Mechanics, engine running rich (2008 GC)

WallyWest

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Joined
Nov 6, 2019
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PA
My wife's Jeep, an 2008 GC Laredo, with the 3.7 V6. Full history to follow, it's a little weird.

Couple months ago, drove into town did some shopping. Came back out, didn't want to start. Crank, yes, start, no. Fired up after a few tries.

Since then every once in a while would feel like it was running a bit rough. No codes, just a momentary thing, usually at low speed in a parking lot or something.

Few weeks ago get my first code. A flashing CEL, with code P0306. Misfire in cylinder 6. Before I can pull over and shut it off the CEL goes off and it runs fine. It does this a few times, at low speeds and once cruising on the freeway. But often it will run fine for the whole trip as well. Fun history, a few years ago basically same thing happened, shop replaced injector in cylinder 6. But, there was an injector code at that time.

So I pulled spark plugs (which are relatively new) from cylinder 4 and 6. They look identical, minimal wear, everything seems fine. I swapped plugs and coils between the two cylinders to see if the code moves with either of those things.

Today, drove it to work which is a short drive. Came home for lunch, back to work, back home. Ran fine, no codes. Picked up the wife at home and went to the grocery store. It starts running rough as we pull into the parking lot. I pull codes, now it's P0175, system rich in bank 2 (the bank with cylinder 2-4-6). No other codes. We do our shopping, go out and it doesn't want to start. It cranks a good bit and finally sputters to life, running VERY rough. CEL flashes now and then on the way home while it chugs along and stinks to high heaven the whole way. Thankfully only about a mile.

From what I'm reading P0175 typically does not come with any noticeable symptoms other than the CEL. Yeah, well mine is very noticeable, and it's the only code being thrown. It also ran fine all day before doing this apparently at random.

If you're thinking fuel system bank 2 is the very tail end of the system. The fuel rail is a U shape running across the front of the engine. The line comes in the driver's side, cylinder 6 is the rear cylinder on the passenger side, dead last on the rail. Seems odd that bank 2 is running rich but not bank 1, if the pressure regulator was the issue.

And a bad injector should throw a specific code, and be limited to a specific cylinder, not just the entire bank 2 running rich. And again, this was not subtle, it was running very rich.

Had to take the intake off to get to the plugs, it looked fine. Cleaned the MAF while I was at it, but it looked ok even before.

Could a faulty O2 sensor cause it to run extremely rich like that?