#2 Misfire at Idle, Random/Multi While Driving

[boy&hisdogs]

98 K1500, 5.7/4l60e, 198k miles (It's the truck in my sig)

Three times now over the past year or so my truck has given me P0302 (Misfire #2) at idle and low speed/rpms, like when I'm pulling out of the driveway. The first time it went away on it's own after a few days, before I could figure out what caused it. The second time it lasted a couple weeks and went away a few days after I replaced the plugs and wires (Probably a coincidence?). It's back again and now it also gives P0300 (Random/Multi cylinder misfire) while driving at road speed. It starts strong and has enough power to drive around on flat land but climbing big hills it feels weak.

So far I have done new plugs, wires, cap, rotor and coil, all AC Delco, Seafoam in the oil and vac hose (surprisingly little smoke), and two rounds of Lucas injector cleaner in the gas. No noticeable change. I don't mind throwing cheap and easy parts at it that it probably needs anyway, but now it's time for more serious diagnostics. I'm worried that it might be the injectors/spider, since as far as I know it's never been replaced or even checked. I'm the 2nd owner, I bought it in 2010 @ 62k miles. I did replace the intake manifold gasket soon after buying the truck and replaced the fuel pump around 6-8 years ago with a Bosch one that runs smooth and quiet.

Is there any way to check the injectors without taking the intake off? What about the distributor itself? I've heard of them stripping teeth around this mileage. What else should I check? Fuel pressure and compression seem like the next step, but if those are fine then what?

I don't know if this helps, but I pulled some interesting data with my scanner. Bank 1 O2 sensors read like a bad cat and bank 2 reads clean despite the misfire. This was with the engine warm and cruising down the highway about an hour long drive.

EDIT: Might be worth noting that the misfires came back recently after about a week of the truck sitting without running/driving.

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[stutaeng]

I see a lean problem with the long term fuel trim on bank 2. Was that at idle?

It's pretty easy to do a good guess if it's a vacuum leak or something else. Rev up the engine and see what the fuel trims do.

With a more advanced diagnostics tool you can do an injector balance test. But even a fuel pressure gauge is a good starting point, but seems less likely in your case to be fuel delivery problems

 

[boy&hisdogs]

I see a lean problem with the long term fuel trim on bank 2. Was that at idle?

It's pretty easy to do a good guess if it's a vacuum leak or something else. Rev up the engine and see what the fuel trims do.

Both were at highway RPM. I forget exactly what but somewhere around 2000-2300ish. Edit: got that backwards. Fuel trim early in the drive and O2 later (same drive)

 

[stutaeng]

Then more likely an injector failing to open.

No lean codes?

My 5.7 always had this P0174 bank 2 lean code. Took me a while to figure it out, but it was injector #6 dead. I also had an intermittent O2 sensor code. Prior to figuring it out, I had changed all 4 injectors...of course, that didn't fix the problem.

My LTFTs we're also around 25 percent...idle or revved, it stayed at a consistent 25. That immediately ruled out vacuum leak.

After finding the bad injector with injector balance test, I double checked that I had injector pulse at #6, which I did.

 

[boy&hisdogs]

Then more likely an injector failing to open.

No lean codes?

My 5.7 always had this P0174 bank 2 lean code. Took me a while to figure it out, but it was injector #6 dead. I also had an intermittent O2 sensor code. Prior to figuring it out, I had changed all 4 injectors...of course, that didn't fix the problem.

My LTFTs we're also around 25 percent...idle or revved, it stayed at a consistent 25. That immediately ruled out vacuum leak.

After finding the bad injector with injector balance test, I double checked that I had injector pulse at #6, which I did.

Nope no other codes for a long time. I'll do what you said about watching the trims when I rev it tomorrow when I drive again, but I guess it's time to start looking at injectors just in case.

 

[stutaeng]

It's weird sometimes on the fuel trim codes. A few weeks ago I was helping my nephew diagnose his 4.3 Vortec S10. The thing was running like crap. Had a hard time restarting when warm. One bank was running like 50 percent rich, but no rich codes!

We hooked up a fuel pressure gauge, and the pressure went from like 60 to almost zero in 30 seconds. Either an injector was stuck open or the FPR was leaking.

He got a new spider assembly (comes with a FRP) and it runs smooth again.

 

[boy&hisdogs]

It's weird sometimes on the fuel trim codes. A few weeks ago I was helping my nephew diagnose his 4.3 Vortec S10. The thing was running like crap. Had a hard time restarting when warm. One bank was running like 50 percent rich, but no rich codes!

We hooked up a fuel pressure gauge, and the pressure went from like 60 to almost zero in 30 seconds. Either an injector was stuck open or the FPR was leaking.

He got a new spider assembly (comes with a FRP) and it runs smooth again.

Is there a way to check the injectors from the top of the spider? Even if it's electrical only and not actually looking at spray?

It definitely wouldn't hurt to put the new style spider in the truck, except for my wallet of course.

 

[boy&hisdogs]

Here's some more fuel trim data I got this morning. Short term was fluctuating constantly, but it consistently had bank 2 higher than bank 1 by at least a few percent, if not more.

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[stutaeng]

Yep, not consistent, but bank 2 seems higher always. This little gadget allows you to do an injector balance test. All you need additionally is a fuel pressure guage. I bought the OTC version, but was much more expensive. . The videos I've seen on YT of guys using this cheaper version say it works well.

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The pressure drop of each injector should be within 1-2 psi of each other.

 

[Schurkey]

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I don't know "Blue Driver".

Looks to me like you've got a dead O2 sensor, or broken wires to it. One would think that would set a code. The other three O2 sensors look lazy--not enough cross-counts. That could be failing/ancient O2 sensors, or an actual problem with the injection system. It could also be an issue with "Blue Driver", using an expanded (very short) time base for those graphs. There's no time scale on those graphs.

When was the last time you replaced the O2 sensors?