94 Truck dilemma

[CLsnyder]

----- Original Message -----
From: <EFISYSTEMS at aol.com>
To: <diy_efi at efi332.eng.ohio-state.edu>
Cc: <gm_ecm at efi332.eng.ohio-state.edu>
Sent: Monday, May 24, 1999 11:36 PM
Subject: 94 Truck dilemma

Carl, have you changed the oil? Checked level and dilution? That would be my
starting point.

> 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
>