Extended crank when warm or hot on crate GM L31

[Fragment]

I'm having a hell of a time with a new GM L31 crate motor that I installed in a 99 K1500 suburban that hadn't been running for about 10 years...
It runs well and everything but when its hot, or warm even, it has a consistent extended crank problem. I've had it do it cold too once or twice, but rarely.
When extended cranking it will usually crank for about five seconds, catch and fire then promptly stumble and stall, then upon turning key again will crank and slowly sputter to life.

Let me list out what was done in the engine install, and what's been done with the parts cannon and diagnostics to try and fix this mess.

During install:​

Wiring harness:​

Went over every inch and re-loomed
Replaced CKP sensor connector
Replaced ICM connector

Things that were new on engine install:​

Delphi lower intake manifold
Good metal band fel-pro lower intake manifold gaskets
Distributor
Cap and rotor
Wire set
Ignition coil
ICM
CKP sensor
EGR valve
Knock sensor
Four O2 sensors
Starter
Upper intake manifold
Spider injectors including regulator.
None of these were junk parts. All either ACDelco or Delphi.
Water pump, radiator, all hoses.
Both ground straps, the battery ground cable, and both positive leads.
Fuel pump, fuel filter.

Things that have been swapped out with known good part from my other truck, or replaced entirely most of these a second time:​

Fuel pump
Spider injectors
CKP sensor
Cap and rotor
Spark plugs
Wire set
Ignition coil
ICM
Checked all grounds again.
Changed Batteries
Made sure EGR valve wasnt stuck open
Checked resistance and wiring connection of temp sensor
Went through service manual ICM troubleshooting steps with no bad voltages or anything.
Set correlation to 0 deg with tech 2 and performed CKP relearn twice.
Watched 02 sensors, and I think they are all behaving as expected cycling from lean to rich etc. (don't think this would affect starting in open loop anyways may be wrong)
No fuel trims seem to be out of whack
Checked exhaust back-pressure (this was a long shot)
Fuel pressure is 61 KOEO, around 52psi after an hour of sitting, spikes right back to 61psi with key on, and idles around 55psi. This is with both fuel pumps I've tried, both spider units, and both fuel filters.


I'm just at a loss. Its been months now I've been trying to nail this down. Been through what I feel like is every test, and every piece of data on a scanner and it just all seems 100% normal. Can't tell if its ignition related or fuel.

It does smell a little like raw gas around the mid to rear section of the truck after running it, but I've looked for leaks and can't find any. Might just be my brain trying to find something wrong.

Only thing left I can think of is the ECM... but I don't see how that would cause a hot hard-start problem and nothing else.
I've never gotten a single code, and the misfires I can see on my tech 2 are not bad... mostly on cyl 2, but still only around 32 counts in a ten minute drive around the block, maybe 1-4 counts on the rest of the cylinders.
there is still a very slight exhaust leak going on somewhere that you can only hear under heavy acceleration, but I don't think this is related.


Any suggestions you think I might have not tried?

Cheers.

 

[Schurkey]

Is this truck equipped with the "hot fuel module"? I wouldn't expect that on a K1500, but I've been wrong before. Turn key from "Off" to "Run", fuel pump primes for ~20 seconds rather than the more-usual 2 seconds?

Connect a scan tool, crank engine, look for RPM signal. No signal = likely ignition problems. Check for spark at the coil wire during cranking. Good signal = likely fuel problems. You have good fuel pressure during cranking when it doesn't start like it should?

 

[Fragment]

Is this truck equipped with the "hot fuel module"? I wouldn't expect that on a K1500, but I've been wrong before. Turn key from "Off" to "Run", fuel pump primes for ~20 seconds rather than the more-usual 2 seconds?

Connect a scan tool, crank engine, look for RPM signal. No signal = likely ignition problems. Check for spark at the coil wire during cranking. Good signal = likely fuel problems. You have good fuel pressure during cranking when it doesn't start like it should?

Fuel only primes for two seconds. I tried hot wiring the fuel pump to run constantly and tried to crank hot with the same result. (Was checking for corroded or inadequate wiring harness)

RPM signal is present at all times with both CKP sensors I have tried.

I will have to try the spark at coil when hot test. I’ve tested the spark with both types of testers at multiple cylinders, but since that passed I didn’t try the coil alone.

 

[Pinger]

Not saying that this is the problem with yours but hot starting problems can be caused by vapour locks in the fuel system. Fuel pipe in the vicinity of say, exhaust could cause it.

 

[Fragment]

Not saying that this is the problem with yours but hot starting problems can be caused by vapour locks in the fuel system. Fuel pipe in the vicinity of say, exhaust could cause it.

That’s a new theory I haven’t thought of anyways. All the lines are stock in their stock locations though… have you ever heard of that being a possibility in the stock location?

 

[Erik the Awful]

I'd suspect a vacuum leak, but there's not enough evidence to support that. If you had that bad of a vacuum leak that it made starting hard, you'd probably have a lot more misfiring.

 

[Fragment]

I'd suspect a vacuum leak, but there's not enough evidence to support that. If you had that bad of a vacuum leak that it made starting hard, you'd probably have a lot more misfiring.

This is why I went the EGR route. Cause it acts like an intermittent vacuum leak. I’ve doused the top end in carb cleaner with no result.
That said all I did was check the EGR to see if it was stuck. I guess this doesn’t necessarily mean it’s working properly though.

Is there a good way to test an EGR valve or should I swap it for the one on my other truck?

 

[Pinger]

That’s a new theory I haven’t thought of anyways. All the lines are stock in their stock locations though… have you ever heard of that being a possibility in the stock location?

I've never heard of it with a GMT400. It used to be more of a problem with carbed motors and mechanical (suction) fuel pumps. The high line pressures these days help in preventing it.
Just a leftfield suggestion given you were running out of ideas. Hard to imagine it is your problem if you have the lines in the stock locations and the exhaust likewise.

 

[stutaeng]

Can you do an injector balance test? Usually the extended crank on hot engine is fuel that leaks into the cylinder and wants to flood on hot starts. All injectors should be within 1-2 psi drop when pulsed. Maybe you got a bad set?

Somewhere at the 24 minute mark he starts talking about it:

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Video is good for other things to check as well, but seems like you covered the basics.

 

[Fragment]

Can you do an injector balance test? Usually the extended crank on hot engine is fuel that leaks into the cylinder and wants to flood on hot starts. All injectors should be within 1-2 psi drop when pulsed. Maybe you got a bad set?

Somewhere at the 24 minute mark he starts talking about it:

xc_hide_links_from_guests_guests_error_hide_media

Video is good for other things to check as well, but seems like you covered the basics.

I did a balance test on the first spider I had in it with results within 1 psi. I have not done the test with the second injector spider, but will add it to the list of remaining things to try.