Hi,

I own a 2007 GT/CS Mustang with nearly 83.000 miles on it. For the last months I've seen like 4 or 5 times the "Misfire on cylinder 8" check engine light. I've already tried to swap coils, but it didn't changed the code: always on cylinder 8. If I dismiss the error code with an OBD the car runs perfectly fine without showing any error message for hundred ofmiles, but sudently it comes back in Every times I've seen it coming back I was cruising below 3000 rpm or in traffic jams, never on full load.

Even with the light on the car runs perfectly fine, no smoke, full power, no hazarding idle, everything works like a charm. Before changing everything I think I'll start with the spark plugs (they've at least 30.000 miles on them I think, changed by the previous owner). What would you start to change?

Thanks ​​​​​​​

 

The mileage on the spark plugs isn't enough to be an issue but you could try swapping no.7 & 8. If the misfire code still doesn't change, try the same with the injectors.

 

Thanks, will try that

 

If you disconnect the #8 injector harness plug with the engine idling and you dont hear the engine's idle characteristics change,the injector or its harness may be suspect or the oring on the #8 injector could possibly be leaking,causing a small or intermittent vacuum leak,which could cause a temporary lean condition in that cylinder & a subsequent misfire.

 

A worn or cracked injector o-ring will also cause a fuel leak.

 

The problem is that it's so unpredictable: I can't know when it'll display the check engine light. Most of the time the engine is running perfectly fine on it's 8 cylinders. I tried what you said once ago when it first started to occur to check if the cylinder was working and yes it was, as the engine sound clearly changed as soon as I disconnected the injector / coil.

I'll try to swap two spark plugs and will get to you both asap.

 

If you're visually able to access the #8 injector,light up a cigar and blow alot of smoke into one of the large vacuum hoses and watch the fuel injector to see if any smoke is visible in its oring area.If it is,that oring is leaking.

 

Hi,

We've swapped two spark plugs, no error code at the moment, but I've not done enough miles as usually the error is shown hundred of miles after reset...

We also checked:
- Unplugged the #8 injector: the car's sound changes and it vibrates: runs on 7 cylinders + error on my OBD scanner on cylinder #8 (but not the misfire one)
- Unplugged the #8 coil: the car's sound changes and it vibrates: runs on 7 cylinders + error on my OBD scanner on cylinder #8 (but not the misfire one)

The spark plug on the #8 seemed a little bit worn (at least, more than the one we used to swap). I'll change them just in case.

I'll get to you if I've more information.

 

Exactly what code are you getting P0308 or P0358? One is mechanical, the other is electrical.

 

I can't remember and now the code is gone, but if it comes back I'll post it there.