have misfires and cat efficiency code after replacing the downstream o2 sensor on bank 2. drove 200 miles with poor fuel mileage and no power plus lots of knocking on any acceleration as 5-8 cylinders all have misfires. Ran a compression check today all 4 cylinders are 150psi so thinking it is not the broken valve spring that this engine is known for. 200k miles, but idled 6000 hours so it is really around 388,000 miles on the engine. I have all new plugs and coils but would it be more likely to be due to bad camshaft (had a ram that idled and it ate up the camshaft lobes), the knock sensor, clogged cat, or electronics

 

You knew to replace the downstream sensor due to the code.

Just in case you did not know, the misfires are the cause the converter may be destroyed.

Can you provide the info we need to get rid of the cause of the misfires ?

WHAT OTHER MAINTENANCE HAS BEEN DONE TO THIS ENGINE ?

Misfire codes , for which cyls ?

 

It had downstream o2 sensor, misfires and cat efficiency below threshold all on drivers side bank. Supposedly all maintenance done as this was a fleet vehicle I just purchased

 

cleared codes, ran compression checks, replaced downstream bank 2 o2 sensor, replaced coils 5 and 6, replaced one plug on 1, 7 and 8, and both plugs on 5 and 6. I can see that there is most misfire on cylinder 6, 7 and 5 but not really any misfires on 8 until it is driven hard. I did a realearn and the tests that Autel would allow me to, so my main question is whether there is a cat clogged, bad ecm, or 3 broken valve springs. It just doesn't make sense to have a broken valve spring when the compression is good, and as I drop each cylinder it will knock so there isn't that much difference in load. what to check

 

Your engine should have no kind of knock at any time. The motor may have more problems than we think.