Tahoe bucks when accelerating at highway speeds but only once!

[Tator]

Hello everyone,

Vehicle:
I've got a 96' tahoe with the 5.7 vortec.

Symptoms:
When:
cruising around 60-80mph in 4th, overdrive engaged and push the throttle to maybe 80%, holding it right before it will downshift while usually going up a hill. Or essentially lugging the engine under high load. It wont do it when I let it downshift and I can floor it and it has great acceleration with no issues.

What:
it will buck/lurch/jump. It's so fast I cant tell if its pushing me forward or im dropping back. it will do this once, maybe twice. Again, fast enough that I dont hear any difference in the engine and there is no change in RPM or speed so it's not the torque converter locking/unlocking.

Parts Replaced Recently:
Spark Plugs
Spark Plug Wires
Entire Distributor assembly
Starter
Fuel regulator (May accidentally have left the small *nub* oring in there, so when putting the new regulator there's now two o-rings stacked)

Codes:
Bank 2 Sensor 2 O2 Low Voltage
(previously) System Too Lean on Bank 1 and Bank 2

My thoughts are maybe that it's either the coil (I know it's factory, and at 170k miles I feel like it could be ready to be replaced), or a fuel issue.
I have no codes that show up, other than low voltage from O2 Sensor 2 Bank 2. I suspect maybe a fuel issue as I have the stock spider assembly and reading out the fuel trims it's hanging out around +23 to +25 fuel trim for both bank 1 and too. No system too lean code however.

One other thing to note that may not really be useful but at idle it will "blub" once out the exhaust, like a cylinder missed once.

Maybe it's all stemming from that fuel regulator having the second oring smashed in there, or something else.
Any advice would be greatly appreciated!!

 

[Hoesgottaeat2]

Hate to just throw parts at things when you’re not throwing a code.

But…it’s inexpensive enough to try this imo.
Have you tried replacing the throttle position sensor?

 

[Doubeleive]

ya the throttle and/or air flow sensor are the likely culprits here. try cleaning them real good and make sure the harness is clean with no corrosion.

 

[Doubeleive]

maybe take a close look at the transmission harness also

 

[Tator]

Hate to just throw parts at things when you’re not throwing a code.

But…it’s inexpensive enough to try this imo.
Have you tried replacing the throttle position sensor?

I have not tried! I know for a fact its factory so it may be worth trying to take a look at it. Is it worth doing a sweep test with a multimeter/oscilloscope or just go ahead and replace?

 

[exp500]

I would lean more toward Fuel. An easy test is to remove fuel filter and do a Pressure and Flow test. If that passes, then recheck FPR. The FPR gasket kits are cheap.
Vacumn leak and MAP sensor another possibility.
TPS is somewhat rare on these other than a dead spot that grows.
Aftermarket distributor caps are always the first culprit to suspect on Ignition. Good Luck!

 

[Hoesgottaeat2]

I have not tried! I know for a fact its factory so it may be worth trying to take a look at it. Is it worth doing a sweep test with a multimeter/oscilloscope or just go ahead and replace?

If you’re confident testing it, by all means do so. A lot of folks don’t have the know-how nor the equipment. But if you do, go for it!

 

[Eman85]

Did it buck before you installed plugs and wires? Or was that part of the attempt to cure it? Try something that is free. Get a spray bottle with water, have someone power brake the truck and mist water around each plug wire and plug and see if it bucks. You don't have to torque it up a lot just enough to load it.

 

[Tator]

ya the throttle and/or air flow sensor are the likely culprits here. try cleaning them real good and make sure the harness is clean with no corrosion.

Replaced TPS sensor, with no aid in the symptoms. Air flow sensor was cleaned and according to my scan tool is reading appropriately (at least increases seemingly linearly with TPS).

I would lean more toward Fuel. An easy test is to remove fuel filter and do a Pressure and Flow test. If that passes, then recheck FPR. The FPR gasket kits are cheap.
Vacumn leak and MAP sensor another possibility.
TPS is somewhat rare on these other than a dead spot that grows.
Aftermarket distributor caps are always the first culprit to suspect on Ignition. Good Luck!

Fuel Pressure Regulator Gasket im assuming is FPR? I may have to open it up again, I just hate removing that top plenum as it's a massive PIA to get the top of the spider injectors pressed back in. Vacuum leak could be possible for sure, will have to test using either propane or spray cleaner around. Distributor assembly is brand new as of a few months ago addressing a different problem (installed by a reputable shop, not me lol). Also MAP sensor seems to be ready correctly (at least at idle, reads 30kPa).

maybe take a close look at the transmission harness also

Anything in particular I should look for?

I would also like to update that I had layering problems, had a fuel line rusting through that was slowly bleeding fuel. Replaced the filter, the lines, fuel tank, and fuel pump/sender assembly. This had no effect on my hesitation issue. The title is WRONG however, it does not miss only once. It will do it several times and surge/buck/hesitate several times in rapid succession. No misfire codes, however and indicator towards EXP500's idea is that it's fuel related as my long term fuel trims are running right around 15% for both banks, it's too soon to see if it'll throw lean codes for that as well but last I checked I had no leaks around the upper intake plenum.

 

[Tator]

Did it buck before you installed plugs and wires? Or was that part of the attempt to cure it? Try something that is free. Get a spray bottle with water, have someone power brake the truck and mist water around each plug wire and plug and see if it bucks. You don't have to torque it up a lot just enough to load it.

Unsure if it did buck before, as the plugs and wires were installed by a shop to solve the no-start condition (ultimately was a worn distributor gear), and before then it had sat for several months due to me being away for school.

Have been driving it since January, cannot believe it's been this long since I've had this issue, with it not getting any worse other than what I can attribute to the fuel leak mentioned in the previous comment)