Intermittent starting gremlin driving me NUTS

Schurkey said:

Sounds to me like this is a two-part problem.

1. Original poster doesn't appropriately specify whether this is a CRANKING problem or a RUNNING problem.

I believe this qualifies as a cranking problem. 90% of the time the truck cranks perfectly with the flick of the key, half second or less of starter engagement before firing, initial RPM’s upon firing reach 1000-1200 then quickly settles down to idle revs around 700. I qualify that as proper cranking behavior. When the stumble occurs, the starter duration feels about the same (half second or so of starter engagement before fire), but initial combustion is rough with RPM’s around 400-500 for about 1-2 seconds, then stabilizes at typical idle revs (~700). After idle stabilizes, the engine accepts throttle normally and runs identically to how it does when it starts cleanly.

2. Diagnosis for "Won't Crank" is entirely different from "Cranks, won't run properly".

The truck always starts, it’s just a matter of whether it fires immediately as expected or stumbles as described above.

3. Folks pile on with "Won't Crank" advice when the problem seems to be that it cranks OK, but won't start and run right.

Most of the diagnostic procedures I’ve researched fall in the “won’t crank” category.

4. No codes? Fine. Connect a real scan tool, look for anomalies in the data stream. Specifically, coolant temperature, cam sensor/crank sensor alignment, (cam offset, cam retard, or however the scan tool identifies it) and idle air bypass position/commanded idle speed. (but don't ignore anything that looks suspicious.)

I have one of the WiFi scan tools that connects to my phone and displays data through one of the various OBD apps. I’m assuming this doesn’t qualify as a “real scan tool.” The app I have however does allow me to monitor live data from various sensors in real time.

5. If it were me, I'd also test the ignition coil with a spark-tester calibrated for HEI ignitions. Available in well-stocked auto parts stores or on Amazon for <$20.

Ignition coil is about 6 months old, replaced with the tune-up I mentioned in the original post. The issue occurred both before and after said tune-up. However I see the benefit of testing to verify it’s function and confidently eliminate it as a culprit.

6. Sure, this could be ignition module, it could be TPS, it could be a heat-soaked ignition coil on the verge of meltdown. It could be a dozen other things. Start with the data stream.

Will a rental tool from the auto parts store suffice, assuming my WiFi module doesn’t cut it?

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