P1351 one time before MPFI conversion, worse after (MISFIRE CYL 4)

[east302]

The plug was dry?

The blackened base ring, to me, means it's running rich. I think that the range of discoloration on the ground strap as it turns 90-degrees to the threaded base is an indication of timing, but I'm not sure how to interpret that.

What brand are the plugs?


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[bigfootchiro]

The plug was dry?

The blackened base ring, to me, means it's running rich. I think that the range of discoloration on the ground strap as it turns 90-degrees to the threaded base is an indication of timing, but I'm not sure how to interpret that.

What brand are the plugs?


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They are AC Delco Iridium. Base looks a little oily too, could be valve? I was thinking timing too, but could the MPFI really throw the timing that far off? Another thought... I have a high volt coil AND very low resistance spark plug wires, which would all cause a hotter spark. Maybe I need to go with a colder plug?

 

[bigfootchiro]

UPDATE!

Turns out the injector on Cylinder 4 is not pushing enough fuel into the cylinder causing the misfire. Going to have the MPFI inspected and flow bench tested to find out why.

 

[east302]

Glad you tracked it down. The injector may not be seated enough. I remember on mine the new style didn't really "click" in as much as I thought they would.

Out of curiosity, what brand MPFI did you use?

Were the lines routed right? May have a slight kink since they're pretty much packed in there.


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[willxfs]

Bigfoot,

Can you post where your are getting your injectors flow tested and any other pertinent info. I installed a set of delphi mpfi injectors several years ago and am thinking I need to get mine tested and/or cleaned. Bran new rebuild of the motor is not quite happy at idle. I think I have maybe 40-50K miles on the injectors. Thanks.

 

[bigfootchiro]

Glad you tracked it down. The injector may not be seated enough. I remember on mine the new style didn't really "click" in as much as I thought they would.

Out of curiosity, what brand MPFI did you use?

Were the lines routed right? May have a slight kink since they're pretty much packed in there.


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I bought the AC Delco brand. When I was installing it, I noticed that they just kind of slid in instead of snap, too. Thought that was weird! They were ran correctly (ones toward the middle of the manifold were routed over, I think.) hopefully it's just a kink and no damage!

Bigfoot,

Can you post where your are getting your injectors flow tested and any other pertinent info. I installed a set of delphi mpfi injectors several years ago and am thinking I need to get mine tested and/or cleaned. Bran new rebuild of the motor is not quite happy at idle. I think I have maybe 40-50K miles on the injectors. Thanks.

Willxfs, I'm having a guy by the name of Adam at Quality Oil Exchange Brakes and Auto Repair in Delray Beach, FL work on the Hoe. He owns a Tahoe with Vortec 350, that is lifted and in pristine condition. He has worked on his since day 1, so he knows these trucks inside and out. Very nice guy, knows his stuff, and very reasonably priced!

 

[willxfs]

Thanks Bigfoot. I've dug around on the internet and haven't found a place that specifically says they work on our model fuel injectors. Most seem to accept the individual style injector. Hope you get fixed up!

 

[east302]

Willxfs,

GM has a procedure (fuel injector balance test) for measuring pressure drops across each injector to evaluate fuel flow. Along with a scanner to command the pump, they use tool #J-39021. Maybe a shop could run the test for you, maybe an hour or so of labor?




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[bigfootchiro]

I am seriously racking my brain right now... Truck runs better, it's much more drivable, but still misfires like a crazy woman. Here are the things that are worse now, and I think they are related:

• Torque converter would have intermittent lockup problems. Every once in a while it would take some time to start locking up at cruise speeds, even if warm. It has become worse now. I have read that this could be related to the PCM reading misfires.

• If I blip the throttle quickly, as to rev in Park (say 0-50% throttle or more) it stumbles/hesitates like it's about to die like a flooded engine would. If I blip it lightly, and it doesn't stumble, there is a delay in its response from my foot to rev. Same thing happens in Drive and Reverse, too. (Used to do it in only Reverse, but not nearly as bad as it does now, so it has become worse).

• When the TC locks up, the jerk/misfire becomes more prevalent, but clears out more as speeds increase (practically unnoticeable about 80 mph). When it occurs, RPMs drop about 200-300, as if someone takes a light switch and turns them on and off quickly just to F*&^ with your brain! One of which is reproducible at the same speed, same RPM, every time, where it drops 200-300 RPMs. The jerk/misfires can happen so hard sometimes that the driveline/u-joints clunk.

• I used to have an occasional rattle/clunk on cold start, which sounded like two lines rattling together, but now it is worse. Could be due to misfire causing the motor to move more, making that clunk/rattle worse.

• If I floor the dang throttle, I can possibly feel that off the start it may be a little hesitant, but after that it runs like a bat outta hell! No misfires, no nothin', just screams like demon during an exorcism.

• Ran DashCommand today, and watched misfires. They happened on #1 and #2 about the same amount, a few more on #3, then much higher on #4, but Cyl 5-8 had 0. Could this be a clue? Where in the world is Carmen San Diego? I have no idea...

• I read on another forum that a guy with a 4.3 was experience exactly what I am, and found that his distributor gear was torn in half. Replaced it and back to normal. Possibility? Maybe. Possible TPS? IAC? MAF was cleaned yesterday, along with replacing the fuel filter, and timing was adjusted.

Today I am going to swap out the high volt coil for the old and see if that works. If not, I'm going to put the old plug wires back on, then look at the ground wires. I dread pulling the distributor, because I don't want to fudge up the timing. Any other ideas would be greatly appreciated, because I am getting close to my last nerve on this thing haha.

 

[east302]

Before you pull the distributor, mark the base vs intake manifold and the rotor position on the side of the housing. Call that mark #1.



As you pull the distributor, the rotor will rotate counterclockwise about 45-degrees. Mark where it stops (mark #2).



When reinstalling, line up the marks on the shaft/intake manifold and position the rotor at mark #2. As it seats, it will rotate clockwise to mark #1 which will closely match the original installed position. Set the CMP offset with DashCommand and tighten down the hold down bolt.

When you swapped the VCM, did the original spark control module get moved to the new VCM? Was the CKP relearn done?


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