Which sensor detects random cyl misfire?

[Colorado_Outback]

Is it the first O2 sensor?

I have a persistant missfire on #4. I have changed out the coil, plugs and I swapped the the wires from 2 to 4 and back again. The wires are less than 6 months old.

The check light will come on randomly and its always a misfire on #4. There is no loss of power, even when the check light is flashing at 70mph. You can lay into the throttle when its flashing and it runs just as smooth as when the check light is off.

So I'm thinking which ever sensor senses the misfire has to be acting up.

Any insight would be appricated, I have read all the threads I can find on random cly misfire and im at a loss..

[02_Legacy]

I think the crank position sensor is what detects misfires. As each piston goes through the four strokes the crankshaft speeds up and slows down slightly. The crank position sensor can detect these small fluctations in speed. If the crank is supposed to speed up because cylinder 4 is firing but it doesn't then it is a misfire.
In your case I don't think it is the sensor because it wouldn't consistently be cylinder number 4 setting the code if it was the sensor.

[ShadowGT]

I say this all the time.. check your exhaust headers. A bad gasket, an improper seating will cause a misfire code for the cylinder that feeds into the exhaust header.

[$400OBW]

I guess you've scanned it and came up with a PO304 code since you mentioned persistent on Cly #4, are you getting any other codes or just the #4 misfire code?

If ShadowGt's suggestion isn't it I would run a compression test and maybe switch injectors I would think you'd be throwing a lean code for it to be an injector but you might try it anyway.

Sounds like you've tried most things I'd try. You say the plugs are new but you can get a bad one once in a while so you might try a new or different one.

You might also try taking a mist spray bottle and spraying a fine mist on coil and plug wires in the dark with engine running and seeing if it is arcing out anywhere.

[Colorado_Outback]

To clarify, I changed the plugs and wires out about 6 months ago, everything was good until Thanksgiving weekend then this Random Cyl Misfire #4 started popping up.
I first swapped the wires between #2 & #4, then replaced the coil, lastly I swapped the wires back and put a new plug in the #4 cyl. Still throws the same code randomly.

#4 misfire is the only code thats come up.

Ill check the exhaust manifold gasket on that side, it sounds like a likely culprit being as that performance is never effected.

Thanks for the replys!

[ouch1011]

Is the car auto or manual?

[Colorado_Outback]

Manual

[ouch1011]

Do you have a lightweight flywheel or crank pulley?

[Colorado_Outback]

The motor is all stock @ 140K