drive, and enough time to show all the cylinders during the misfire (with a cylinder trigger)
- Secondary ignition pattern with all cylinders during the misfire
- Injector current waveform (preferably at a common point like a fuse or harness), injector drive, and enough time to show all the injectors during the misfire (with a cylinder trigger)

I would also like to know what the firing order is. If you have access to a gas analyzer, I would recommend recording the readings during the misfire and sharing those as well.

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Sleuth

Ex Member

Re: Misfires on BMW M20 Motor (Oscilloscope Pics)
Reply #4 - Jul 24^th, 2008 at 5:51am

Hey Bob,
Noticed the plug wires you have on the car..motronic has an uncanny ability to learn the(internal) correct fuel map to get you on. The way I have found best is to let it idle for 5 minutes--complete a warm up cycle, turn the a-c on and idle for 5 minutes(load sensor) then drive normal till you get in 3 rd gear , then do a full tilt hard pull on the engine. May want to drain the learned values before you attemp this procedure. I have setup a scirroco CIS turbo`ed car once using the Motronic controller--would blow off a vr-6 with the 100hp supercharger...talk about torque steer

What are you using for fuel enrichment and boost control? I noticed a few hoses coming from your intake plenum? I used a couple of fuel injectors (cis style) fabricated into the plenum controlled by the merkur turbo boost control devices available back in the day. Once I got all that setup the motronic controller took over and did a sweet job of getting me on the right curve. Your valve`s do sound noisy-Is that the spec for the cam?..perhaps a relash--critical on that engine. Perhaps you should try the OEM wire set-grounded.Ought to be some sleeper when you get it tuned

You have alot of device`s there i`M NOT familiar with..hard for an ole dog to hunt with new technolgy sometimes. SLow down with the camera and give us some more pics of your basic set-up with an explanation of controls. You using a MAF or Airflow sensor or what? May find some info to help you at http://www.splitsec.com/

Nowdays they make kit`s....

Nice laptop software?? Got any uef or dbc datbase files to share?
Looking for any vector/canoe databases files I may be able to use in my playing?
Dave

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joecar+

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Picogroup

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Posts: 443

Southern California
California
USA

Re: Misfires on BMW M20 Motor (Oscilloscope Pics)
Reply #5 - Jul 24^th, 2008 at 11:01pm

Hi Bob,

Your secondary voltage trace shows the coil charging for ~2.4ms and then firing...

the spark line is ~1.2ms...

your primary current shows the coil being charged;

I'm still new to diagnosis-by-oscilloscope... those waveforms look ok to me...

Dave indicates you need to capture some waveforms with motor under load... a bit hard to do driving around with a "real" oscilloscope.

Cheers
Joe

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« Last Edit: Jul 25^th, 2008 at 9:11am by joecar+ »

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