Diagnosing problems with NemiSys scan tool

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Ed - 10 May 2006 00:46 GMT

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. 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

Ed

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Kevin - 10 May 2006 03:43 GMT

> I've been trying to diagnose an intermittent drivability problem in a
> '95 Buick Park Ave Ultra (supercharged 3.8L V6)
[quoted text clipped - 47 lines]
>
> Ed

Interesting problem. I'm not sure there is enough information here to
formulate an accurate diagnosis, but I can venture a couple of guesses.
Perhaps a glitch in the speed sensor signal. One that might not show up on
the scanner, but might if you scoped the sensor signal wave form. I would
also like to see a pattern of intake pressure synced with ignition during
this episode. Maybe you are just having a plain old induction back fire due
to a mechanical or ignition miss. I don't suppose you have a portable
ignition scope to watch the secondary events during the problem. That sure
would shed some light on what is happening. Another thing in your graph
puzzles me. Why is the speed and engine RPM holding while throttle position
is decreasing? Where you going down hill? There again, a TPS or MAF signal
glitch that might not show on a scanner. You said some parts are new, but
how about the wiring and connectors. Many times, intermittent problems are
due to connection problems. Is this problem predictable or repeatable under
certain circumstances? In other words, does it occur only once warmed up or
mostly at certain load conditions etc. There in may lie the clues to finding
the solution.

I guess I could think of some other possibilities, like maybe a sudden rise
in fuel pressure, or even some alternator frequency induced PCM malfunction
(that's kind of exotic), but the lack of a DTC makes me think I would be
looking for something more basic like Ignition system or valve train. Maybe
even fuel contamination (water?).

Good luck,

Reply to this Message

Ed - 11 May 2006 04:19 GMT

> Interesting problem. I'm not sure there is enough information here
> to
> formulate an accurate diagnosis, but I can venture a couple of
> guesses.

I know. I feel the same way :-( Somehow I get the feeling this
shouled have been
carried out in a GM research lab!

> Perhaps a glitch in the speed sensor signal. One that might not show
> up on
[quoted text clipped - 3 lines]
> during
> this episode.

I've been trying to figure out if the car id fitted with a MAP sensor.
I don't think so,
since a search in Alldata for it brings up nothing. GMPartsDirect.com
brings up a part
of that description, but when I sent them a question about it I was
told "not in stock,"
so i think it doesn't exist. It does have a MAP sensor, which I've
always understood to
be an alternative to the pressure sensor. the MAP sensor was replaced,
a $300
experiment by my trusted garage. No help whatever.

>Maybe you are just having a plain old induction back fire due
> to a mechanical or ignition miss.

I don't ever hear anything like a backfire.

>I don't suppose you have a portable
> ignition scope to watch the secondary events during the problem.
> That sure
> would shed some light on what is happening.

I have little Hitachi scope that I use in the garage, but don't think
it would work
very well plugged into the cigar lighter.

> Another thing in your graph
> puzzles me. Why is the speed and engine RPM holding while throttle
> position
> is decreasing? Where you going down hill? There again, a TPS or MAF
> signal
> glitch that might not show on a scanner.

Could have been a slight grade. The scanner will record up to 5
episodes, and
I have trouble remembering what was going on at the time episode #3
occurred,
if you know what I mean.

>You said some parts are new, but
> how about the wiring and connectors. Many times, intermittent
> problems are
> due to connection problems.

I am also thinking along those lines, since there has been a lot of
work over the
years that may have given techs the opportunity to tug and twist. Hard
to know where
to begin. I've spent some time looking over ground path schematice in
Alldate, but
maybe I should do some more.

> Is this problem predictable or repeatable under
> certain circumstances? In other words, does it occur only once
> warmed up or
> mostly at certain load conditions etc.

I've been looking for things like that. Here is all I'v ecome up with.
If I drive down the freeway
about 20 miles nothing happens on the way. If I stop for a latte
(drinking it there... 20 minutes or so)and then come back
it will act out on the way back. Also, sometimes it will stumble
pulling away from a stop on the first drive of the day...
2-5 minutes away from the house. Trouble is, these two scenarios are
diametrically opposed.

> I guess I could think of some other possibilities, like maybe a
> sudden rise
> in fuel pressure,

Some people at alt.autos.gm jumped on fuel pump right away. That's a
$400 experiment, not carried out.
I did replace hte regulator and filter. No help. Also put a gauge on
it, in the garage. In spec. It really
irritates me that OBD-II does not give a feul pressure signal. Must be
a safty issue.

>or even some alternator frequency induced PCM malfunction
> (that's kind of exotic), but the lack of a DTC makes me think I
> would be
> looking for something more basic like Ignition system or valve
> train. Maybe
> even fuel contamination (water?).

I was thinking alternator problems for a while, for 2 reasons. One,
the red light comes on occassionally at start up
and it never used to do that. Second, somewhere I found a GM bulletin
about a S-10 having a TCC problem
due to a wierd frequency interaction with the alternator. The fix was
a special part that I suspect was a filter
of some kind on the alternator output. However, people on some forum
told me the scans I was getting are normal.

Never though about fuel contamination. We liv ein a moderate climate
(So. Cal) and buy fuel only from major
stations. How woudl I follow up on that?

> Good luck,

Thanks. Your interest is appreciated.

Ed

Reply to this Message

Kevin - 11 May 2006 10:54 GMT

> > Interesting problem. I'm not sure there is enough information here
> > to
[quoted text clipped - 115 lines]
>
> Ed

You could check for fuel contamination by emptying the contents of the fuel
filter into a clear container and letting it sit. Any contaminants (other
than alcohol) should separate and become visible (including water). You can
check the alcohol content by adding a small measured amount of water to a
measured amount of fuel. After it sits and separates the volume of water
will have increased by the amount of alcohol present in the fuel. Use a
calibrated beaker so you can see the difference. OTC also sells a kit to
measure vapor pressure in order to determine fuel volatility, but It might
just be easier to switch fueling stations for a while to see if there is an
affect on the symptom.

Signature

Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

Reply to this Message

Marsh Monster - 10 May 2006 15:08 GMT

> I've been trying to diagnose an intermittent drivability problem in a
> '95 Buick Park Ave Ultra (supercharged 3.8L V6)
[quoted text clipped - 47 lines]
>
> Ed

=============
=============

Research the system you're working on and understand the input/output
of the computer functions before you do anything when it comes to
diagnosing a symptom that's not readily identifiable using "quick" scan
and performance checks.

with that said............

scrutinize the cam/crank signals.........with a lab scope.
synchronize the signals with the injector signal and you may
find that the sync signal is glitching at the moment of the injector
glitch.

Injector timing is output using engine sync. signals.

~:~
MarshMonster
~:~

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