Diagnosing herky jerky problem with NemiSys scan tool

[jag_man653]

I've been trying to diagnose an intermittent drivability problem in a '95 Buick Park Ave Ultra (supercharged 3.8L V6) for some time. Briefly described, there will be a sudden jerk or shudder while driving. I call it the herky Jerky. It occurs most often at freeway speeds, which sometimes causes the TCC to pop in and out which is VERY noticeable. But also sometimes happens when accelerating away from a stop when the TCC is not involved. I'm pretty much convinced it's an engine problem, and I've replaced a bunch of stuff... all the usual suspects, so to speak.

The last week or two I've had the use of a very nice scanner, the SPX NemiSys, that allows me to capture a frame of the OBD-II datastream at the time of an incident. After peering at a lot of frames I think I may be on to something. It seems that when it happens there is a sudden drop in injector pulse width. The following URL is an example:

http://www.efsowell.us/ed/ParkAve/FrwyCC05012006_04P18.jpg

At the top you can see I'm cruising at about 63 mph when there are 2 blips in vehicle speed, the second of which is followed by a dip in speed. The throttle angle is constant through all of this. What I'm now focusing on is the sudden drop in injector pulse width that corresponds to the speed glitches. The time steps here are about 1/4 second (I think), so the pulse width changes take place over 1/2 to 1 second, dropping from around 7.5 ms to about 0.9 ms. Not shown are the coolant and air temperatures, but they are constant. The O2 sensor output is shown. It is jumping around a lot, which I believe is normal, but it does seem to go lower at the time of the pulse width drops. My guess is that means the O2 sensor is just reporting the lean condition the results from the pulse with drops.

So, is there anyone here that can tell me where to go from here? What could be causing the pulse width to fall off like that? FYI, the coolant temperature sensor and MAF sensor, fuel pressure regulator and filter are new, as are the plugs, wires, ignition module, throttle position sensor. The O2 sensor has not been changed. BTW, I get no trouble codes of Check Engine lights. Finally, let me say I'm doing all this because the dealer and my long-time independent shop has had their go at it several times.

TIA

[HotZ28]

This link will give you some tips on how to diagnose fuel related problems;
http://www.autotap.com/articles/Diag...ems_0106ta.pdf