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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.

KivPanther 02-20-2021 05:13 AM

With the loose gas cap on the way home I noticed a few things. The stumble didn't go away, but it definitely seemed less intense. It also didn't start doing it until the engine had fully warmed up to operating temperature and only gradually came on once it did. Seems like I might consider replacing the gas cap and cleaning out the filler neck at the very least while hunting for the rest of it.

LugNut1990 02-20-2021 09:18 PM

Interesting! If not a venting problem, next _guess_ is the fuel pump or sock. Before that (PITA job) I'd check out the wiring, grounds, etc. Be careful!

KivPanther 02-21-2021 08:02 PM

Shall do. I'm also going to run another can of seafoam in the crankcase since I noticed that the rockers and inside of the covers were covered in crud when I replaced the valve cover gaskets. Gotta love farm trucks driven by 90-something year olds!

KivPanther 02-22-2021 07:13 PM

I did a quick test on the gas cap valves per the FSM and found out that while the intake is working, the relief is not. So new gas cap anyway.

kenmack 02-23-2021 08:48 PM

You might consult the factory service manual (FSM) in the EF&EC section which has some good flow charts for servicing. You should start on page EF&EC-45 which will direct you to the flow chart specific to your issue.

KivPanther 02-23-2021 10:08 PM

I have practically memorized that section of the service manual. I'm still at a loss, hence my request for out-of-box thoughts.

Deerhurst 02-24-2021 01:00 AM

I got 2lbs of tannerite when you are fully fustrated!

Got a quick recap of what has been done to job our memories?

KivPanther 02-24-2021 07:07 PM

Sure thing. To date:

  • New distributor
  • New NGK Iridium plugs
  • New NGK plug wires
  • All new injectors and seals
  • Replaced a bunch of vacuum hose and can't find a leak with my current test methods (smoke)
  • New fuel pressure regulator
  • New fuel filter
  • Throttle body cleaned
  • MAF cleaned
  • Throttle Position Sensor tested working
  • Throttle wires adjusted correctly
  • Timing double-checked and steady at 15 BTDC
  • New gas cap
  • New air filter
To re-iterate, the stumble does not happen at idle or under full throttle acceleration. It only seems to do it when my foot is barely to lightly on the gas and only between about 2500-3200 RPM.

Deerhurst 02-24-2021 08:58 PM

Does the TPS have a dead spot? Easiest to see with an analog meter.

KivPanther 02-25-2021 04:30 AM

It seemed steady with my digital meter but I'll double-check. I don't have an analog meter anymore so I'll just go extra slow.

Deerhurst 02-25-2021 05:51 AM

Digital meters have a tendency to smooth things out I always try to keep an old simpson on hand because analog cannot be replaced sometimes

KivPanther 03-01-2021 04:25 PM

I couldn't notice a definitive dead spot on the digital meter, but I DID notice that I could consistently get the engine to start doing the stumble parked under no load if I veeeery carefully held the accelerator at just the right spot. Coincidentally, the same spot I usually cruise at. I'm thinking you've nailed it, Deerhurst, and my TPS has a worn out spot. I'll report back once I've had a chance to install the new one.

KivPanther 03-09-2021 07:02 PM

No dead spot, but the wide open throttle position sensor wasn't working so I went ahead and replaced the TPS anyway. Haven't had a chance to test it under cruise yet but it does seem that the throttle response is a bit smoother. Will report back after some highway cruising today.

KivPanther 03-09-2021 11:21 PM

Started out fine but stumble returned on the highway. It is still much more muted than before, but I’m back to square one.

KivPanther 04-05-2021 02:03 AM

On a hunch I removed and thoroughly cleaned the Mass Airflow Sensor. The stumble is just barely noticeable now. Still hunting down the final cause, but it's looking like whatever it is was being worsened by a bunch of other stuff I've been tweaking.

KivPanther 06-12-2021 06:56 PM

So, update. The stumble hasn't gone away. In fact, it's pretty noticable now.

The fuel pump, all injectors, pressure regulator, fuel filter, fuel lines, air filter, all six spark plugs, spark plug wire, ignition coil, distributor, throttle position sensor, and the entire exhaust system have all been replaced with new components. The only things that are still stock (and tested working supposedly) are the MAF and coolant temperature sensors.

Since there is sometimes a noticable clunk from the rear end when taking off in 1st gear I'm wondering if maybe I'm barking up the wrong tree and should be looking for something worn out in the drivetrain. It has a new carrier bearing and the driveshat doesn't seem to have any play in it. Any thoughts along those lines?

diosmuertos 07-13-2021 02:56 PM

dumb question but did you reset the ECU after those sensor changes? it'll still be mad with the new ones if you don't.

KivPanther 07-14-2021 09:46 PM

By which you mean disconnect the battery for a few minutes? No, I suppose I didn't. I feel like the stumble itself is mechanical at this point, but I'll give that a shot and see what happens.

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