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Not to detract from the excellent electrical troubleshooting advice George is giving and maybe I missed it somewhere in the thread, but have you pulled the plugs to see if there is an abnormality in any of them on bank 2 as compared to bank 1 which seems to be running properly and/or comparing them each other on bank 2 which could indicate a problem affecting only one or two cylinders on that side of the engine to the detriment of the other(s)? Differences here could indicate the source of your problem being something other than an issue with the electrical side of the engine controls.
As far as non-electrical sources for this behavior, have you tried capping off the vacuum lines that attach to the brake booster, HVAC/4wd system, & PCV hose and seeing if that has any affect on the idle speed? A drop in idle speed when capped off that is below that of the accessory being connected normally would indicate that it is possibly flowing air that may not be accounted for by the PCM.
A leaking vacuum accessory can affect the B2 side of the engine as those intake runners pull air from the driver side of the plenum right under the fittings for the booster & hvac/4wd which is combined with the PCV valve hose. The booster line connects into a fitting that is directly over the #6 cylinder runner while the other fitting is kind of baffled, but does have 3 ports that line up quite well with all 3 of the bank 2 cylinders.
Just food for thought. I have seen this issue be a number of different things over the years. Most of the time it has been vacuum related with only a few times that it was an issue with the driver for the O2 sensor (PCM issue).
Normally a vacuum leak would be one of the first things to check with a lean condition but because this effects only one bank I thought that this would be off the table. The reason I put a compression test on the list is because a mechanical problem could be a defacto source of air for just one bank. Same with exhaust leak. .Fuel injectors are on the list because this could be the opposite problem and the bank is starved for fuel.
That said, Swartlkk has been doing this for a very long time and his advice is usually excellent. We should add ruling out a vacuum leak as the source of the problem and maybe we will both learn something. Besides isolating the vacuum circuit as he suggested, another thing you can do is run your fuel trims at both idle and 2500 rpm and compare them.
We have started with the sensor circuit to be sure that this is an actual fuel trim problem and not a measuring and reporting problem but where you start is dealers choice and experience can often help you choose the most likely cause first.
LTFT won't change with just a quick jaunt up to 2500RPM, but the STFT for B2 does show as positive at idle and zero at 2500RPM.
Just a quick side note, I would not mess with B1S2 at this point. This sensor is for catalyst monitoring only and does not affect engine performance unless for some reason it was kicking out the driver for the other two upstream sensor heaters...but I think that would trigger a different code.
One thing that I did not fully notice until Kyle added his vacuum leak theory is that the catastrophic fuel trims that occurred during the P0151 code as seen in the freeze frame data is a random anomaly. All the other fuel trims for bank 2 are anywhere from perfect to marginal. So this problem has to be something that can go from near perfect operation to a significant fault condition rapidly and randomly.
This points in the direction of a wiring continuity problem, voltage or ground problem, pcm issue, a sensor/injector crapping out intermittently. I must say that most vacuum leaks would also not change this violently except a leaking brake booster when the brakes are applied so even though I don’t have experience with vacuum affecting only one bank, this idea still has legs along with some of my other ideas. The brake booster theory is easy enough to test by blocking off the line but there is no problem most of the time. A better test would be repeatedly applying the brakes and watching O2 sensor output and fuel trim data (and rpm) , and codes of course. If this does not prove to be the issues then we get back to looking at intermittent issues either on the fuel calculation side (MAF, ECT, …), the fuel delivery control (O2 sensor circuit) or the injectors. Compression and exhaust leaks are probably not the issue either at this point.
Here are a few pictures that show what I am talking about. 98-02 SCFI/MFI Lower Intake Manifold w/ EGR valve location 98+ SCFI Upper Plenum - Underside
As shown, the runners for cylinders 2, 4, & 6 are on the driver side of the intake plenum, right under where the PCV/4wd/hvac port is (outer walled off area) and the booster port (directly over the #6 intake runner in the LIM.
You may very well be correct, I just wanted to see what the plugs said about the situation as that is fairly easy to review and can push in other directions.
I am all for following your vacuum/plugs idea, this isn't your first rodeo. The challenge is that the problem is intermittent so closing off the vacuum may not show anything in the OP's driveway. That's why I think pumping the brakes may be a good test.
Been thinking about this thread. I had always thought of a vacuum leak as a global problem. The unmetered air would mix in the intake plenum and be available to all cylinders. This idea that the extra air could be consumed preferentially by selected cylinders is interesting. The throttle body is after all in the middle of the intake structure. We still have the interesting challenge that the problem is intermittent but could a bad LIM gasket leaking on that side only cause the same thing? Another case for looking at the plugs.
Thank you very much for great explanation and things to try. I am on travel for a couple of days for work but will be able to do some testing tomorrow.
1. Back probed and checked pins on B2S1 with incandescent light and got power on one of the green wires and nothing on others. Checked B1S1 with same results. Engine was at operating temperature when I tested.
2. Pulled a spark plug from both banks. B2 plug had considerable more carbon build up on anode and porcelain than B1. Plugs are only about 18 months old.
3. With engine at idle, LT/ST fuel trims did not change when I depressed brake pedal or pinched vacuum line to booster. I do not have 4WD but there is a vacuum line running to HVAC controls which I did not pinch.
4. I went for a test ride with live data (below). Wide range of LT/ST fuel trims especially on bank 2 but also on bank 1. System A was in closed loop but system B was open. About 20 minutes into drive System A opened and I got CEL, combine FT for Bank 2 was ~74% when light triggered.
The purpose of the incandescent test light is to be sure that the heater power circuit can provide enough power to run the heater. The light gives a visual indication that the entire circuit is functional at multiple watts of power. The probe is used to complete the circuit as the heater load so you unplug the connector and at key on with a cold engine (the heater only comes on to warm up a cold sensor) you connect the probe tip and the ground clip across both green wires in the vehicle harness. Do not ever use an incandescent probe on computer sensor lines, power or grounds unless you know what you are doing because some of them are unprotected and you can blow the driver transistors with a current that is much higher than designed for and destroy the PCM. That's why led probes are called computer safe (very low current) and a multimeter is safe because of its high impedance when measuring voltage. The measurement of the other two wires is not normally necessary if you are getting good sensor waveforms on your scanner but because you have a catastrophic intermittent failure, it may become necessary to measure the continuity of those sensor wires to make sure that you don't have a wiring problem.
Does the O2 data or rpm change when you tap the brakes or pinch off any vacuum lines?
It sounds like you have problems with both banks, not just bank 2?
03-21-2022, 08:18 AM
