Thanks, Walt for setting us straight on the base idle screw. I got my trucks mixed up. The Workshop Manual show idle spec is 700 +/-20 RPM (all loads off), so czerta's 97 is right on spec.

To reset adaptive values, I was able to load a GEMs Lite app on the same tablet that runs the Torque app. The GEMs app was very useful any time I worked on the throttle position sensor to reset the ECU and prevent the very high idle condition. It also resets Long Term Fuel Trims back to zero.

 

Current mileage 190k. I've been using the gems android app to reset the adaptives anytime I touch intake or sensors on this truck so they should be fairly clear. Injectors replaced and fuel rail blown out/cleaned at 185k
Data pulled from schrader valve:
Fuel pressure idle 32psi
Fuel pressure 2k rpm 30psi
Fuel pressure with regulator unplugged 39psi, made a very nice hissing sound and lots of vacuum pressure

Some data pulled from torque:
idle rpm ~700rpm
O2B1S1v at idle 0.9v, under load keeps rapidly oscillating between 0.9 and 0.0
O2B1S2v at idle 0.8v sometimes other times 0.0-0.1, under load 0.0-0.1
O2B2S2v at idle 0.9v, under load keeps rapidly oscillating between 0.9 and 0.0
O2B2S2v at idle 1v, under load 1v (bad sensor?)
Longterm Fuel Trim 1: -14.06%
Longterm Fuel Trim 2: -14.06%
Shortterm Fuel Trim 1: 21.88% idle, 3.12% at load
Shortterm Fuel Trim 2: 23.44% idle. 10.16% at load

So it looks like O2B2 sensor is bad and im running very rich?
Difference between shortterm fuel trim 1 and 2 at load seems concerning since they are roughly in sync at idle. Could this indicate a blocked injector or similar fuel rail issue?

Worth noting that when I got the truck I had downstream (bank 2) codes and replaced both downstreams before doing the cats so its possible the downstreams are fouled, but its my understanding that the upstream controls mixture and the downstream exists just to make jobs for emissions shops. Both bank 1 sensors are brand new and done within past 150mi.

 

More or less correct about the downstream sensors. The ECU doesn’t use them to modify fuel delivery. They only check for a difference from the upstream sensors to ensure that the Cat is doing something. The downstream sensors will throw a CEL but it won’t modify the fuel.

 

That's good data! (Except you typed B2S2 twice. That's OK, I think the first one is B2S1, easy to fat finger).

Short term fuel trims: Adding lots (25% is maximum) of fuel at idle, adding little fuel at load; typically indicates a vacuum leak...somewhere. I needed to re-seal my intake plenums, but could be at an injector O-ring as well, or somewhere else. Air leaks in more at idle when the plenum vacuum is maximum, then the ECU tries to compensate by adding fuel. Interesting thing for me was after the plenums were re-sealed, some ticking went away.

Long term fuel trims: Subtracting fuel indicates engine running rich on average.

Fuel pressures look good. Start with fixing the vacuum leak, the rest might follow.

2 cents worth: My favorite sealant for the plenums is Loctite 577. 567 works good too. I apply Loctite SF 7471 primer (activator) first, that helps speed up and strengthen the anaerobic cure. Wipes off clean for a nice look. I have also used silicon sealants, but harder to look good. The Loctite is recommended by LR.

By the way, I have both downstream O2s disconnected. Even though I understand the theory that they do not effect fueling, I have a sneaking suspicion that they do under certain circumstances. Disconnecting them removes them from the equation. Maybe I'll plug them back in some day? The truck runs great without them.

Maybe focus on finding a vacuum leak.

 

yeah, vacuum leak now at top if list (or the new MAF is a lemon). Those fuel trim levels are no bueno.

How is the hose from passenger valve cover to plenum?

Maybe a smoke machine in your future.

 

Passenger valve cover to plenum was replaced and is solidly seated.
MAF wise I have an original (96 production) maf and two aftermarkets. Originally the 96 maf was giving me a MAF sensor code. Ill swap around all the MAFs later today and report back on those.
Vacuum leak wise I dont hear any hissing and anytime I disconnect a vacuum line while running the car will stumble more as expected but the vacuum pressure is strong.

After swapping injectors I had a small sealing issue where I was leaking fuel between injector and rail on one and injector and block on other. I tweaked the injectors a couple degrees of rotation in both cases and fixed the leaks. So my current suspicion is that it could be the o-rings for the injectors?

 

Test results are in.
Vacuum leaks:
Checked all the major lines around intake/valve covers they are all seated firmly and all lines have been replaced as part of this ongoing problem solving
Sprayed "extremely flammable" non-chlorinated brake cleaner around various points near intake/throttle/injectors without any discernible shift in RPMs

MAF:
Basic process was; bring car to temp, kill engine, swap maf, start engine, clear adaptives, stop engine, start engine, take STFT values. Process repeated for each MAF. Values taken at idle.
Aftermarket MAF #2: See post #12 for values
OEM 1996 MAF: STFT1: -10 to -15%, STFT2: -6 to -10%
Aftermarket MAF #1: STFT1: +0 to +3.3%, STFT2: +0 to +4%

Aftermarket MAF #1 installed currently, but miss remains.

 

Not quite getting the whole picture. Does STFT change a lot between idle (adding fuel) and highway (adding less or subtracting) driving? If so, that indicates a vacuum leak, which obviously is playing hard to get. It could be one small, or several small leaks that do not make any hissing sounds. Remember, Long and Short fuel trims add together, so good to record Long Term trims as well, even though the adaptive values were reset. Long Term changes are driven by Short Term values out of whack for long enough times.

The upstream O2s should oscillate at both idle and highway, with temporary rich or lean events during transitions (cold start, accel/decel). Have you viewed the Timing Advance in real time data? How about Engine Coolant Temperature? I had a loose connection on the ground splice for ECT that caused a signal for really cold on bumpy roads. Same ground point for MAF, TPS, and fuel rail temperature sensors. Seems you're finding all the places the problem isn't. That's real diagnosis and troubleshooting.

 

STFT changes significantly between idle and highway driving.
With MAF #2 I was seeing readings as high as +24% STFT under highway load
Upstream o2s are oscilating when at temp
Timing advance was 28deg when I pulled data for post #16, any conditions I should check under for it?
Ive been using a scangauge to read coolant temp sensor data for ~3mo and the output seems to match reality. Reflects thermostat opening, cold starts, etc.

 

More hard data and a video:
Throttle: 11.37% at idle
Timing advance: 28.5-30.5 at idle
Vacuum (As reported by ODB2): -29.87 to -29.88in/Hg at idle and under load
Quick video in bay of it being revved up and down, audio sounds a bit distorted under load due to fan blowing at the mic but the idle is fair to reality: https://files.catbox.moe/1rejcm.mp4