5.3l LT1 Misfire Under Load

2017 Silverado 5.3l, has code for oil pump solenoid which I tested and found it to be bad but am also chasing code p0300. Misfire data shows a lot of counts for cylinder 4 under acceleration and load. I removed valve cover to inspect valve train as I suspected a sticking AFM lifter but found no obvious issues, rocker and pushrod okay and lifter not stuck when cranking. The spark plugs on that bank were oil fouled, I looked inside with a borescope and saw a lot of oil coming out of one of the valves when opening. Engine also was 3 qts low of oil. Threw a pcv valve and new spark plugs and vehicle still misfiring on #4. Compression 190 psi and leak down test passed (other cylinders on that bank around 200psi) Relative compression does show #4(green trace)and #3 to be lower than the rest. Swapped coil and wire and misfire did not follow. The amount of oil that came out the valve has me suspecting bad valve seals but the misfire should have at least been resolved temporarily with a new plug if that was the cause. Also did an injector balance test and got bad results for the other bank injectors. Anyone have any suggestions or seen this issue before? Sticking valve/lifter?

Make sure the misfiring cylinder is identified correctly (probably he is, but anyway...)

If you don't have access to a pressure or pressure pulse sensor: the engine features both MAP and MAF sensors. You may use these signals to see what is going on in the intake manifold, at cranking, idle and snap throttle and go from there. You may want to employ a math channel on the MAF frequency as well.

The relative compression capture looks like there is significantly more difference in peak pressure than only 10 psi. Take another capture with a sync signal to identify cylinders

The green trace is cylinder 4, and yes, that is what's confusing me. Relative compression shows different results than the static compression. I will look into scoping map and maf since i don't have a pressure sensor, any links to some videos on this?

Is this a misfire under load only? Or does it misfire at idle and under a load? Not trying to be nitpicky, but it matters.

Also did an injector balance test and got bad results for the other bank injectors.

That's pretty typical on these engines, in my experience. Bank one always reads lower than bank two. I've always attributed this to the location of the fuel rail pressure sensor.

I don't place a lot of value on the automated injector balance test on GM's. These are the results I got from a '15 Silverado that had a lean injector on #3:



And these are the results after I replaced all the injectors:



Both shots show bank one lower than bank two, and the before doesn't highlight an issue with #3. But injectors fixed it. :silly:

EDIT: Are you sure this is an LT1 engine? The '17 Silverado 5.3L should be an L83.

You are correct, its not an LT1 engine, I fixed the title. And i stated that it misfires under acceleration and load. Idles just fine, do you suspect a bad injector?

Teranluis1991 wrote:

You are correct, its not an LT1 engine, I fixed the title. And i stated that it misfires under acceleration and load. Idles just fine

Sorry, I know you said it misses under load, I just wanted to clarify if it missed at idle. :blush:

do you suspect a bad injector?

Possibly, yeah. I like secondary ignition for finding these. If you're comfy with using the secondary leads included with the Ultra, you can hook up to two cylinders (#4 and a known good) and try to power brake the engine into misfiring. If you have to, you can stuff the VCMI into the engine bay and have the scope running in the passenger seat while you make it miss during a test drive.

A misfire from a lean condition (failing injector) will be pretty distinctive compared to a normal cylinder. I stole a screenshot from an SD video as an example:





But, on the other hand, there's not a whole lot of suspects left. :huh: You've moved ignition parts around, with new plugs. If you're sure that the #3 valves had equal lift and duration to other cylinders, then that'd pretty well rule out pushrods and camshafts.

I've seen these lifters stick and unstick during a test drive, but usually once they stick, they stay that way until the engine is shut off and restarted. That's part of why I was looking for clarification on a misfire at idle. Broken valve spring, maybe? Can't say I've seen many 5.3's with broken valve springs, though.

No need to apologize, I should've clarified better. I always miss important details when I type out my thoughts. And I will definitely scope the secondary when I get a chance, that's a great suggestion. Thank you for your assistance, I will keep you updated.

I went ahead and took some secondary captures. If you would be kind enough to review them with me I would appreciate it. Red trace is the misfiring cylinder #4 and green is known good cylinder #8. From what little knowledge I have regarding waveform analysis, cylinder #4 definitely resembles a lean misfire, would you agree that this leads us to believe the injector is faulty?

Teranluis1991 wrote:

I went ahead and took some secondary captures. If you would be kind enough to review them with me I would appreciate it. Red trace is the misfiring cylinder #4 and green is known good cylinder #8.

Absolutely perfect captures. :woohoo: Classic lean miss on #4. Were you able to get those while power braking, or did you have to drive it?

I grabbed a screenshot of your capture for anyone else following along:

From what little knowledge I have regarding waveform analysis, cylinder #4 definitely resembles a lean misfire, would you agree that this leads us to believe the injector is faulty?

I'm sold. 100% injector problem.

If it's me doing this job, I'm gonna strongly suggest doing all eight injectors, but settle on doing the four injectors on bank two if the customer doesn't like the price. That way, you won't have to pull the bank two fuel rail again if another injector decides to act up in the future. These rails can be a real pain to pry loose.

I had to drive it and do hard acceleration runs to get these results. I went ahead and ordered injectors and will confirm the fix tomorrow. Ive only been scratching the surface on this scope stuff and it is pretty amazing. It also definitely helps to learn by having great technicians like yourself that are willing to help others, thanks again for the help!

Teranluis1991 wrote:

I had to drive it and do hard acceleration runs to get these results. I went ahead and ordered injectors and will confirm the fix tomorrow. Ive only been scratching the surface on this scope stuff and it is pretty amazing. It also definitely helps to learn by having great technicians like yourself that are willing to help others, thanks again for the help!

Happy to help! We'd love to have a follow up after injector replacement.

Update on this one, replaced all injectors on bank 2 and misfires are no longer present. Even hooked up the scope again just to see the after results and the waveform on cylinder #4 remained normal and no longer had that second voltage spike. Learned some cool stuff on this one and I hope this post helps others.