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KivPanther 02-19-2021 02:15 AM

Stumble troubleshooting advice
I’m having a bastard of a time tracking down this intermittent stumble in my V6 so I need to pick some brains.
  • It only happens when under light acceleration or cruising; doesn’t do it coasting under power, idling, or under heavy acceleration.
  • It seems to only do it around about 3k RPM.
  • It does it both cold and hot with no seeming change in frequency.
  • It happens very quickly, often a couple times in succession. Not enough to cause the tach to jitter even.
  • It also does it while cruise is active and doesn’t seem to affect its performance.

I have replaced many vacuum hoses and couldn’t find a leak using smoke. The O2 sensor has been replaced. I cleaned the MAF while the manifold was off the truck. All the injectors are brand new and have brand new o rings and seals. The distributor is new, the spark plugs are new, and the plug wires are new. Any thoughts?

Deerhurst 02-19-2021 03:10 AM

Any codes? How old are your plugs and wires?

KivPanther 02-19-2021 03:32 AM

No codes. Plugs and wires are brand new. NGK iridium plugs (real ones) and NGK blue wires.

Ftown 02-19-2021 03:50 AM

It happens while in cruise control? Maybe the cruise mechanism is going out. Try smoking the cruise control mechanism. It also runs off vacuum

Ftown 02-19-2021 03:55 AM

Also, I'd check your injectors. I have an f150 that I chased a miss I thought was a vacuum leak. Turned out to be a bad injector

KivPanther 02-19-2021 05:08 AM

Sorry, I should clarify: It doesn't happen ONLY while in cruise control, it happens ALSO while in cruise control. I actually just replaced the ASCD vacuum hose since it had rotted, but it has its own vacuum pump separate from the engine control system. The fuel injectors are, as mentioned above, all brand new, tested working, and getting pulse.

89'HBV6 02-19-2021 08:31 PM

If you replaced the vac line to the cruise, what condition was it in? Could be some smaller particulates got pulled in? ...from the inside of the old rotted hose? (thinking outloud)

KivPanther 02-19-2021 09:46 PM

The ASCD itself is old it in good shape. The only thing bad was the vacuum hose. Still though, since it does it regardless of whether I’m in cruise control or not I doubt it’s related.

LugNut1990 02-19-2021 10:57 PM

So no change between before/after the new injectors?

It seems that the problem would be more evident under hard acceleration if fuel-flow was the cause, but did you change the fuel filter too?

If it is repeatable, try cracking the gas cap open and trying it again. Might generate an error code, but is the miss gone?

KivPanther 02-20-2021 01:01 AM

Correct; there was no change after the injectors were replaced. I did replace the fuel
filter at the same time. I had bought a new TPS but hadn’t gotten around to installing it yet since I wasn’t sure it would need it. I also haven’t been able to get the idle to drop all the way to 750 after warming up even after adjustment. I might just go ahead and swap them just in case.

I’ll give the gas cap thing a go on my way home from shift tonight and see what happens.

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