94 Truck dilemma

[Bruce Plecan]

Clear ecm.  Unplug O2 and drive for a while.  If the O2 had been poisoned,
putting a new one in while out the poison being burned out would kill the
new one to.  See if having no O2 report makes a difference, better yet
disable 13-44-45, an drive.  Any fresh gaskets?.
   What for mufflers they use?.  Really quiet?.  Excessive backpressure?.
High miles, cam off a tooth?.
Sneezy


| Hi People,
|           I have spent an entire day trying to diagnose a problem, to no
|  avail.  When the engine starts it runs fine for approx 10 seconds,then
| starts
|  running extremely rich.....I then rev it up to "clean" it out and block
|  learns and integrators are reasonable (123-125) then gradually returning
to
|  idle all numbers stay within reason and again will idle fine now for
about
|  3-4 seconds then whammo rich again, I was fortunate enough to have the
exact
|  year and truck to start swapping parts with.....here is a list of what I
| have
|  checked or changed....
|  Fuel Pressure      steady at 13 when problem is occuring or not
|  TPS voltage         steady, no intermittent highs or lows
|  Map Voltage        steady, no intermittent highs or lows......even tried
|  another
|  Coil                     coil was tried from other truck....supply
voltage
| 13+
|  Tach lead disconnected from coil to verify no shorting of tach
|  Throttle Body and injectors were exhanged for the other truck
|  BHMX (matching)computer chip tried from other truck
|  Computer tried from other truck
|  Distributor tried from other truck
|  EGR checked for leaks and even blocked off at one point to verify idle
| problem
|  Temp sensor checked on scanner to verify no intermittent jumps
|  Knock circuit checked on scanner to verify no intermittent jumps
|  Timing verified and verified 10 degree jump with rpm(wire pulled) still
|  changed dist later
|  Spark plugs changed....compression checked ,all within 20 lbs of each
other.
|  Another shop had already replaced cap, rotor, wires, and plugs... to try
to
|  fix problem.
|  Customer had just come from muffler shop and had new o2 sensor, y pipe
and
|  cat back system installed since previous shop had told him he had a
plugged
|  cat.
|  Stored data on the scanner of 75 frames prior to engine stumble and 25
after
|  ....looked at every frame and compared all data points to see what
voltages
|  had moved or even if vss had somehow gone crazy......
|  All this and no codes on the scanner (unless you let it idle poorly so
long
|  it sets the high map voltage code)
|  The above is not in the order of which things were changed...just trying
to
|  recall from a very frustrating day.
|  At the end of the day I got the idea it has to be some sort of a
grounding
|  problem and the only grounds I know of on the engine are the two small
ones
|  near the thermostat, the two on the passenger rear of engine(one goes to
the
|  chassis from the engine and one goes to the frame)and the main battery
| ground
|  cable......All seems ok.....but maybe there's more I am missing
here......I
|  then unhooked the battery for a few minutes (last ditch effort) and truck
|  started and ran normally this time for 7-8 minutes,,,not
| seconds..........One
|  last thing is this truck has had lots of stereo put in it and lots of
|  capacitors for noise......if anyone can give me a spot to look tomorrow I
|  would sure appreciate it and get back to the group with my
|  findings......Thanks in advance.
|  -Carl Summers
|