I've got an '03 2wd 5.7/Auto, just hit 100k. No 'mods' per se, just a CAI and single in/dual out Flowmaster. I have a CEL that i've been dealing with recently.

It has been ran for a while on 87 octane (i know shame on me. I have since done some research and found 89 to be the fuel for ideal performance) and got a CEL for Cyl 8 misfire.

-i ran the last tank down to 'E' and filled up with 93. I had a CEL for 'too rich' prior to fill up.

-Plugs were changed 2k miles ago. (Copper, researched that on here)

-Figured it was due for wires, so i changed the plug wires and coil boots.

-PCV valve changed with wires.

i can manually shift, from second to 'D' (~5-5500 revs) the CEL will blink as I'm letting off the throttle shifting into drive then go solid again.

-Mileage has been 9-11 mpg.

Searched on here and found no hits. If you know of a good thread, point me that way

Thanks for any help with this.

 

Have you done a compression test? and/or Leak down test?

If not, I'd go ahead and do the following:

1) Swap coil on #8 with another cylinder, if the misfire carries to the new Cylinder replace the bad coil.

2) Swap the #8 injector to another cylinder and if the problem carries to the new cylinder replace the bad injector.

If neither of those methods pinpoints your issue it's time for compression and leak down tests.

Also, what did you set the gap to on the spark plugs when you replaced them? (have you pulled the plugs to check if they got smashed or anything?

 

did you get the misfire CEL before or after you changed the plugs/wires?


If the misfire is still on #8 after you do the swap tests that Izero suggested, try swapping the spark plug to another cylinder and see if the problem follows. Also, make sure the wires on #8 are snug...you can even swap the plug wire as part of the swap tests...

 

the misfire was before plugs/wires. i had ran it on 87 since i bought it. i ran a 1/3 can of seafoam in the last tank before filing up with 93. i went to check the code again and thats when it gave me a 0172 code (which is the current code) it blinks at WOT and goes back solid when i let off the throttle.


-on a side note, my wife and i took it on a long road trip (~650 miles, highway driving with cruise on) when the misfire code was coming up and it stayed off the entire trip. coming back into town, we filled up (thats when i added seafoam) and the 0172 code appeared.

 

That could be an o2 sensor. Is it giving you a specific bank that is rich?

 

bank #1

 

I would replace both o2 sensors. Some have bad luck with Bosch. I had boschs in my 04 and never had any problems.

 

Replace with NGK or NTK (same company) Oxygen Sensors..... you will need to replace the Up-Stream sensor(s) [before the cat(s)]

I still suggest that you do the swap tests I recommended to narrow down your problem a bit more.... If you are getting a misfire due to a spun bearing or slightly bad compression you can get that code too, so we can't rule out internal engine damage just yet...

 

so, i replaced the crankshaft position sensor, and both o2 sensors with NTK replacements. cleared the codes and drove it a few miles. came home and let it sit idling in the driveway for 20 or 30 minutes. got back in and ran to the store about 30 minutes after i had shut it down and its throwing p0172 again. misfire code went away.

 

If you have a multimeter you should now check the resistance of each injector.

Write them all down for each Cylinder and post up your results.

Have you done any of the swap tests I posted yet?