Misfire with LTFT negative bank 2, codes P302, 304, 306

DRFIROJTADAVI

MEMBER
2010 GMC ACADIA
3.6L
6 CYL
4WD
AUTOMATIC
75,000 MILES

Misfiring cylinder with check engine light, codes P302, 304, 306 with -13% ltft bank 2 and -8% bank 1, change ignition coil, spark plugs just now and 2 months back change fuel injectors, since 1 year rpm fluctuations on cold start, clean TB, mass airflow, still misfiring with codes. Adding freeze frame.

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3 Replies

KASEKENNY

MECHANIC
18,907 POSTS

So if you have both banks rich that are not able to go lean then we have one of two issues. Either the fuel injectors are being commanded to shorten their pulse width to reduce the amount of fuel in the exhaust which would increase the amount of oxygen creating a leaner mixture, and they cannot do this because they are leaking. Or we have less air in the combustion chamber due to an air restriction. Based on the MAF it doesn't look like that is the issue.

The main thing that stands out here is the fuel rail pressure. It says this is 9970 kPa. This may just be the scan tool issue but that is way too high. You should be around 50-60 PSI and this is saying it is a 1446 PSI.

We need to start with measuring the fuel pressure with a mechanical gauge and seeing what it is when the engine is running and then what it is after you shut it off. If the pressure is high then it will cause a rich condition.

Let me know and we can go from there. Thanks

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

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Saturday, February 15th, 2020 AT 11:24 AM

DRFIROJTADAVI

MEMBER
33 POSTS

I have ordered mechanical gauge for fuel pressure measurement, mean while I have checked fuel pressure with obd 2 scanner 1) fuel pressure- key on engine off:-48 psi and fuel rail pressure :-600 psi. 2) fuel pressure key on engine on :- 41-45 psi. Fuel rail pressure :- 600-1700 psi, and bank 1( cylinder 1, 3, 5) spark plugs good Bank 2 (cylinder 2, 4, 6) which are misfiring, spark plugs shows carbon fouling (dry).

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Sunday, February 16th, 2020 AT 10:47 PM

KASEKENNY

MECHANIC
18,907 POSTS

Okay. I am thinking your scan tools scale is off because it should have around 60-70 PSI when running. However if that side is carbon fouled then that confirms the rich condition.

It will be interesting to see what the fuel pressure says on the gauge.

Not sure why I didn't think of this before but can you run a compression check on each of those cylinders? It would be great for all 6 but at least one or two on bank 1 but all on bank 2. If each on bank 2 are low then I suspect we have an issue with the intake or exhaust came.

Clearly it is highly unlikely for all 3 to have separate issues so we need something that effects just that side. This would be cam timing. If you have low compression then run a cylinder leakage test to see if the air is escaping from one of the valves.

Low compression from a cam explains all of it, including the carbon fouling and rich exhaust.

I included the compression specs.

https://www.2carpros.com/articles/how-to-test-engine-compression

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