1997 5.7 Misfire under load, bunch of parts replaced

[Dud1f3r]

Hi, this is my first post, but I've been reading the forums since about April, when I bought this truck.

Some backstory, just to get you into the mindset of why I did all this to the truck. The truck is a 1997 K1500 with the 5.7L engine. I bought it off a friend's dad, who parked it for 6 years. Got a good deal on it, I figured it'd be a pretty easy truck to work on, and a good first domestic. Well, after getting it running (the dizzy was mistimed, I just pulled it out, retimed it, and put it back in, along with a new cap and rotor), my first goal was to clean out and replace the fuel system as good as I could. I replaced all the gas in the tank (for obvious reasons), replaced the fuel pump with a new Delphi unit, replaced the fuel filter with a NAPA brand filter, and replaced the spider injection unit with a "TRQ" brand (The 1AAuto brand) MPFI conversion, all of which are working great.

The issue now is it misfires only when warmed up, only under moderate/heavy load. First thing in the morning, go fire it up, and it runs great. Drive around for a bit, maybe 5-8 miles, and it's warmed up. Once it's warmed up, you can't do anything but just barely touch the gas pedal, or the truck will sputter and misfire, and it'll get worse and worse until it starts to misfire at idle.
I've replaced:
Fuel pump
Fuel filter
Spider injection unit (with MPFI conversion)
Intake plenum gasket set
Plug wires (But had to go back to the old wires as the new ones were making it miss constantly)
Dizzy cap
Dizzy rotor
Coolant temp sensor in the thermostat housing

When reading live data, the truck is getting up to temp fine, and all of the sensors are reading what you'd expect them to read. I've checked for vacuum leaks and found none. I've double checked my ignition timing, and without a proper GM tool or anything, it's fine. Only code is a P0300.

Can anyone point me in the right direction in what to look for? Despite what it looks like, I'm not too fond of just throwing parts at issues.

 

[RichLo]

How does the o2 sensors look in live data stream? And what does the LTFT and STFT look like for each bank?

 

[Laredo1307]

I had a similar issue lately except it was the opposite of you. Would misfire really bad my whole 30 mile commute in the mornings and then run fine at lunch and my drive home. All I would get is P0300 codes as well. Ended up being my fuel pressure regulator the MPFI I installed a few years prior.

Honestly if you just put in a new injection system you may have to double check all the injectors are seated properly. Pretty sure my truck was doing something similar to yours a few years ago because the original poppet injectors were leaking fuel into the intake......which lead me to install the new MPFI system.

- Have checked you MAF sensor?
- Checked your MAP sensor?
- Is it dumping fuel or starving fuel to cause the misfire?
- Can you see which cylinders are misfiring?
- What plugs are you running and what is the gap. Best to run AC Delco Iridium plugs with .060" gap. These trucks can be finicky
- Are the old wires arcing or have cracks?
- Is the ignition coil cracked or warped/ bulging?
- Can you see the CAM/ CKP offset? I use Dash Commander app with a bluetooth dongle connected to the OBD2 port. You can access the reading in the Data Grid area. Don't look at the timing in the dashboard. You can also see individual cylinder misfires.

- Are you missing any coolant? Could be an intake gasket leak allowing coolant into the combustion. Very common on these trucks. Need to upgrade to the FelPRO gasket. If it sat that long there's a good chance the gasket dried up and cracked once you started driving again.

 

[Dud1f3r]

How does the o2 sensors look in live data stream? And what does the LTFT and STFT look like for each bank?

Both LTFT's are either 9.4 or 9.8, I forget but they're not crazy, and they're equal.
I haven't checked STFT's after clearing codes.

1 Have checked you MAF sensor?

I have. It's reading fine, but I cleaned the pins to the connector. Made sure not to actually touch the MAF itself with any chemicals, from fear of messing up it's readings. (EDIT: I had a picture of the MAF/MAP readings. MAF reading at idle was 1.512 lbs/min)

2 Checked your MAP sensor?

I have, and verified it's readings with the MAF to confirm both are in good shape. I don't remember the numbers off the top of my head, but they were very strong, leading me to believe it's not a vacuum leak of any kind. (EDIT: MAP reading was 10.9 inHg)

3 Is it dumping fuel or starving fuel to cause the misfire?

I'm not entirely sure, I haven't watched live data when driving it, but I suspect it's starving for fuel since it almost sounds like knocking.

4 Can you see which cylinders are misfiring?

No. I get the sense that it's all of them at random, or maybe one of the banks missing at random. The only code I have is P0300.

5 What plugs are you running and what is the gap. Best to run AC Delco Iridium plugs with .060" gap. These trucks can be finicky

I'm running I think some Denso platinum plugs. I gapped them down to 0.045" and it made the truck a lot happier.

6 Are the old wires arcing or have cracks?

Not that I've seen, but I haven't checked either. The old wires look factory, they're Packard wires and numbered. Super old, but the truck seems to like them the best.

7 Is the ignition coil cracked or warped/ bulging?

No, the coil is looking fine. I was gonna replace it with the same brand as the new wires I got for the truck, but after those wires sucked major doodoo I'm hesitant to replace the coil. (I also don't have spare rivets to replace the ones I'll need to drill out)

8 Can you see the CAM/ CKP offset? I use Dash Commander app with a bluetooth dongle connected to the OBD2 port. You can access the reading in the Data Grid area. Don't look at the timing in the dashboard. You can also see individual cylinder misfires.

I cannot, not with my current scanner. I may end up getting a lil BT live data scanner sometime soon though.

9 Are you missing any coolant? Could be an intake gasket leak allowing coolant into the combustion. Very common on these trucks. Need to upgrade to the FelPRO gasket. If it sat that long there's a good chance the gasket dried up and cracked once you started driving again.

I've only driven it about 150 miles since I bought it, and I'm not sure if the missing coolant is from taking the thermostat housing off to do the spider or anything else. But that is a good point that I forgot to consider, and might do when I double check my spider since I'll be in there (and really wanna clean out the lower plenum anyway, it's so gross)

 

[Dud1f3r]

Update, found out one of the dizzy cap screw holes broke off, so the cap is kinda half assed on there. Gonna replace the dizzy with an aluminum body one with some new wires (my coworker told me the heat could be making the wires resistance go high enough to cause misfires, and that's why it's only doing it warmed up) and see how she does. Intake manifold gaskets and plugs are coming in a bit, whenever Rockauto ships them. Made sure to get Fel-Pro brand gaskets since I know that brand from my VW stuff and Laredo1307 recommended them. I'll keep y'all updated.

 

[Erik the Awful]

my coworker told me the heat could be making the wires resistance go high enough to cause misfires, and that's why it's only doing it warmed up

No. It would be more likely that there's no thermal paste between your ignition module and your distributor, and that could cause a misfire when warmed up. There should be a thin layer, as if you're trying to glue the module on without having a bunch of excess squish out the sides.

 

[Dud1f3r]

It would be more likely that there's no thermal paste between your ignition module and your distributor

You mean the ICM and it's heatsink? It sits beside the coil on the passenger side of the engine, kinda between 6 and 8 but up top. I recently re-applied thermal paste thinking that was the issue, and no fix. Also cleaned up the mating point between it and the heatsink, making sure it's got a good ground (read somewhere that it grounds through the screws? Didn't make sense but I'm a VW guy so I figured they knew more than me).

 

[Erik the Awful]

Doh! Vortec truck. You'd think I'd catch that. The TBIs have a module in the distributor that requires thermal paste.

 

[Dud1f3r]

Doh! Vortec truck. You'd think I'd catch that. The TBIs have a module in the distributor that requires thermal paste.

If it makes you feel better, I know my AEG family engines like the back of my hand, and I still seem to forget what years each one came out.

 

[Dud1f3r]

So I replaced the intake manifold gaskets, and put the dizzy in wrong (d'oh)

After getting that sorted out, I have P1345 coming back, along with it not running at all if the cam position sensor is plugged in. I swapped out the old dizzy's cam position sensor, and same thing. Current codes are P1345 (Crank cam correlation code), P0340 (Camshaft position sensor circuit A malfunction), and P0300 (Random/multiple misfires).

Replaced the ICM, to see if it does anything, and it *kinda* did, but wasn't much. Certainly made it mildly better. So as it sits:
New plug wires
New Dizzy
New ICM
New fuel pump
New fuel filter
New MPI conversion
New intake manifold gaskets, both upper and lower

Verified compression is within 1% of 150PSI on all 8 cylinders
Verified the MAF and MAP are working as they should
Verified the TBPS is working as it should
Verified no vacuum leaks
Verified fuel pressure at 62psi key on/engine off/55psi key on engine on. No excessive bleed off.

Thinking maybe a crank sensor? I mean, either that or an internal fault in my PCM at this point. Nothing else I can thing of to cause a misfire only under any moderate/heavy load, that gets worse when it's warmed up.