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
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
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
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.