Issue has gotten worse over time. When I snap the throttle, sometimes the motor stumbles/cuts out and fails to rev strongly. When driving, I let off the gas to slow down. When I come back on the gas. I randomly get a slight stumble. Motor runs strong otherwise. Also seems to be more noticeable when cold.
I've tried 3 different MAP sensors and 2 different IAC Motors. No difference. Any thoughts ?

Thanks, Kevin

 

I'm gonna take a WAG and "assume" this is the 350 with the SuperRam.

Has it ever worked right? Does it come and go, or is it pretty consistent?

Sounds like a fuel problem. What's the fuel pressure WHILE IT'S IN THE ACT OF SCREWING UP? That is, if you watch the gauge, what does it do AT THE MOMENT THE ENGINE SCREWS UP? NOT, "it's normal"; NOT, "it's xx psi idling"; WHILE IT'S IN THE ACT OF SCREWING UP.

 

I'm gonna take a WAG and "assume" this is the 350 with the SuperRam. Sorry, Yes it is

Has it ever worked right? Yes it did long ago as I remember. seems to have progressed over the years . Does it come and go, or is it pretty consistent? Pretty consistent now

Sounds like a fuel problem. What's the fuel pressure WHILE IT'S IN THE ACT OF SCREWING UP? That is, if you watch the gauge, what does it do AT THE MOMENT THE ENGINE SCREWS UP? NOT, "it's normal"; NOT, "it's xx psi idling"; WHILE IT'S IN THE ACT OF SCREWING UP. With Vacuum connected it idles just above 35psi. When I rev it, it goes up. With vacuum removed, it's around 44psi. I'm running a Walbro GSL392 Pump, with -8 lines in and out. Has a 100 micron pre and GM 25171792 -10 micron post pump filters. All 3 have been replaced several times.
JFYI....It runs wide open just fine.

 

So, exactly NOT what I asked you to measure.

What's the fuel pressure WHILE IT'S IN THE ACT OF SCREWING UP? That is, if you watch the gauge, what does it do AT THE MOMENT THE ENGINE SCREWS UP? NOT, "it's normal"; NOT, "it's xx psi idling"; WHILE IT'S IN THE ACT OF SCREWING UP.

 

I did....It goes up from 35psi as I rev it

 

Was it IN THE ACT OF SCREWING UP at that instant?

 

Yes. Multiple revs resulted in no drop in fuel pressure. Some revs would act up and once or twice it revved normal.

 

Seems like a potential lack of acceleration enrichment. Is that adjustable in your control scheme?

 

If your referring to the TPS rate of change AE, I believe so.
Why would that be an issue if the tune has not been changed. Or anything tune related for that matter, if its the same base tune?

 

Apparently, something has changed. If the parameters are the same as when it ran properly, that's good, but something has changed. As you know, air, fuel, and timing are the critical items supporting good combustion and power production.

It seems that fuel pressure has been verified to function about normally, or at least within the ± tolerance of a generic pressure gauge. However, that is fuel rail pressure only, and there is no measurement of the fuel flowing through the injectors. If the injectors are rated 2.8 g/s (common for a 350) and the screens are clogging or tips are restricted to the point where they flow 2.4 g/s, that could create some lean conditions and the ECM wouldn't necessarily accommodate the change in accel mode. The O² could adjust for idle and cruise once in closed loop, but that doesn't change anything in AE to my recollection.

The opposite could also be true if inlet air flow is restricted. Does it run differently with the throttle body disconnected from the intake ducting and filter?

Another consideration is possible spark blowout under high chamber pressures. You probably know all the usual suspects there.

Timing of both the ignition and valve operation can allow acceptable idle and lower RPM cruise, but be more noticeable at higher RPM and under acceleration. I've seen SBCs start, idle, and light cruise in a way that seems completely normal, and around 2,500-3,000 RPM or under heavier load begin to stumble and even backfire into the intake slightly when the chain has slipped a couple teeth on a sprocket, or 35-40° late on vale timing.

 

Thanks for the info.....Got some things to check out. Kevin

 

So I was advised by George Landis on the Facebook Accel site to up the fuel pressure and see if that clears up the stumble. So I bumped it to 47psi and hesitation was all but gone. He said To raise the "TPS rate of change AE" table and I should be able to lower the fuel pressure back down.
Not sure why it needs more fuel coming off idle, but it seemed to like it. Whos to argue.
Thanks for everybody's assistance sorting this out. Kevin