Sometimes when I accelerate at part throttle, I'll have a stumble. If I give it more gas it'll go away, or if I just wait, like a light switch, it'll disappear. This seems to happen sometimes, but not all the time.

Oh yeah... 91 Corvette 6 speed. Minor mods and an AS&M Stage II or Stage III prom.

So anyway... I put the scan tool on it and watch what's going on. That's when I learned that the stumble is related to a very lean condition. when the car snaps out of it and starts accelerating normally, the o2 voltage snaps up immediately.

So... something is making the car very lean under certain acceleration conditions. MAP looks fine, Intake air temp sensor is fine, coolant temp sensor is fine, TPS is fine, knock counts and knock retard aren't out of the ordinary, and I've even driven around with a fuel pressure gauge taped to my hood to verify that the fuel pressure is ok.

It seems that everything is working as it should, except that I'm still very lean under certain conditions. About all I can think of is that the AS&M prom is at fault, though this doesn't really explain why this situation doesn't consistantly occur every time. I can go in and start messing with the fuel tables on the PROM, but I'm concerned that since this issue is intermittant, that doing PROM changes is just going to put a band-aid on it rather than fix the problem.

Anybody have any ideas for what I should look at? Anybody have this sort of issue before?

 

Jim. how are you doing?

What about the O2 sensor? Could it be getting "lazy".

 

It doesn't appear to be lazy. When it's not lean, crosscounts go up rapidly. :shrug:

 

Did you ever fix it? I might be having the same problem.

 

My guess is acceleration enrichment is too lean. The accelerator shot may be right on the edge of being too lean all the times but sometimes the engine responds and sometimes it does not. Find out when the stumble occurs and have someone change the chip add more acceleration enrichment. There will be adjustments to make the entire curve richer,and there will be adjustments to make individual temperature areas richer without moving the entire curve.