2006 Yaris 1.0 Hesitation / Limited revs under load

[Schakal]

2006 Yaris with the 1.0 litre 3 cylinder engine with 137k miles on the clock.

Recently started burning oil (last 12 months or so ) then eventually engine light came on which after reading on the OBD2 turned out to be the blocked catalytic converter which makes sense with burning oil . I have used a catalytic converter cleaner in the fuel tank to clean up which did work and the engine light went off for a while but came back again. Now the car hesitates on acceleration and when the steering direction is changed ( fuel related ? ) I tried to give it a good high rev run today but it wont go over 3.5k rpm under load , however in neutral it will happily go all the way to redline.

OBD2 read codes that are related to cylinder 3 misfire and upon removing the spark plug here ,it appears to be very black where as the other 2 are  more of a grey colour.

Am i right thinking there is an injector issue here with cylinder 3 ? 

thanks for replies in advance 

[Stevie J]

Black spark plug could well be oil fouling, which would make sense considering the car is burning oil. Eventually the plug will not fire if it becomes severely oil fouled. It could well be that that cylinder is low on compression due to worn or damaged piston rings allowing oil to get past and get burnt. I would see what the compression readings for the 3 cylinders are, and if the reading for the one with the bad spark plug is much lower than the other two, then it will likely mean that the engine requires dismantling to see why the compression is down, but at 137K miles, it is probably down to defective / worn piston rings, ie, the engine is pretty much worn out.