Weird Misfire '99 GMC Sierra K1500 5.7L

[Schimpfy]

Looking for some guidance with a weird misfire in a '99 GMC Sierra K1500 5.7L. I've got a P0300, but no cylinder specific codes.

At idle while in Park cylinder 1 has an occasional misfire. When I put it in Drive and still idling cylinder 2 starts misfiring pretty heavily and 1 still has the miss once in a while. Finally, when under load the heavy misfire moves entirely from 2 to 3.

Once in a while I'll get a P0130 for an O2 sensor that's becoming more frequent. I'm going to change both upstream sensors.

New plugs, wires, cap, rotor and coil, but the problem persists. At this point I'm out of ideas.

 

[PWC Repair]

Check for a cracked reluctor wheel in the distributor and ohm out the pickup coil. I'm guessing one of those is causing your issue.

 

[Schurkey]

WHAT VEHICLE??? WHAT ENGINE???

Looking for some guidance with a weird misfire. I've got a P0300, but no cylinder specific codes.

Is your scan tool lying to you? Is is capable of displaying cylinder-specific misfire codes? Have you seen cylinder-specific misfire codes using this scan tool on other vehicles?

At idle while in Park cylinder 1 has an occasional misfire. When I put it in Drive and still idling cylinder 2 starts misfiring pretty heavily and 1 still has the miss once in a while. Finally, when under load the heavy misfire moves entirely from 2 to 3.

You're seeing misfire counts for each cylinder? How do you know which cylinders are misfiring if the scan tool says it's "random"?

How old are the intake manifold gaskets?

Once in a while I'll get a P0130 for an O2 sensor that's becoming more frequent. I'm going to change both upstream sensors.

Good plan. How old are the downstream sensors?

New plugs, wires, cap, rotor and coil, but the problem persists. At this point I'm out of ideas.

What kind of fuel injectors? How old (time/mileage) are the injectors?

What is the fuel pressure at prime, at idle, and under load?

How old is the distributor, and have you checked for distributor gear wear, and distributor shaft bushing wear?

 

[Schurkey]

Check for a cracked reluctor wheel in the distributor and ohm out the pickup coil. I'm guessing one of those is causing your issue.

"P0130" indicates OBD-2.

OBD-2 on a GMT400 web-site indicates '96--2000-ish Vortec, probably 5.7L although that's not confirmed.

Vortec V-6, 5.0, 5.7, 7.4 don't have a reluctor wheel or pickup coil in the distributor.

 

[Schimpfy]

WHY ARE YOU YELLING AT ME!

My truck is in my sig, but I forgot that on mobile browsers it doesn't show up by default. Anyway, added info to OP and thread title.

I'm using the Torque app to monitor so I can see the count. I've used the same app (with a different profile, obviously) on my 2006 Silverado and have gotten cylinder specific codes. After resetting faults and a 20 minute city drive the misfire indications are as follows: Cyl 1 = ~250, Cyl 2 = ~2,500 and Cyl 3 = ~3,000. I get the P0300 within a few seconds of starting the truck, but I find it odd that it doesn't throw any cylinder specific misfire codes. After about 10 minutes I get the P0130 as a pending fault.

Downstream O2 sensors were "deleted" in a tune a few years back and I never had a problem with that. As far as the upstream O2 sensors I'm replacing them since after an incident with a bent cylinder 6 exhaust valve I'm wondering if either of them has been fouled.

I upgraded the OE spider injector to MPFI about five years ago and at the same time I replaced the distributor. They've got about 3,000 miles on them. I'm not for sure about fuel pressure numbers as I haven't had a chance to check them yet. As a side note, it does smell like it's running pretty rich.

 

[Schurkey]

"Smells rich" means little or nothing. A rich mixture produces heaps and piles of CO, and CO is colorless and odorless. Mixture has to be enormously rich to throw much in the way of HC.

A misfire produces heaps and piles of HC, and HC is what makes your eyes water and throat swell. And we know you've got misfire.