Extreme Rich Spikes / MAP Spikes

[gmpearson]

For starters, I have an AEM Standalone ECU (Series 2) and I'm running a speed-density setup - No MAF sensor.

I've had an issue recently develop where, once the car has warmed up, it'll start bucking under low to medium load, like when cruising in town or entering an on-ramp.

AFR spikes WAY rich, enough to cause the engine to stutter and totally cut out repeatedly (it sounds like a Baahh-buh-buh-buh-buh-baahhh-buh sorta thing) - it's very violent, the whole car jerks in and out of acceleration.

Looking at logs, I noticed that the MAP sensor will rapidly spike from 4 PSI steady up to 9 or 10 PSI for a fraction of a second, drop back to steady, then spike again. This causes fueling to go crazy rich, of course, and seems to be the cause of the stuttering.

What I'm trying to figure out is...what would the cause be? Could a failing MAP sensor spike like that, or would it just hold a steady voltage? Theory number two is that valves are leaking and as the piston compresses, it's blowing enough pressure back into the manifold to cause the sensor to spike upwards. (There's actually fuel coating the inside of the upper plenum, which further leads me to believe this is what's happening).

Any ideas? I know this is a pretty off the wall one...

[sns00]

Was work being done to the car before the issue came up?
Have you checked your vacuum hoses?