Stumped on misfire

Mooseman

Original poster
Moderator
Dec 4, 2011
25,257
Ottawa, ON
This one needs the collective knowledge of the Nation. If I'm stumped, it's a total mystery.

On my '02 TB 4.2L I6, misfire on #6 with P0306 set. During diagnosis, I did the following:

- Swapped #5 and #6 coils, misfire stayed at #6
- compression test good (it was even better than #5)
- ran #6 coil out with an old plug, got spark
- replaced #6 coil twice with spares, all had spark
- Fuel pressure good at 55 psi when running
- ran injector balance test on all the injectors using the Tech 2, all dropped fuel pressure equally when individually activated, including #6.
- swapped #5 and #6 plugs, misfire stayed at #6
- swore at it and my son (whores know what I'm talking about)

About the injector balance test, this primes the pump and then cycles one injector while you watch the fuel pressure drop with fuel pump off. You can hear the injector working while the pressure drops and #6 dropped just like all the others so it is working.

Any and all comments and suggestions welcome.
 

littleblazer

Member
Jul 6, 2014
9,265
So you get spark by grounding it? How does it compare to others. In my expirence just because it sparks doesn't mean it's strong. I'd chase the wires at the harness/compare to others if you haven't already and check the head in the area, maybe the plug isn't making a good ground on the head? Since everything else mechanically checks out it has to be on the electrical side of the spectrum imo.
 

freddyboy61

Member
Dec 4, 2011
276
Another possibility is that the intake valve is not operating properly. Don't know how to check for that.
 

Sparky

Member
Dec 4, 2011
12,927
Well intake must be working (at least somewhat) if he's getting compression.

I'm assuming you cleared the codes and/or viewed live data to determine the misfire stayed at #6 each time, right? In past experience, moving a problem coil/plug/whatever to another cylinder would not change the code very quickly, it would take the computer some time to determine the misfire moved.

Are you getting full 12V feeding the coil, and a good ground, and good connections? Weak voltage at the coil can result in a weak output.

How did the plug look when you pulled it compared to others?
 

Mooseman

Original poster
Moderator
Dec 4, 2011
25,257
Ottawa, ON
So you get spark by grounding it? How does it compare to others. In my expirence just because it sparks doesn't mean it's strong. I'd chase the wires at the harness/compare to others if you haven't already and check the head in the area, maybe the plug isn't making a good ground on the head? Since everything else mechanically checks out it has to be on the electrical side of the spectrum imo.

I'm thinking the same way. I did find the spark on it kinda wimpy but that's just me. I'll try to compare to #5, which would mean running it with two cylinders misfiring.

Another possibility is that the intake valve is not operating properly. Don't know how to check for that.

If the compression is any indication, I wouldn't think so unless one of the valves isn't opening, but you'd think it would still run? I also don't have any mechanical noises pointing this way.

I'm assuming you cleared the codes and/or viewed live data to determine the misfire stayed at #6 each time, right?

Yessir!
Are you getting full 12V feeding the coil, and a good ground, and good connections? Weak voltage at the coil can result in a weak output.

That would be my next step. I'll print out the schematics to guide me on that.

How did the plug look when you pulled it compared to others?

Normal, same as #5, not wet or anything, which would make me think that's it's the injector but I tested it. Now I'm wondering if the injector works when commanded full tilt but won't work at partial.

I'm heading out to tinker with it some more....
 
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littleblazer

Member
Jul 6, 2014
9,265
I'm thinking the same way. I did find the spark on it kinda wimpy but that's just me. I'll try to compare to #5, which would mean running it with two cylinders misfiring.



If the compression is any indication, I wouldn't think so unless one of the valves isn't opening, but you'd think it would still run? I also don't have any mechanical noises pointing this way.



Yessir!


That would be my next step. I'll print out the schematics to guide me on that.



Normal, same as #5, not wet or anything, which would make me think that's it's the injector but I tested it. Now I'm wondering if the injector works when commanded full tilt but won't work at partial.

I'm heading out to tinker with it some more....
I think @MAY03LT had said he did have something similar with injectors on a customers vehicle? I think it was more of a stumble though. It took multiple tests to see the pressure not work properly iirc.
 

Mooseman

Original poster
Moderator
Dec 4, 2011
25,257
Ottawa, ON
OK everybody, stand down. It's fixed.

6a00d83451b6fc69e2016767dbc29c970b-800wi


It was that bad a fail on my part. Try to guess what it was and click below for the full story.

Went into GM-SI with the troubleshooting charts for a P030x, things suggested, check each injector resistance. So went back and started delving deeper into this. First checked the voltages and grounds, all good. Pulled the #5 coil, sparked strong. Checked #6 again and it was just as strong. Then went to the injector connector to start checking the injectors, which is located at the back of the valve cover. Pull the plug and start peeling the tape to see the wires, I glance at the coil just next to this plug and I just hung my head. All this time, I was dicking around with the NUMBER 1 COIL! For whatever reason, I started working on the #1 coil and swapping with #2.
:duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh::duh:

I will now go into exile. Maybe they'll take me at TrailVoy :hahano:
 

Sparky

Member
Dec 4, 2011
12,927
LOL

That may be why nothing changed!

So, swapping the actual #6 coil, what's the results? :biggrin:
 

DAlastDON

Member
Apr 6, 2014
5,550
Kentucky
When working from the drivers side coil #6 is on the right, under the windshield cowl. When working from the passenger side coil #6 is on the left, under the windshield cowl. When working from the front #6 is all the way in the back, under the windshield cowl.

flat,800x800,075,t.u1.jpg
 
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Mooseman

Original poster
Moderator
Dec 4, 2011
25,257
Ottawa, ON
So, swapping the actual #6 coil, what's the results? :biggrin:

It was dead. Replaced it with a spare, instantly running fine :Banghead:
 
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MAY03LT

Member
Nov 18, 2011
3,412
Delmarva
Dang I was looking forward to injector balance testing discussion but glad to hear it's resolved.

Also LB I was just talking about that s10 from hell the other day. It took multiple tries of the IB test to get them to fail, which threw me through a diagnostic loop.
 
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littleblazer

Member
Jul 6, 2014
9,265
On the inlines I count front to back... I had a fail as well...:rotfl:
 

Mooseman

Original poster
Moderator
Dec 4, 2011
25,257
Ottawa, ON
That's the way you're supposed to count them. For whatever reason, I counted from back to front. I think I may have used the axiom of determining position of parts from the sitting position in the vehicle (left=driver, right=passenger) and went from there.
:dunce:
 

littleblazer

Member
Jul 6, 2014
9,265
Yea.. I just failed.
 

djthumper

Administrator
Nov 20, 2011
14,950
North Las Vegas
Chalk this one up to counting backwards.
 

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