2002 GMC Savana 3500 w/ 5.7L Vortec - Random misfire (all cylinders), hard start

I have a 2002 GMC Savana 3500 15-passenger van with a 5.7L Vortec motor in it.  The motor is a Jasper rebuild that was installed May 2023 with most components and hardware transferred from the dead motor (only the water pump, distributor cap and rotor, plugs, plug wires, and starter were replaced with new components).  Van has about 145000 miles on it.  Here are my problems:

As van warms up, engine starts to miss at higher RPM and the "SERVICE ENGINE SOON" light starts flashing the misfire warning.

After van is parked and motor is shut off for 10 minutes to about an hour, the van is very hard to start.  I have to crank the van on-and-off for about 5 minutes before it starts, and then it runs rough for a few minutes after starting.  During this hard start, the motor seems to try and fire, but it kicks and stalls the starter motor periodically while cranking the van.

What I've tested so far:

• Collected OBD2 trend data from all available van sensors and some phone sensors from the Torque app
  • It seems vacuum is fine at about 20 in. Hg
  • Upstream O2 sensors seem to be working properly
  • Long-term fuel trims between 10% ad 15%, sometimes hitting 20% at high RPM
  • Coolant and intake air temperature sensor readings seem reasonable
• Cleaned MAF sensor
• P0300 Code - Random misfire, all cylinders
• Tested fuel pressure while parked:
  • With fuel pump priming, fuel pressure went up to 65 psi, then dropped to 60 for a second, then further down to 55, then started to slowly drop more from there
  • With van motor started and running, fuel pressure remains at about 54.5 psi.  According to my Haynes manual, the fuel pressure should be between 60 and 66 psi.  Van reached operating temperature.
  • After shutting van's motor off, fuel pressure dropped to 49 psi.
  • After about 5 minutes, I restarted the van's motor and the motor fired up right away.  Fuel pressure was back at 54.5 psi at idle.
  • Shut van motor down and let it sit for 15 minutes.  During this time fuel pressure dropped to 44 psi.
  • After restarting the motor, the van started right up and idled fine, but started missing at higher RPM.  Dropping down to idle, the van started missing a little bit at idle but not enough to trigger a misfire warning.

I have not attempted any fixes yet.

I have attached the CSV file of data trends and a few pictures.  I have the file loaded in Excel as a Pivot table, so if anyone wants to look at anything I don't have posted as a picture I can get it graphed and posted.

 

 

 

 

 

For some reason the PNG charts are really small in my post and can't be magnified, so here's a zip file with the full-size PNG charts, just minus the O2 chart since that's the biggest file.

An update:  I cleaned the distributor cap and rotor and the misfire problem remains.  Today I was driving the van for about 2 hours, an hour of it on the interstate.  Before the interstate driving, I was getting misses and engine pinging, but it seems that as the motor was getting thoroughly warm (as opposed to just the coolant hitting 190), the missing stopped and the van ran great for the rest of the trip, no hard starts or anything.

Any thoughts?

I love the warm reception here, thanks everyone - LOL! 

I think the problem now is the EGR valve on the account that the fuel pump seems to build pressure just fine and I don't smell any gasoline in the intake.  I think the check valve on the fuel pump is bad and the injectors aren't leaking, but that will be a problem to solve another time.  The symptoms I've listed seem consistent with a poorly-functioning EGR valve, so I'm going to install a new one this week and see what happens.

Busy time of year for everyone, thanks for not taking it personally.
Kicking back on the starter while cranking is generally an ignition timing issue. (All though even with such a simple starter, I have seen them installed poorly and cause binding)I don't know a whole lot about adjusting the ignition timing on those old small blocks, so I haven't opened my mouth, lol.
One thing I would want to to verify is the engine coolant temperature sensor signal since you said it seemed to run much better once warmed up.
Again, sorry for the low reply rate.

I replaced the EGR valve. Turns out that the guy who installed the motor didn't install the gasket with the old one. Regardless, the EGR valve didn't make a difference - the van still runs lousy!

I haven't checked ignition timing yet, but I do have trends for timing advance available - would that help?

As for the engine coolant temperature sensor, its readings seem to be consistent with how warm they're supposed to run, so between 180 and 190 degrees F when warmed up.

check timing as Noah mentioned then plug wires for the firing order to the dist.. cause you said they were new . , may be even vacum leaks intake and exhaust
GOOD LUCK

After about $1000 of diagnostic repairs and opinions from two mechanics, the answer was that the new distributor cap that was installed with the Jasper motor was shorted.  A new AC Delco cap solved the problem and now the van is running great.

Thank you for updating us with the fix.

You're welcome! I hate to leave people hanging without a resolution.

One of my mechanics finally consulted with a friend of his who's a GM master tech. The master tech said that he's seen even brand new distributor caps short out after very little use, sometimes even right out of the box. So the moral of the story with these 5.7L Vortecs that are running very badly and won't start? Start by replacing the distributor cap! And if that doesn't work, swap it again! If that doesn't work, then you probably have another problem!

Also, the starter needed to be replaced. The multiple backfires put a lot of energy into the starter and straight-up shattered the motor's gearbox and bent the mounting bolts. Fortunately the bolts could be pulled from the bell housing without too much trouble and the flywheel was undamaged.