Cylinder six misfire at idle only

I just had a vehicle (different make) doing this exact same thing. First, what are the short term fuel trims at idle? Then, one at a time unplug one of the injectors on the opposite bank and watch the o2 sensor short term fuel trim when you do it. If that injector is no longer firing the short term trim should show a very lean exhaust because it is just pumping air. Then unplug one on the same side as the misfire.

Let me know what the result is of the short term fuel trim and we can go from there.

The one I had it was injectors were damaged. We swapped them around and it remained on the same cylinder. No matter what we did it remained on the same cylinder. Finally, replaced all the injectors and it was fixed.

We need to see what happens when they are unplugged to the fuel trim but just make sure you get a baseline before you unplug them.

So if I understand that correctly, when you unplug number 6 there is not much change? Does the engine start running rough or does it run rough with it plug in and there is not much difference when you unplug number 6?

As for the STFT that is an output of the PCM so it makes sense that it goes to 0. Other OEMs keep that output so you can monitor it but the PCM will just ignore it.

Since this is the case, if unplugging the injector makes no difference we could still have an injector issue but if you swapped it let's move on and come back to it if all else checks out.

We need to check to see if we have a valve sticking. The exhaust valve check you can use a dollar bill over the tail pipe and see if it is pulling the dollar into the pipe. It won't be much but it should feel consistent pressure outwards.

Then put a vacuum gauge on the intake to see if the intake valve is sticking. The needle should be steady and not shaking.

Depending on that we can run a compression check and cylinder leakage but that is if the valves are not sticking.

Okay. Sounds good because fingers don't seem to work at 12 degrees.

I would not rule out a valve issue just because it is a new head. So if you can get a vacuum gauge on it we can rule it out. In fact, that may be the issue because of it being a new head. I have learned the hard way, new parts are not good parts.

Let me know what you find and we can go from there. Thanks

Does this misfire at idle at all times? Even during cold start and after it warms up? If it does then I suspect it is a mechanical issue. Also, can you feel the misfire or do you only know because the scanner tells you it is misfiring?

Sorry. Mechanical meaning it is a part failure and not just an electronic induced misfire. Normally if you feel them they are a mechanical component and not a faulty PCM or sensor that sets a check engine light because it thinks it is misfiring but you never feel it.

Just to recap, short term is between 0-3 positive with no vacuum leak shown on the gauge, no exhaust valve issue shown, swapped injector and plug, misfires at idle warm or cold. If this is all the case, swap the o2 sensors side to side (appears they are the same) and see if the misfire moves banks. If not, then I would replace all the injectors. If the short term on that side is showing to positive 3 when it have a misfire then it is commanding more fuel. Granted positive 3 is not much but I just had a vehicle that was doing something very similar. Basically we found that when we unplugged the injector the misfire did not change. However, we only had single cylinder misfire but replacing all the injectors corrected it.

We found out that the PCM was adjusting all the pulse widths to compensate for the single cylinder. It may no sense at the time but does after the fact.

One way to try to confirm this is to monitor the o2 voltage when you unplug the injector. I understand the short term went to 0 but that is because that is a PCM output. You should still get o2 voltage. You should see the o2 respond lean since you have a cylinder on that side pumping nothing but air. If you don't then the injectors are leaking.

So when you say it dropped to 40 what was the reading measured in? Was it volts or a percentage?

Typically the sensor voltage is from.1 volts (lean) to.9 volts (rich). I expected to see just that side go lean so it is odd that both sides would move equally.

You may need to let the sensor soak in penetrating oil overnight so it can loosen up.