1996 K2500 Sub L29 454 Hesitation off idle

[inconceivablemotors]

Has anyone ever found the reason for the stumble off idle?
The engine fully warmed up, at idle 650rpm, at a stop... then press the gas pedal to start moving and the engine will surge / hesitate, then jump up to about 900 RPM and the truck drives perfectly normal from there.

Live data all looks good, timing is correct, did a smoke test no vacuum leaks. So far I have replaced:
-Intake gaskets
-Fuel injectors and regulator
-Fuel pump
-Distributor, wires and plugs
-Idle control valve - It seemed lazy
-Timing chain and sprockets
-Crankshaft sensor
-O2 Sensors

And so far have not been able to resolve this..... Any inside greatly appreciated.

 

[Keeper]

I think I read something by @L31MaxExpress where he said that it is part of the stock tune. It's annoying though, for sure.

 

[6sand9s]

7.4 hesitation, can anybody write this out of 96 3500? or know a way to get around this annoying tip in stumble? L31max express?

 

[Supercharged111]

I think I read something by @L31MaxExpress where he said that it is part of the stock tune. It's annoying though, for sure.

It happens with a marine intake on a 350 or, to a lesser extent, a 454 with a 411 swap.

 

[Schurkey]

Throttle body nice 'n' clean? BOTH sides of the throttle plate, and the housing it seals against? IAC passage clean?

"I" would be looking next at the EGR timing. Is this a vacuum EGR valve, or one of the newer solenoid-style? Does it engage directly off-idle? Correct EGR valve? IF this is a vacuum valve, does the big-block come with a pinhole in the vacuum nipple, that the aftermarket valves don't have, as is the case with (at least some of) the small-blocks?

Then, I'd make sure the electronic spark advance was working properly, and the cam offset was correct.

Is there such a thing as a lazy MAF sensor? I know they can get dirty, and they can fail and act funky. Twice as common if there's an oiled-gauze air filter in place. Worth checking out.

 

[6sand9s]

Well, weird thing unplug map sensor, everything defaults open loop runs fine., only after sitting still and taking off or powerbrake between idle and 1100 rpm , acts like a dip in fuel volume or not rich enough ,monitored fuel pressure ok at 59, acts just like egr flys open for a second but have blocked that off. believe to have 502 marine cam when rebuilt, only dips once when it happens and takes off. no code ,fuel trims look perfect tps iac have no spikes ,maf and o2 sensors been replaced. Wearing my a$$ out.

 

[Cadillacmak]

@inconceivablemotors Is your profile picture from Alaska?

 

[Supercharged111]

Well when you unplug the MAP it runs solely off of the MAP, but if you put in a cam with 225/225 duration you're in need of a tune period.

 

[Keeper]

One of mine has been doing that for a long time with a stock cam/intake (336k). Falls flat on it's face for a full second. It's worse in spring and fall. The OP has done everything I have, plus some. I need to do intake gaskets again and a rear seal, so I'll be chasing it again soon, especially since I just cracked the front axle on the tractor and have to pivot on projects.

Watching intently.

 

[6sand9s]

@inconceivablemotors Is your profile picture from Alaska?

no sir n.c.