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Diagnosing herky jerky problem with NemiSys scan tool

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

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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

Ed

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Ed
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Your engine ECM controls all system functions it determines injector on off times I would check power and grounds also alternator output, sometimes the ECM has to be reflashed or updated for service bulletin recalls etc. Check dealer for recalls.

Ray

Reply to
Raynaud

Thanks, Ray.

The scans include the "battery" voltage, which I understand is actually the voltage at the ECM (it's called the Powertrain Control Module- PCM, on the Park Ave Ultra). It shows a lot of fluctuation, but seems to be between reasonable limits. For example, in this frame 13.4 and 13.7. I posted it earlier and someone said that was normal for alternators of this type.

I have also done some work chasing the bad ground theory. A few days ago I replaced the ignition module. Since the main engine ground and the grounds for all the PCM stuff are right below that module I took them all apart to clean them up. In fact, I replaced the one of the lugs that I broke during removal. No help. Nonetheless, I have not yet dismissed an electrical problem in that area. One reason for suspicion is there has been a lot of engine work in that area. Several years ago the crank position sensor ring came loose causing a terrible racket and had to be replaced. More recently the supercharger was replaced, twice. All the main harness wires are snaking around the pieces that must have been removed, and the techs probably pulled and yanked they around quite a bit. So, there may be one that is just holding by a thread. Still puzzling over how to track it down if that is indeed the problem.

Ed

Reply to
Ed

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