K2500 goes into and stays in Open Loop

Engine is warmed up and runs fine in Closed Loop until hard acceleration like up a hill. Then goes into Open Loop, stays in Open Loop and runs poorly until stopped and restarted.

1997 K2500 5.7l.

All 4 O2 sensors are new.

Connect a scan tool. Read the data stream. The fault will be identified in the data.

Sounds like something is bad enough that it's setting the "limp-home" mode on the computer.

I'm surprised it doesn't trigger the SES light.

More info if it helps with any potential suggestions.

I've been chasing error codes down the rabbit hole for a while.

Got Lean errors (both banks) and with almost 100,000 on the truck decided to replace all 4 o2 sensors just as a normal service action.

Didn't make any difference. But at least I'm sure it's not them.
Next replaced PCV valve/grommet/hose as it seemed to be loose and a possible vacuum leak.

Got slightly fewer errors after that.

Was getting LEAN error much more on Bank 2.
Noticed leaking where exhaust manifold joins exhaust pipe.
Leaking on both banks but definitely much worse on Bank 2.
I sealed the leaks on both sides.
Engine ran much better.
Took it for a long drive on highway and it ran fine with no errors but I had now bought a live scanner (Blue Driver) and noticed both banks would give normal STFT and LTFT readings but jump to 25% LTFT at about 2700 RPM (Blue Driver seems to max out at 25%). Closed/Open Loop was normal. Ran normal.

Next day went out and ran fine until I accelerated up a hill and noticed it had gone into Open Loop and stayed Open for the entire drive home. (running poorly).

I disconnected the battery overnight thinking maybe everything just needed to reset from default.

Out for drive next day, started and ran fine again until hard acceleration again, then into Open Loop mode again for the entire ride back to house (running poorly).

I was not scanner-monitoring TEMP on those later runs but engine was fully warmed up by the dash gauge. (So I have no record of temp on the data downloads)

Is it possible it just takes a LONG time for the PCM data to reset to accommodate the lack of exhaust leaks when jumping into Open Loop from acceleration? Like a dozen or more long trips time?

Does disconnecting the battery overnight ZERO OUT the PCM data tables such that it has bad data to default to in Open Loop mode?

It did trigger the Service Engine light.

With Lean Error and Low O2 sensor voltage codes.

What is your fuel pressure when you stab the throttle at higher RPM (Heavy load)?

Aren't those Vortec engines known for having problems with the Fuel Injection "spider"?

post all the codes .. my guess the engine is running lean . O2 sensors ALL will not cause this. the fuel pressure will .. so when the miss fires occur it will drop into open loop.. 25% LTFT is the max so if it is over that max then the PCM shifts to the open loop.

vacuum leaks and low fuel pressures will do it .. fixing the vac leaks will move the issue to a slightly better run .. so my guess is 2 problems .

What is your fuel pressure when you stab the throttle at higher RPM (Heavy load)?

Aren't those Vortec engines known for having problems with the Fuel Injection "spider"?

Fuel Pressure didn't appear as a monitoring option on my Blue Driver scanner so I don't know that.

The spider pump is what I am thinking as a possibility as I replaced the fuel pump recently with OEM and the fuel filter just before that. It is in the shop right now having the brakes done and I will discuss this with them once they finish that. I assume every shop is capable of diagnosing that?

spider is common failure on these at this age ..

Update.

Had the brake work done and they also ran the vehicle through their scanner computer.
Everything looked fine to them and their scanner guy doesn't think it is the spider based on this and other observations of the engine, though not 100%.

Anyway it seems to have corrected from not exiting the Open Loop mode after acceleration. Last few drives it drops back into Closed Loop just as it's supposed to do.

So I'm thinking more and more that the exhaust leaks had set the Long Term Fuel Trims so far out of whack that once the leak was corrected it took quite a number of rides for the data to reset to something that didn't look to the computer like a major foul up whenever it tried to recover from a temporary Open Loop mode. Like 8 or 10 rides.

I still haven't run it on a longer highway trip to see if they still go to 25% at 2700rpm or if that corrected as well. Reads as it should for short rides at that RPM.

well they said it looks fine so the vehicle fixed itself . you cannot have exhaust leaks of any kind with O2 sensors ..

well they said it looks fine so the vehicle fixed itself . you cannot have exhaust leaks of any kind with O2 sensors ..

Yep, that was the biggest problem.

Still want to test it on a long highway run as I suspect it might still be getting restricted fuel under extended high draw conditions. And everyone does cite these for injector spider problems at this age/mileage.

Anyway it seems to have corrected from not exiting the Open Loop mode after acceleration. Last few drives it drops back into Closed Loop just as it's supposed to do.
It should not be going into open loop under acceleration. Still has problems to be resolved.

It should not be going into open loop under acceleration. Still has problems to be resolved.

Actually it's normal for hard acceleration to drop the car into Open Loop. The problem is when it doesn't return to Closed Loop ASAP afterwards.
https://www.enginebasics.com/EFI%20Tuning/Closed%20Loop%20o2.html