HELP!!! Intermittent spark 2002 suburban 5.3
 

I used to frequent the 67-72 forum, but haven't been active for a few years. I will apologize in advance this will be a long one... I bought a 2002 Z71 suburban a few days ago that has me stumped. The seller was upfront that it had issues and as such I got it for a good deal. It starts and runs good, has a slight misfire at idle but drives great once moving. Only code showing is P0300-multiple random misfires. He said it was initially running very rough and he changed several parts trying to fix it including fresh oil, all 4 cats, oxygen sensor B2S1, plugs and wires, and intake gaskets. He said that made it run much better, but still getting P0300. He said he took it to a mechanic and they weren't able to diagnose the problem but said the majority of the misfires were on cyl 1 & 8 and the compression was good. Vehicle looks like it was used as a construction rig (dents, dings, trunk had sawdust, mortar, paint) but seems like mechanically it was well taken care of, I have the invoice for new trans less than 2 years ago, good tires, all new brakes/rotors, etc. My first thought was ground issue since I know this generation has issues with ground points, or injectors. i brought it home and ran some injector cleaner through it with no change, checked all the plugs were plugged in and not corroded, swapped some coil packs, pulled plugs and verified gap, checked compression (all cyl 135-145 psi) and no change, if I clear codes p0301 usually pops up within a minute, then changes to p0300 confirming cyl 1 is the worst misfire. I hooked up my timing light to each spark plug wire and what i found is #1 is firing less than half the time, #3,5,7 are firing about 75% of the time, #2,4,6 are firing 100% of the time, #8 is firing about 75% of the time. all the coil packs seem to be getting solid ground, hot, and reference voltage, but I'm not sure how to check the trigger pulse from the ecm. At the moment all I have is a test light (my multi meters are at a job site in the mountains with a foot or 2 of snow...). I have tried attaching the test light to the trigger wire attached to hot or ground and it doesn't flash at all. I've searched everywhere on the internet but havent found if the trigger pulse is 5V or 12V and thought it might be too fast of a pulse to work with an incandescent bulb. So first question is how do you test the coil trigger pulse/signal? Second question is if the trigger pulse is erratic what would cause that? ECM? If I need a multimeter I can go get a cheapy from harbor freight. I've started digging through the main harness and grounds looking for loose connections and I'm going to keep looking into that, but any other suggestions/help would be greatly appreciated!

Re: HELP!!! Intermittent spark 2002 suburban 5.3
 

I checked the trigger pulse with an LED that I had laying around, good consistent pulse. So I changed coil packs again with a known good coil, and #1 is still firing less than half the time. Looked at grounds, g103, g104, and g105 all appear to be in good shape. I also ran a temporary ground from the bank 1 pigtail to neg terminal with no change. I pulled most of the looming off of the harness with no frayed/broken wires found. I Forgot to mention in the original post that the other reason I thought ground issue is some random dash lights that come and go including ABS, brake, and Service 4wd.

Re: HELP!!! Intermittent spark 2002 suburban 5.3
 

Your dash lights are likely one of those unrelated "It does this too." things. It's probably worth looking at and cleaning the ABS & 4wd controller grounds.

You'll need more than a multimeter or test light to check the signals from the PCM to the coils. They're self contained ignition systems with control signals off the PCM.

Anything made much after 1998 needs a scope to test some systems.
A Picoscope is nice but you need a reasonable Windows based laptop and the scope itself is pricey.

Single channel handheld Oscilloscopes are not that expensive anymore. Hantek sells some in the $60-$150 price range.
Here's a review of the Hantek handheld scope. Adrian Black has reviewed several inexpensive handheld and laptop connected scopes. You can find more reviews if you look at his channel.
This is his review of a Hantek handheld scope-meter.

Owon is compettitor to Hantek. This is Kerry Wong comparing an Owon to a Hantek handheld scope.

I have a Quad DSO and DSO Nano I purchased back in 2014ish. They're useful but I wouldn't recommend em for anything more than looking at waveforms. Add to that the non standard MMCX connector that limits your selection of probes. Measurement of frequency and voltage leaves more than a little to be desired as well.

Re: HELP!!! Intermittent spark 2002 suburban 5.3
 

More stuff specifically targeted at automotive.
These all require a laoptop.
If you're going to get a 2, 3, 4 channel scope you probably want one specifically setup for automotive.
My Rigol digital and my analog HP & Tek bench scopes share the ground across all channels.
Automotive specific scopes have floating grounds meaning every channel has its' own ground.
Here's yet another video about this stuff and how not to smoke your PCM, scope, or laptop.

Re: HELP!!! Intermittent spark 2002 suburban 5.3
 

How many miles are on this thing? Two things come to mind, if the injectors are dirty, injector cleaner is most likely going to do nothing, either new injectors or getting them ultrasonically cleaned are you're options. Sounds like you have spark issues though, I would try to find a scan tool that can show you live data in a graphing type way and see what the cam & crank sensors are doing. You could have a cam sensor going out, if so replace with one from the dealer, the parts store ones will give you more problems 9 times out of 10. The grounds on the back of the cylinder heads for each coil bank are good correct?