Rough running

[Hib Halverson]

Originally Posted by Billy Mild

I replaced the fuel pumps last night and the car overall runs better. Smoother idle, feels like more power up top. The stumble still remains.

Conditions when it happens, decel from 3000+ RPM slow down to around the 2000-2200 RPM range and give it just a little throttle 10-20%. It will stumble. I noticed while on decel the left side INT drops drastically from 120-125 range to 64, while the right side stays around 120-125ish maybe 113. I notice the lower INT on the left side the worse the stumble is. If I got WOT the stumble stops right away, but I think that has to do with it is enriching the mixture immediately and ignores the sensors. This stumble happens with power key ON or OFF

So on the scanner I'm not sure what side of the engine is the left vs. right side. Could this be an exhaust leak, O2 sensor issue, or something else?

Just guessing, here...

If the bank 1 integrator is going down to 64, that means the system is taking a whole lot of fuel out. If the stumble is worse, the lower the bank 1 integrator gets--that makes sense.

I'm going to guess that the stumble is being caused by a suddenly way lean AFR (64 integrator) and that the system, while in closed loop, cannot change states (from a pulse width typical of a 64 integrator to a pulse width wide enough to provide proper fueling) fast enough to prevent a lean sag during a modest throttle opening.

You don't get the stumble when you go straight to WOT because the engine goes instantly to open loop with both integrators at 128.

Let me know what you think.