94 Truck dilemma

[Shannen Durphey]

EFISYSTEMS at aol.com wrote:
> 
> 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

I try to eliminate as many sensors as possible when something like
this happens.  You never mentioned O2 readings.  Don't fall into the
trap thinking a replaced part is a good one. I agree with Sneezy, try
disconnecting O2 while running rich, see if fuel delivery returns to
normal.  I've seen problems with aftermarket sensors.  Sometimes they
do strange things.  Also had trouble with ground at thermostat housing
causing problems.  ECM O2 circuit grounds there.  Make sure all
threads and water outlet are clean, and that the wire isn't corroded
inside the terminal crimp.  Sounds like you're using a Snap-on
scanner, so data refresh rates should be fast enough to catch an
intermittent problem.  Can you force the engine to go leaner by faking
signals to ecm?  Is it responding to a rich O2 signal?  Can you make
it respond to one?  Exhaust system prividing suitable ground?  Is this
an old problem, or something that just came about?  
HTH.  If I think of anything else, I'll post it.  Let us know what you
find.
Shannen