Another Cylinder Misfire....

[dvmotorsports]

This time it's on a friends sons car. He picked up a 1991 318is for $250 because of a cylinder misfire. Pretty good score, but it has some rust which is typical of an original Hawaii car.

Some of you may remember this "Misfire" thread about my wifes car.

http://m42club.com/forums/showthread.php?t=10406

This is similar to hers. But her car was a complete no fire in the cylinder. This one is intermittent.

Anyhow, here is where I am at with this. Cylinder 3 has an intermittent misfire. I pulled the plug wire and you can here the spark for 3 to 10 consecutive duty cycles and then it will drop out for a few seconds, repeat.

I diaged it for about 5 hours today.

I have tested and or replaced the following parts

DME
Coil Pack
Cam Pos. Sensor
Plug Wire
Spark Plug

I have voltage and ohm tested all parts but the misfire is still present. All the values I got back from the meter are within the acceptable range. I also swapped the coil and plug wire between cylinders, but #3 is still intermittently missing. So the problem has to be before that. But even when I swapped the DME and tested the front and back side of the connector at the DME harness, still present.

I am ruling out the Crank Pos Sensor as the motor won't run without it.

What else is left in the system?

[jscribble]

Could there be a short in the wiring to the coil?

[dvmotorsports]

Could there be a short in the wiring to the coil?

That is what the consensus seems to be. I am thinking the voltage is grounding somewhere due to a break in the wire insulation. And I am guessing this is in the loom somewhere. I am going to rewire everything for cylinder three. That should do the trick.

Where is Desktop Dave.......

[jscribble]

Desktop Dave is my hero too. Lol. Good luck to you.

[dvmotorsports]

Okay, so after snipping some wires, I found out the Black wire from the DME to the coil is shielded. After stripping the shielding, it appears there was some internal arcing. Am I seeing things, or could that be the source? I'm thinking so.

[jscribble]

It could be. If it shorted somehow, it could have heated the wire and damaged it, even inside the shielding. maybe a trip to the jy or some rewiring is the answer. Can you test with a multimeter? See if you can test the resistance on the wire.

[DesktopDave]

That is what the consensus seems to be. I am thinking the voltage is grounding somewhere due to a break in the wire insulation. And I am guessing this is in the loom somewhere. I am going to rewire everything for cylinder three. That should do the trick.

Where is Desktop Dave.......

Undeserved, but thank you anyway.

Desktop Dave is my hero too. Lol. Good luck to  you.

I am honored.  I figured I should add some pithy Zen saying but I can't think one up.

I'm thinking the wires too.  I was unaware that the common was shielded.  I'd assume it's a bit of very high-quality work by the Bavarians if it's lasted this long.  I'll do some snooping to see where a substitute might be found.  

I'd assume an e36 could provide a nice bit of loom for splicing, but those little coax cables are troublesome.  I've spliced some VGA looms in my day and it's not an easy job.

[dvmotorsports]

I was unaware myself. And this is not shielded in a sense similar to a household coax. They way this is configured is as such

There is small gauge 18ish with the insulation. Around that insulation is another woven wire set followed by the outer insulation. Then end result appears to be a stranded 14 gauge. So there is no metal shielding. It's wire, insulation, wire, insulation.

On another note, 400 posts! Kinda sad it took me over two years to get there.

[DesktopDave]

I'm sure your M3 build will help with the post count...  :cool:

Found this odd little tidbit...http://www.picoauto.com/tutorials/trigger-signals3.html

I'm off to dig a bit more.

[dvmotorsports]

I really wish I had an oscilloscope.

[DesktopDave]

I was thinking about one of the sound card 'scopes.  This one is only about $30 (I'm not sure of quality as I've lost my bids):

http://cgi.ebay.com/Sound-Card-Oscilloscope-Spectrum-Analyzer-Probe-audio-/120580967632?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item1c132f8cd0

[jscribble]

It sounds to me to be similar to many computer peripherals use for wiring. They use a braided shield to protect low voltage signals from radio interference. It would stand to reason that the wire you're tackling has a similar function.

It should be repairable if the damage is a short section, strip back the outer cover, separate the inner wire from the outer "shield" wire, and make any repair solder joints staggered. Be sure to use heat shrink generously on the inner wire, and over the outer.

Otherwise, donor wiring would be nice to have laying around. I thought I might have some wire similar to what you described in my project box, but it was super light gauge. Unless this is 20+gauge, I don't think it'll fly.

[DesktopDave]

Analog PC video cables have three nice coax cables internally.  I'm not figuring it'll do well at high temperatures though.  I'd suspect some RG6 wouldn't do the trick either...

[jscribble]

Agreed. Wasn't there a harness is the fs? I wouldn't even know where to source that kind of wire, and I run about 2-300 feet of wire every month. mcm electronics has oodles of wires, coax and shielded, but nothing I know of for higher temps.

[dvmotorsports]

You guys are confusing the hell out of me.

I will say this, the wiring has a name on the insulation. "KabelMetal"