Engine Backfiring While in time

[Clutchmaster404]

Recently dropped in a crate motor from Jeggs into a 2000 Chevy C3500 SWB with the 5.7l 350 Vortec. Everything was running pretty good before all this mess. After 2k it was time to readjust the valves after they fully seated (roller cam with rocker arms and pushrods). Stripped it down past the lower intake manifold re did the valves. Removed the lash, cleaned block of residue and old gasket material, new intake gaskets, torqued down intake bolts accordingly to specs, timed my distributor (cyl 1 on TDC making sure intake just closed rotating crank.) set cap pointing at #1 cyl, put the rest back. Fire it up and it wont turn over. For giggles enjoying the gun shot noises I turned the crank with distributor out a full 360 thinking just maybe it was 180 degrees out and still nothing....Put it all back to TDC and still just backfires through the throttle body and exhaust. Plug wires are in the correct spot, distributor points at #1 cylinder on TDC and it just sits there and back fires over time. Fuel pressure is reading 50-55 psi when primed and 60psi when trying to start. Plugs are firing when grounded, Fuel is there and it is in time. I have no idea why it wont at least sound like it wants to turn over. Any help is appreciated. No compression test has been done yet, but I did have great oil pressure before I did the valves. Im starting to think its the Ignition control module acting up.

 

[SAATR]

Two things:

1. The distributor is still out of time, and that's causing your backfire and afterfire.

2. You adjusted the valves too tight and they're hanging open, causing the backfire and afterfire.

Either way, start over from square 1 and you'll probably find the issue.

 

[Clutchmaster404]

Two things:

1. The distributor is still out of time, and that's causing your backfire and afterfire.

2. You adjusted the valves too tight and they're hanging open, causing the backfire and afterfire.

Either way, start over from square 1 and you'll probably find the issue.

The distributor is in time ive checked it 3 times along with my valves they are not too tight or too loose and have been set according to a service manual. Ive set the timing on these motors several times before with a flawless turn out. Its honestly not that hard.

 

[SAATR]

The distributor is in time ive checked it 3 times along with my valves they are not too tight or too loose and have been set according to a service manual. Ive set the timing on these motors several times before with a flawless turn out. Its honestly not that hard.

You disassembled what I assume was a good running engine, did your maintenance, put it back together, and now it won't run. Do you think it's more likely that a sensor or module coincidentally failed, or that you made a mistake or pinched a wire? In my experience, the latter is much more likely. Recheck everything you touched. Loose wire? Bent pin on a connector? Shorted wire on the distributor harness? My money is still on one of the other two possibilities.

 

[Clutchmaster404]

ok, well, again. I have gone over everything over and over again with what I did to ensure accuracy. It still wont turn and back fires. I get what you are saying and trust me it has frustrated the hell out of me because I keep reverting back to the same thing you are, but it has to be something else. Im still leaning towards the ignition control module or maybe possibly the coil. I have pulled the coils boot to see if there is spark and there was, but not sure how to test the ignition control module.

 

[Schurkey]

Cranking compression test. My guess is you got the lifter preload too tight. However, I question you "set cap pointing at #1 cyl". Did you point the rotor at #1 CYLINDER, point the rotor at the #1 spark plug wire at the distributor, or did you point the rotor at the #1 terminal on the inside of the cap? The three are vastly different.

You say the "plugs are firing when grounded". You've removed the plugs, grounded them, and seen the spark jump the gap? Or you've tested the plug WIRES, not the plugs themselves?

Not likely to be the ignition module, since you seem to have reliable spark.

I don't know why you'd remove the intake manifold to set lifter preload.

I have no idea why you'd think you need to re-set lifter preload when using a roller cam on a stock or near-stock engine. If it was assembled properly to begin with, the lifter preload is set for the life of the engine.

 

[tpass]

probably time to get a second set of eyes onsite to go over everything.

Its like writing then reading your own paragraph 16 times and seeing no mistakes, until someone else reads it and spots them right away.

we've all been there.

ETA: you said turning engine til #1 intake valve just closes... are you then putting the damper timing mark at zero? Obvious question but it wasn't noted in your post.

 

[1998_K1500_Sub]

Fire it up and it wont turn over. …. Ihave no idea why it wont at least sound like it wants to turn over.

It still wont turn and back fires.

What exactly do you mean by “It won’t turn over?”

Are you saying it cranks until it backfires, at which time the starter stalls, e.g., because of cylinder pressure?

???

A complete and accurate description helps immensely.

 

[1998_K1500_Sub]

Will a backfire ever lead to a throttle shaft / plate (or other) deformity on account of atypically high manifold pressures?

Might it damage a MAP sensor?

 

[Pinger]

Will a backfire ever lead to a throttle shaft / plate (or other) deformity on account of atypically high manifold pressures?

Might it damage a MAP sensor?

Theoretically an inlet backfire could bend throttle plates and damage a MAP sensor. The fuel pressure regulator also. The biggest fear of an inlet backfire I have is of an inlet valve being popped open and either coming in contact with a piston or allowing the pushrod to to fall out of place - or both!