So I’m having this frustrating hard start issue. I’ve researched for days with no luck, here’s the issue.
swapped a gen 3 4.8 into my 70 c10, hasn’t been driving yet but has been started about 30 times just idling. Been running amazing, starting very well, over the past month…. It’s been getting harder to start, long cranks, finally one day I got it running, then it stalled on me and I couldn’t get it running again. Un hooked battery for a couple days and now it will start but it’s extremely hard, takes 30 seconds or more of cranking. I’ve changed plugs, checked iac ohms, no cel’s. Cannot figure this out.

4.8
Aeromotive 355 pump
58psi verified with 2 gauges on my setup
stage 2 Texas speed cam
ls1 injectors
vats removed with base tune

 

Chech the fuel pressure? Are the plugs firing?

 

Ok this is gonna sound stupid, but how much gas is in the tank? What is the condition of the tank itself? Hows the sock on the in tank pump? Checked the fuel filter? There was a LS swap thread that was like a year of a guy messing with things before he realized there wasn't enough fuel in the tank.

 

Honestly the fuel in the tank was my first thought, so I added 20 litres, pump and tank are brand new. I’m starting to wonder if it’s something internal… lifter or something.

 

Roger that. Does it spin over by hand easily/not a lot of resistance?

 

I’ll check…

 

Does it just crank and crank with no sign of trying to run?

 

A lifter wouldn't keep it from starting and running. Start checking the easy stuff 1st. Fuel, spark, and spark at the right time, compression. Maybe the fuse to the fuel pump blew.? Do you hear the fuel pump run when you turn the key to the on position?

 

No it will eventually start but takes multiple tries of extended cranking…… and it will idle and run ok, maybe a slight misfire but nothing crazy. If I turn it off, it goes right back to hard starting

 

Plugs are firing, not sure if the ignition timing is on, how’s best to check that? And the pump does stay on

 

Have you checked the ECM for codes?

 

No codes

 

Use your scanner

 

Kinda sounds like ECM not receiving a good crank position sensor signal

 

My torque app said 17.5 timing advance at idle.

 

Will it display timing while you are cranking the engine?

 

I’ll check that

 

What kind of things have you done to the engine since it was starting/running good? Maybe something is loose or not positioned correctly or not connected properly?

 

So I don’t think I’m getting a timing advance signal during cranking…… is that normal?

 

When you said you have spark... is that because it will run sometimes? Or have you checked spark while the engine is cranking and has not started yet? Make sure there is spark while cranking.
Without knowing how the harness was run and powered it's hard to say. Did you check all fuses for circuits powered during the key cranking position? Not the run position.