Gen 3 – P0171 and P303 in 07 H6

Hello to everyone! My name is Matheus and I have a 2007 6 cylinder car. The car has had a recent head gasket service and the temp seems fine now but I have a problem with fuel and low rpm behavior. I know there are several posts like this as I’ve been looking at it most of the time for research, but I wanted to direct it to the most personal solution to possible problems.

I bought the car last year with a head gasket problem, we repaired it and replaced all gasket sets + water pump for maintenance purposes. I got the car from the shop and it ran almost fine for about 80 miles. I am currently working in a city 680 miles from my hometown where my trusted mechanic did the repair. Everything seemed normal, but as I stopped during the drive, I noticed that the car was running rough, especially at low rpm. In addition, the engine began to vibrate in a way that you can even feel inside the cabin.

Did a scan with an OBD and got the following codes:
P0171 system bank 1 too weak
P0303 cylinder 3 misfire detected

Eventually when I clear the codes it gets a little better but they persist. Sometimes it’s just one, sometimes it’s both again. After 3500rpm the car doesn’t feel too harsh, although it does feel like acceleration is somewhat slower.

From what I read on the forum it is most likely related to the A/F mixture or possibly a vacuum leak. I’ll list what’s been done so far and what I plan to check out next. If possible, I’d like to hear some suggestions on what to do next, starting with the simplest solution.

  • When the car left the shop, the mechanic said the battery was not healthy enough and the car was misfiring. We let it charge for a while and it seemed fine, so we stuck with the old battery (it was bought last year and used for about 600 miles).
  • My question: could the battery have a voltage problem and cause this problem once again? Although the car starts without a problem and the voltage on the OBD seems fine

The injectors were then cleaned and equalized while the engine was out of the car, but would it be possible for one of them to cause the problem once again?

The spark plugs are new, we bought an NGK kit when we did the head repair. Not sure about the coils, should I check them again after the injectors?

I have checked the MAF sensor and the connections look good. I didn’t want to mess with it so I didn’t try any kind of cleaning. By the way, the air filter is new.

Plus – I noticed that fuel consumption has increased significantly. I did the trip at 6km/L where it used to be 12km/L

Sorry for my bad english and thanks in advance!

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