08 running rich, misfires and foul plugs on one side

[Sad_ant]

Hi!
I bought an 08 with 250k miles on it, but struggle to make sense of an error…
It shoots a p0300, it misfires badly but only on bank 1. I get like 1000 misfire on cyl 3-5-7 and maybe a few hundred on cyl 1. 0 on the right side.
BOTH side have fuel trims around -25% it smells unburnt exhaust from the car.
Both sides have o2 sensors more or less stuck at 0.8-0.9 but they do flip up and down at startup and if gas is let off.
Car has no power.
I have cleaned throttle body and maf sensor. Switched plugs bank 1 side was sooty and black bank 2 looked fine.
Did switch an ignition coil from a bad cyl to a good one problem dis not move.
Any ideas?
I struggle to combine running rich, one bank and negative fuel trims. I guess o2 sensors could be bad but they already say exhaust is rich, which it is, so they are right…?

 

[EXT4ME]

A leaking intake manifold gasket can cause a random misfire.

Performing a smoke test can help find if your engine has a bad gasket that is leaking.

Intake manifold gaskets are well known to fail and leak air, especially on higher mileage engines.

Not to terribly difficult or expensive to repair.

Use a Fel-Pro intake gasket gasket kit.

 

[Sad_ant]

A leaking intake manifold gasket can cause a random misfire.

Performing a smoke test can help find if your engine has a bad gasket that is leaking.

Intake manifold gaskets are well known to fail and leak air, especially on higher mileage engines.

Not to terribly difficult or expensive to repair.

Use a Fel-Pro intake gasket gasket kit.

But wouldn’t vacuum side give lean condition?

 

[MS03 2500]

That p0300 is difficult to problem solve a guess is the catalytic converter since it only on one side. Any strange smells. ?

 

[Sad_ant]

That p0300 is difficult to problem solve a guess is the catalytic converter since it only on one side. Any strange smells. ?

Yeah quite annoying. I forgot to mention i did a compression test that was OK.
Thought of the catalytic converter too, but a bit of a challenge to test/confirm.
Smell wise nothung except unburnt fuel smell

 

[MS03 2500]

The easiest way to test it, is to check the back pressure if it's over 5 psi it is probably bad. Just remove the front O2 sensor and install a gauge. Some compression testers will go down that low, do both sides to verify.

While your at it use a noid light to see if a injector is not stuck open.

 

[2004Slickside]

But wouldn’t vacuum side give lean condition?

yes.

On these, flooding is either injectors mechanically stuck open or injector control is not working correctly. More than one injector sticking open at once is doubtful. Is the injector wiring harness for the one bank melted on something between the injectors and PCM? But that should set it's own codes. Injectors work by the PCM grounding the circuits.

 

[Sad_ant]

Thanks good advice! I will have a look but i think injector cables are fine, i will verify though.
I bought new injectors so hopefully that fixes it, but it is so many cyl, so not sure its that. Backpressure is maybe a good guess… see if i manage to remove o2 sensor and find some guage to mount.

 

[Sad_ant]

Thought i’d update the group with the verdict. Shoutout to MS03 2500! It was the catalytic converter and using the o2 sensor to measure was fine, i measured 45psi at 2500rpm!
Simply umplugging the o2 sensor and leaving the hole open removed the misfire. My cat had been replaced and all is well!!!
Thanks!