I'm beginning to wonder if you might possibly have a bad PCM. If you do it will be the first of this vintage I have personally heard about. Lots of them get replaced needlessly. Lets initially focus on the fact that yours should be controlling and adjusting fuel trims when in closed loop and go from there. Its all I got right now. Please put all 4 fuel trims, rpm, and both fuel system status PIDs in Dash Command and screen video capture the result! Of course pursue whatever fuel pump tests you wish to conduct, but I dont think the pressure is different enough to make any difference. But I have been wrong before!

 

Ahh thanks Les, I will put my efforts most into fixing the fuel trims now and go from there after as I know there is a problem with them now. I have the video here, went into closed loop fast because I just had it started for a bit. I also put some other pids up to check some sensors, it seems all the sensors are working well, just not the fuel trims. Any idea about how I can test this more?

 

OK - there seems to be multiple fuel trim choices in Dash Command. The most basic ones are basic "SAE" Fuel trim PID that was mandated by the government. The others are enhanced GM PIDs that GM made for some reason. I thought Dash Command had the PIDs filtered for the ones that don't work with your vehicle, but that may be incorrect

1) So Long Term Fuel Trim - Bank X and Short Term Fuel Trim - Bank X are the basic SAE PIDs that are supposed to work with everything.
2) Long Term Fuel Trim Average - Bank X and Short Term Average - Bank X are GM enhanced PIDs. These read zero on mine for a few minutes (apparently collecting data) before it started to show an average similar to the SAE PIDs.
3) The Oxygen Sensor Short Term Fuel Trim Bank X - Sensor X that you showed is a SAE PID. I suspect it is the same as the SAE Fuel Trim PID, but a different name because it is grouped with the Oxygen sensor PIDs. So maybe your issue with fuel trims is only the PIDs that you chose. Have you tried graphing them in Torque Pro? But anyways, please give them another try in Dash Command. You may be just fine with them!

My Fuel Trims in Torque Pro stayed steady (long term at 0.8, short term at zero) until closed loop hit.

 

I wanted to add that when I have more than one PID to choose from for a particular reading, I typically try to choose the universal SAE PID. Just remember that the SAE PID set is much smaller than the extended GM set - but the GM PIDs often have to be matched with a particlar make/year/model vehicle. The SAE PIDs are supposed to work universally. This is where Car Diagnostic Pro is a pain (no filtering at all - just a big mix to try). I always thought Dash Command had done this filtering when you paid for the GM PIDs. I guess both PIDs do work on my vehicle. Question is if the average GM Fuel Trim PID is what you want to use. Apparently not.

 

Thanks for clarifying that Les, I will put up the basic PIDs on the way home later and see if that makes a difference, I will also try plugging in my wired scanner that has read fuel trims on my blazer before (and I've seen them working). I was looking over my video from yesterday again and noticed the o2 sensors reading 0v more than once, and many times reading a very low voltage below 0.1V, if I'm not mistaken and they are working properly, this correlates to a lean condition? Furthermore, it seems like both bank 1 and 2 sensors will dip near zero together. Hopefully Les you know more about how the o2 sensors operate on these trucks and can guide me in the right direction here if something is out of whack with the o2 readings.

 

Pre converter O2 sensors cycle rapidly. Only way to really see what they are doing is to plot them (Torque Pro or Car Diagnostic Pro), and only plot the S1 O2 sensor PIDs for both banks (that's not what I did below - but it was the screen capture that I found on my phone - in any case it shows one O2 sensor cycling). As long as they are cycling across 0.45V in general, then they are controlling the fuel mix (may or may not be right mix, but they are being used for control and the computer thinks it is right). Any single displayed value could be at any point on the plot that happens to be sampled. Then if the fuel trims are reasonably close to zero, one can assume the correct amount of fuel is actually being delivered to each bank. Note this is all true in closed loop. Open loop the PCM has to rely on programming, but if this is good then open loop should also be good.

0.1v is lean but only if it stays there and doesn't cycle at all

 

Okay thanks a bunch, Les... There has been some new developments here, so on my way home I plotted the o2 sensors and immediately saw inconsistencies, both sensors would stay low then go high at around the same time, but it seems to only happen at idle, while driving my o2 sensors look good both oscillating up and down pretty quickly. In particular, after I would give it some gas at idle, both o2 sensors would always go rich then lean right after, I made sure to put the sound on in this video so it's easier to see when I rev it up, in the first bit I went for a quick drive to get it fully up to temp then was just idling in the last part, I think it's pretty clear to hear. To add to this, in torque pro I saw my fuel trims working, I'm not sure what's up with dashcommand not reading them! Not even the short term or anything! I will look into that more. Anyways still not convinced I plugged my wired scanner up and took a reading of the fuel trims, well let's just say they are not looking good! Picture below. So, with the LT trims pinned so negative, I am actually chasing a rich condition here? Hmmm.

 

I'm on a trip to visit my kids and grandkids through Monday - please bear with my slow responses

 

I'm on a trip to visit my kids and grandkids through Monday - please bear with my slow responses

 

No worries Les, feel free to jump back in whenever. I did some more diagnosing yesterday, and plotted short-term fuel trims, again not seeing anything concrete other than a bad running engine. Then I went over and took a look in the tb, in the area I could see around the cylinder 1 and 2 intake ports it looked clean and dry which I would expect since I just cleaned everything in there. Then I hooked the fuel pressure gauge back up and had it running and did another leak down test, it all checks out, I stab the throttle and the pressure immediately hikes up to around 60psi which is normal as far as I believe? 50psi idling and will hold 55psi for 10 minutes after shut off. I am almost sure now my fuel delivery system is good, it's passing all the tests I throw at it, the thing is I can smell raw gas from the tb, but there's no leak? This is almost pointing in the direction of something mechanically wrong here, I'm thinking about getting the compression tester back out and doing all the cylinders again to see if I can find something. It's just weird as I did a compression test on all the cylinders before it went back in and they all checked out? In addition, misfire counters are not telling me anything! They will kind of bounce around like yours do Les showing maybe 0 to 15 total misfires at a given time. Also, when I had a dead cylinder and had it running with the fuel pressure gauge hooked up I could see the fuel pressure jumping all over the place because the vacuum in the intake was all messed up from the dead cylinder, this is not happening now, the fuel pressure stays steady while idling. This indeed has me stuck here as I'm not sure where to go from here other than doing a compression test, I'm also thinking a crank sensor relearn might help to tell me something on the misfire counters? I haven't done that since taking it out to replace the front cover. I guess I could also have a closer look at the ignition system, hmm. Here are some videos of me doing some of the things I was talking about above.