Idle missfire, What to log

[Biggsy]

Ive been chasing a Gremlin in my car now for a few weeks now:bash:

It's a missfire when car is idling (VX LS1). I would drive the car around without a hitch, come up to a stop, then every so often it would get a small missfire. It doesnt create a code, it drops rpm about 50 then stall saver kicks in with some timing and it goes back to idle normal again.

This happens once every 30 secs or so. The more I load the motor up electrically (fan, blower, high beems etc), the more often it misses (batt voltage is lower)

I'm thinking along the lines of maybe a coil/module problem?

Is there any PID I can log that will tell me if this problem is happening on the same cylinder everytime?

Anyone with thoughts as to what this problem is?
1392

Cheers,

[joecar]

Dave,

MFCYLnC and MFCYLnH where n = cylinder number.

Possibilites:
Spark plug wires
Spark plugs
Intermittent air leak
Intermittent ground
Other

[Biggsy]

Tried those PID's, not registering a thing.

Cheers,

[limited cv8r]

have you tried using the bidi controls - spark/fuel to see if it makes any difference. I had a similar condition and found that my VE table had too much of a difference between a couple of idle points and was causing a stumble/misfire type situation,it was alternating between a good cell and a very lean one.

[Biggsy]

have you tried using the bidi controls - spark/fuel to see if it makes any difference. I had a similar condition and found that my VE table had too much of a difference between a couple of idle points and was causing a stumble/misfire type situation,it was alternating between a good cell and a very lean one.

I don't think its a tune problem, haven't modified my tune for months.
This problem only started in the last couple of weeks.

Going to pull the plugs tonight and test the leads (one of joe's possible suggestions). Maybe I am looking for something more complicated than what it really is.

Still would be nice if I could narrow down the cylinder its doing it on.
If I had an engine analyser I could see for sure, but I was hoping the scan tool could help.

Was loading up the electrics last night and was getting a miss every couple of seconds, its getting worse:bawl:

Cheers,

[joecar]

The "Midnight" Test:
Engine running,
Pitch Dark (no lights),
Do you see any spark plug wires glowing in the dark...?
If you spray water mist, do you see any glowing...?

[joecar]

...
Was loading up the electrics last night and was getting a miss every couple of seconds, its getting worse:bawl:
Dave,
What loads do you have...?
Might also be a loose/dirty ground.

Cheers,
Joe

[Biggsy]

The "Midnight" Test:
Engine running,
Pitch Dark (no lights),
Do you see any spark plug wires glowing in the dark...?
If you spray water mist, do you see any glowing...?

About to go to my mates w/shop and change the plugs (just for the hell of) and read the plugs to see if I can pick the cylinder. Will test the leads while I am at it.

If problems persist, I was thinking about doing your "midnight test" anyway.

Cheers,

[Biggsy]

Dave,
What loads do you have...?
Might also be a loose/dirty ground.

Cheers,
Joe
1: Ok, it will miss a little if I just have the motor running.
2: Turn A/C and fans on, misses twice as much.
3: Turn high beams on as well, misses 3 times as much.
4: Turn Demister and crank stereo/subwoofer up (found a song that really makes subwoofer work) and every few seconds a miss happens.
Will miss slightly more if trans is in gear.

Haven't had a chance to put my clamp meter on (being repaired) so I don't know the current, but it would be a shitload.

Batt volts 1: 13.8v
2: 13.5v
3: 13.2v
4: 13.0v - 12.9v (depends on subwoofer)

I don't ever have these sort of loads on at once, high beams and stereo cranking while staionary never happens.
Worst case: demister, A/C cranked, low beams and mild stereo

I was just seeing how load affects it.

Cheers,

[Biggsy]

:jump: :jump: FIXED!!!!! :jump: :jump:

Pulled all leads off, measured their resistances, found one measuring 20-40M Ohms!

Didn't have spare, modified v6 lead to get it going. Put new plugs in anyway.

Smooth as8)

Thankyou All :thankyou2:
Thankyou Joecar
Thankyou limited cv8r

[Biggsy]

Still find it interesting that the Missfire PID didn't pick up the cylinder that was having the problem???

[joecar]

Misfire fixed... :cheers:

[joecar]

Still find it interesting that the Missfire PID didn't pick up the cylinder that was having the problem???Hmmm... I'll check mine after I wake up later this morning (...I'm up late again...).

[Biggsy]

Is there any aftermarket leads you can get that are better than OEM?
Or is it not worth the effort?

I noticed the leads were quite low in resistance than say a 9mm lead of equal length.

[Biggsy]

Hmmm... I'll check mine after I wake up later this morning (...I'm up late again...).

LOL, keep of the caffene!

[joecar]

Is there any aftermarket leads you can get that are better than OEM?
Or is it not worth the effort?

I noticed the leads were quite low in resistance than say a 9mm lead of equal length.My opinion (backed by my experiments) is that low ohm spiral wound spark plug wires show no external magnetic field (which I can easily explain using EM theory, and I can demonstrate using an inductive clamp and scopemeter), so no voltages can be induced in other conductive objects in the engine bay (head, block, header, etc), and therefore no energy is lost (it takes energy for a varying magnetic field to induce a voltage).

So the ignition coils only have to work at driving spark and not at losing energy to the engine bay. :D

[Biggsy]

:cheers: