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Old 05-21-2010, 11:22 AM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by Trainwreck
I recently finished up a rebuild on my 1989 C2500 TBI 5.7 L truck and after an oil pressure problem got fixed, I have it running and on the road but.... I've been trying to figure out a problem with stumbling, hesitating, and dying. It has been going on from the first time I drove it after the rebuild.
The truck hesitates and stumbles BAD when I lightly apply the throttle. If I put my foot in it when leaving a stop it will accelerate okay but if I just ease away in a normal fashion it will stumble, buck, and even die if I don't get deeper into the pedal. I thought I had it fixed after replacing the EGR solenoid but qapparently not. I drove to town this morning and the problem is still happening. I nursed it to the parts store and, since it is the only thing I could think of, bought a new ($60) TPS. I got home and installed the TPS and went for a drive. No change at all. I'm completely baffled.
Here is what I have done so far:

Fresh build on engine- 250 miles so far
New plugs
New plug wires
New cap
New rotor
New fuel filter
New air filter
Timing set to 0 degrees with tan wire unplugged per the book
No vacuum leaks- checked every line and around throttle body with propane
Fuel pressure rock solid at 12.5 psi
EGR valve tested and passed
New MAP sensor
New EGR solenoid
New Throttle Position Sensor
New Coolant Temperature Sensor

Truck runs fine when cold, problem becomes noticeable as engine approaches normal temp.
Anybody got any suggestions?
The injectors are aging out, they're just not operating well when the flow demand is low, probably not shutting the fuel flow off completely so they run a bit rich for the low speed condition where there is more "closed" time in relation to open., so the dribble is a bigger proportion of the total fuel demand. You don't see it when the engine is cold and running on "choke" as the engine wants to rich at this point; then don't see it again when the RPMs or power demand is higher as the injector is passing the off phase so quickly that the dribble is inconsequential against the total flow and the computer can probably still trim for it so it isn't throwing any codes.

I'd start with a couple doses of injector cleaner, if that doesn't improve it, then some rebuilt injectors are in order.

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