1998 no load stumble

[Srg1016]

I’ve posted a couple times here with no resolve. My problem is a slight stumble or mis-fire at highway speed upon a slight reduction in throttle pressure. No codes and all new sensors and plugs, coil, wires and distributer. Black bear sees nothing unusual in the spark tables so I’m chasing the ignition and fuel circuit.
Is anyone able to explain what happens when you slightly reduce the throttle while at highway speed. As in what sensors see the throttle reduction and how is the spark and fuel compensated at just that moment? Been 4 months and the only thing progressing is my frustration!
Anybody I can really use the help?

Ex: Maf pressure reduced signals ecm to …………… do what to spark and fuel table? Vacuumreduction causes what?

 

[docstoy]

Don't guess you access to a scanner ? How about distributor shaft , how much up and down play do you have ? Also is transmission tcc trying to disengage ? Just a few thoughts , hope it helps .

 

[Srg1016]

Thanks D, although I have replaced the distributor I have not explored the tcc. At least I have a clue. Had a friend put an older scanner on it but. It was to slow to catch the problem. Yep I have no scanner. I’ll research the tcc today.

 

[Road Trip]

Thanks D, although I have replaced the distributor I have not explored the tcc. At least I have a clue. Had a friend put an older scanner on it but. It was to slow to catch the problem. Yep I have no scanner. I’ll research the tcc today.

Knowing that a *light* touch of the brakes while cruising (ie: just enough to activate the brake lights, but not enough to create drag)
will disconnect the TCC, it would be a easy ad-hoc test to gently left foot the brake pedal while right foot on the accelerator, take the
TCC out of circuit and see if this changes the behavior of the vehicle.

It's an outside chance, but sometimes the only way to track something like this down is to methodically move all the involved parts from
the 'could be the cause' pile to the 'proven good' pile. And eventually the last part remaining ends up being the root cause.

 

[docstoy]

Road Trip is correct about light brake pedal pressure , good thought. As to your question about sensors just about all of them will a change instantly and others soon thereafter . Also just because you install new parts doesn't make them good , not being smart but I have had gm parts that weren't as good as the old part I had just removed . That happens more now than ever with our global economy and not just China parts they get them from everywhere . Anyway when changing parts inspect what your about to use and will ck back if anymore questions I'll try to help and others here will also .

 

[Srg1016]

Just a short post on my progress. Fuel map, no issues. Spark and advance tables no issues as verified by Black Bear. All sensors, coil, distributer, plugs, wires, replaced or no issues found. A visit to my transmission rebuilder with 30 years experience building transmissions informed me that the transmission is working as it should with no issues. His advice after explaining all I have been through is “ look at the transfer case”.
Is it possible that my transfer case could cause the slight stumble I am experiencing as in a mechanical problem. That’s the direction I’m going now with the NP243.

 

[Srg1016]

Problem solved!
After chasing the slight stumble at throttle reduction it has finally been determined. The transfer case output shaft bearing were shot causing the shaft to “wobble” enough to reduce load on the driveshaft for a split second. Remanufactured transfer case resolved the problem
Thank you all for your thoughts and ideas. Happy to be part of this forum.