Off idle stumble

[dkrace67]

We are working on a 94 Buick Estate Wagon, 5.7L. The car runs excellent EXCEPT just off idle there is a hesitation/stumble. Once you get past that it has good power and exhibits no more symptoms or issues. We've noticed the spark advance drops to zero when the stumble occurs (from usually around 19-23 BTDC) but comes right back after the hesitation. We have sprayed some intake system cleaner through it, added injector cleaner (Sea Foam) to the tank and even tried a new TPS sensor, still the stumble persists.
The IAC has good values/feedback and the EGR system seems to be functioning properly. Any ideas/suggestions? The intake does have an excessive amount of carbon buildup.

[suzqlar2]

Hi, I am a GM mastertech and I have found many EGR valves bad on this engine. 94-6 LT1 are the only years that do it. Standing to the side of the driver fender, if you have an assistant lightly torque brake the engine in gear, about 20% throttle, and the problem is present, reach around the back of the intake and pull the hose off the EGR and the problem will probably stop. If the engine immediately smoothes out your valve is bad. This assumes that you do not have an exhaust restriction, which you most likely do not if the car has good hard accel power.