1997 5.7 Vortec misfire/stumble

[DaHoe]

Hello everyone I have a 1997 Suburban that's giving me a fit. Generally can diagnose anything but after thinking for a few weeks and trying a few things I'm interested to hear some ideas outside the box. Truck only has 120k original miles. The issue I have is when it warms up and goes into closed loop (converter locks up) I get a stumble of I modulate the throttle and stay in lock up. For example cruising 60 mph.. lightly press the throttle to accelerate while keeping the converter locked..truck will make a single "buck" like you turned the key off and on really quick. Also if you are steady accelerating with the converter locked and then quickly back off the throttle it will give the same "buck". Wide open throttle runs great. All other times it runs great. Starts right up immediately cold or hot. No codes currently but did have a cat monitor O2 heater code previously. I have done a full tune up with quality parts, replaced the EGR, and verified cam retard at -1*. When I checked the cam retard I was excited to find it at 12* and thought I found my issue. I reset the cam retard to -1* with my snap-on scanner hoping that was my problem but it remains. I don't believe it's a fuel issue because of the violent nature of the stumble. It definitely feels electrical/ignition. I have also monitored the crank sensor with the scanner and don't see any drops. TCC slip rpm also doesn't really seem to jump around during the stumble. I really don't want to throw parts at this thing. Thanks for any ideas.

 

[Schurkey]

Verify TPS.

Verify wire harness providing power to coil/ignition module

Verify spark power with a spark tester calibrated for HEI.

Record the data stream while deliberately trying to induce a "buck", see what the data stream shows during the time that the engine stumbles.

 

[DaHoe]

Ok so I have been looking at all of the relevant data and can't see anything on the input side of things. Crank signal holds steady, TPS sweep looks good and doesn't fall out when the buck occurs. MAF readings are within spec. When the "buck" occurs I get a few misfires on the counter and the graph shows that all misfiring cylinders occur at the same time. The cylinders are random but in time with each other. I have since replaced the whole distributor and verified cam offset dead at 0, replaced the ignition coil, module, cleaned and verified battery terminals are tight (battery and alternator is brand new) with no change at all. Next step is clean all the ECU and frame grounds just to rule that out. Then try to scope the ignition output to the coil and see if I can get a bad signal there. At this point I'm leaning towards a bad coil driver or something in the ECU. Thanks Schurkey for your input

 

[DaHoe]

It has been the same for a month now but finally it randomly stalled out in a parking lot and will not start. Yay now I can fix it! Lol! It will cough and sputter it definitely seems spark related. I have crank signal on the tach while cranking. I'll have a chance to mess with it today I'll post up what I find.

 

[DaHoe]

So after diagnosing the now current no spark issue, I found that the brand new NAPA coil failed after a few hundred miles. I temporarily installed the original coil and it started right up and runs exactly as before with the misfire/stumble still present. 5 steps sideways, one step back, one step forward lol