Misfire in Cylinder #4 Compression Questions

2002 2.7L Tacoma with 256,000 miles. Cylinder 4 has been misfiring when the engine gets to temp. Changed plugs, coils and check wires. Completed a compression test which might show a different problem. Cylinder #1 has 182PSI #2 153PSI #3 153PSI and #4 has 183PSI. Thinking that it might be a fuel injector but not sure if that is the case since it only happens when the engine gets to temp? Should I chase the misfire or look into the compression?

Compression test was completed with all plugs pulled.

 

Well you said its 4 thats missing, thats one thats on the high side so dont think thats it....
Not sure how you "checked" your wires.....

You happen to still have the original #4 sparkplug?
Any evidence of coolant deposits on sparkplug electrode tip. discoloration same color of the coolant, etc....

 

I visually checked coil wires for any irregularities. We had a problem with a rodent chewing my wife's O2 sensor wires last year. I unfortunately do not have the old spark plugs. I haven't noticed any off color exhaust smoke either. Possibly running rich while idling.

 

If it was a fuel injector the problem will indicate right away not when it goes up to temp. Because you did a compression test looks fine I would say inspect the o2 sensors those ten to fail if they bad when it goes to to temp.

 

Last time valve clearances were checked?

Swap the position of the coils and see if the miss follows the coil.

 

bad 02 sensor is going to cause a misfire, ever.........

 

Mice nibbling on wires?
An old timers trick... use a hand pump misting spray bottle of water at night on the wires while running (thr car).
Shorted/leaking wires give quite the light show

 

I've never had the valve clearances checked.

Swapped coils and the miss stayed in #4.

 

Looking into O2 sensors. I replaced both this past summer.

 

never seen an 02 cause a misfire.....

 

For the next "test" I would loosen the radaiator pressure cap, not take it off all the way, just put it on enough to keep the coolant in the radiator and warm her up at idle then take her for short trip to see if she misses, is she doesnt then put the cap on to where the cooling system builds pressure and repeat the drive. If it misses with the cooling system under pressure you have a coolant leak into a cylinder, if this is the case you would see a slow loss of coolant from the overflow tank.

 

or pt the system & send a camera down into the cylinder for a look-see, could very well be coolant intrusion....
usually wouldnt miss all the time though, unless really bad...

 

Yes, for you and I that have a borescope and cooling system pressure tester with the right adaptors.