94 Truck dilemma

[Programmer]

Hey--did you check for any AC output from the alternator ?? Anything above
400mV AC can be interpreted by the ECM as an increase in RPM--and fuel is
pulsated to match. When you disconnected the alternator you may have
"discovered" the problem...but you didn't mention how long or if the problem
returned with the alt disconnected. Remember that a low voltage will trigger
an extended pulse width to try to get the engine to "trigger" the alternator
into an output situation by increasing the RPM.

Keep us posted...seems like you've checked everything else. You didn't
mention if the engine vacuum changed as the pulse width at the injectors
changed (geez--did the pulse width change ?). If the valve lifters get
pumped up, vacuum drops, map tells the ECM to richen up !!

Lyndon IPTECH
-----Original Message-----
From: EFISYSTEMS at aol.com <EFISYSTEMS at aol.com>
To: gmecm at efi332.eng.ohio-state.edu <gmecm at efi332.eng.ohio-state.edu>
Date: Monday, May 24, 1999 11:50 PM
Subject: 94 Truck dilemma


> 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
>