2000 GMC Yukon 5.3l Bank 1 LTFT 25%

I’m scratching my head on this one. When I first received the vehicle it ran terribly. Had p0300, p0307, and p0171 on top of the other codes it still has. I did some work to it and since the work they haven’t reset and the truck has been driven about 100 miles. However the weather also significantly warmed up since.

Intake gaskets, fuel pressure vacuum line, plug wires, and number 7 injector we’re replaced which smoothed the idle out however it usually sits around 500rpm regardless if in park/neutral/drive. Which seems a bit low.

Long term fuel trims on bank 1 run between about 14 to 25 and occasionally come down to 10ish during cruise. Idle is typically high. Anywhere from 14+. It fluctuates during cruise sometimes to 0 but quickly heads back to the higher end where it spends the majority of it’s time at. Bank 2 stays between 0 and 8ish typically.

The intake manifold absolute pressure PID is showing 8” of mercury even though 2 of my vacuum gauges are showing 14” (I’m at about 6,000 elevation if that helps.). Not sure what that is about. The PID will show up to 22” while driving around.

I tried leak detecting with carb cleaner and brake cleaner but found no change. Tried soapy water, nothing. So I did a smoke test. All I found was a little trickling out of the oil fill tube after running for about 10-15 minutes with the throttle body closed off (sealed off with a nitrile glove.). I checked everything under the hood. I paid extra attention to every around the port, injector, plug, connection, bolt, etc. I tried with the booster connected and with the port capped off. Removing a vacuum line would make smoke pour out so I know the machine was working properly.

The engine has 126k miles. Pulled the plugs on the drivers side. They were completely dry with whiteish deposits. It has some codes for the trans, ambient light sensor, and ignition key nothing I am concerned about as I have a replacement harness and solenoids coming for the trans. Ambient light sensor is just waiting for me to install it. I am guessing the ignition key code has something to do with the aftermarket remote start.

I am also adding some screen shots from my phone scan tool.

Any ideas to point me in the right direction would be appreciated!

Can you add all 4 oxygen sensor readings. Trying to see if one is sticking lean. Also what do the trims do at 2500 rpm

Hopefully this helps. I switched over to the laptop to get a little better idea what is going on. Also I have tried 2 upsteam o2 sensors. I also checked the fuel pressure even though I was doubtful. It rests at about 54-56psi fuel pressure. There are no abnormal sounds coming from the engine.

Have you tried a fuel trim reset with your scanner? After repairs that effect fuel trims gm recommends fuel trim reset. Also might be worth checking your purge valve to see if it’s stuck open.

Some quick thoughts.

The intake manifold absolute pressure PID is showing 8” of mercury even though 2 of my vacuum gauges are showing 14”

The MAP is ABSOLUTE pressure. Your gauge is BARO minus manifold pressure.

I am also adding some screen shots from my phone scan tool.

I suppose someone can help you with this, but there isn't much to work with here. It's just one moment in time and you've already eliminated the obvious stuff.

A baseline of your engine readings is the best starting place in order to give an informed diagnosis.

With the engine full warmed up to operating temp and at idle, graph both upstream 02 voltages, both downstream 02 voltages, STFT and LTFT for both upstream 02's and RPM.

Increase the idle to 1500 then to 2500rpm. Record the values.

With a bank specific problem, if that's what it is, maf and mass won't matter as they affect both banks.

Let's see the values and go from there.

I am probably not the best at explaining what I am seeing. So based upon suggestions I made two seperate videos showing idle, 1500, 2500. Hopefully these give some direction.


Video 1


Video 2

Parade graphs are hard to look at. Having trouble with the colors. But what I'm seeing is at idle LTFT on bank 1 going from +1 to +25 with increase in rpm and the B1S2 oscillating. You did not graph the bank 2 sensors. Need to see those as well.

Sorry. I am using an ObdLink MX+ with OBDWiz. It only allows me so many options per screen. Here are two videos going from idle, 1500, to 2500. Bank 1 and Bank 2.

Bank 1

Bank 2

Do B1S1 and B1S2 voltages in one video and then B2S1 and B2S2 voltages in a second video. 1 minute at idle and 1 minute at 2500 rpm.

Well after many a frustration trying to get my scantool to show me GM enhanced info I was unsuccessful in fact the majority of the time it failed to connect (ObdLink MX+ and ObdWiz Professional). I really hope to upgrade to a better scan tool in the future. However I did pull out the ole usb scope. I didn't have an attenuator because it ran off in the rubarb or something, but I went ahead and scoped the injectors with a 1x probe to see if I could see anything. This is what I caught. I added Cylinder #2 in there as a reference because that bank is running perfect. Looking at these I suspect I probably should have replaced #5 also.