If all the plugs look like #6, except #4, you've already got the evidence. More idling or high traffic driving will continue to darken #4 until it fouls.

So theres no other way out other than changing the seals?

So theres no other way out other than changing the seals?

As Mark says, just do it! You’re not going to solve it any other way.

 If it was mine, I'd start with removing the #4 valve seal and inspect it for damage. I suspect it was compromised during installation. Swapping out the other seals depends on what you find and the desire to do all of them.

Ok. Question: why does it gets oily and dark when the car is idling for long periods while the other dont? 

The simple answer is, one of the valve seals (I'd guess the intake valve seal) #4 cylinder is leaking. The surface of the seal that rides on the valve stem is damaged, probably during installation. That allows oil to run down the valve stem and get sucked (intake valve) into that cylinder.

I've always called it loading up. Fuel/oil gets by idling but burns off at higher rpm. Boats are bad about doing that.

Edited June 15, 20213 yr by siteunseen

That once was called an "Italian Tune Up".

Just now, Mark Maras said:

That once was called an "Italian Tune Up".

"FIAT"

 

Fix it again tony

Normal vacuum at idle will pull oil into the combustion chamber due to a compromised valve stem seal.  At higher revs, it tends to seal a bit better, or a bit more.

On 5/18/2021 at 10:58 AM, Mark Maras said:

@jalexquijano Everyone agrees there's no need to remove the head to change a valve seal and if you do you may be opening a Pandora's Box of trouble and expense. The compression is good on all cylinders so we know the rings and valves are sealing. The only problem is a fouled #4 plug which, by process of elimination, we've come to believe is a leaking valve seal. Installing one valve seal is a piece of cake compared to pulling and reinstalling the head. 

Ok. Dday is tomorrow. Im removing the valve cam and connecting an air compressor to cylinder number 4 to keep the valve from falling. Any caution or consideration i should take in the process. I really need a guide in order to get this thing right change the seals and avoid opening any pandora box, sort of a step 1 , 2 and 3 guide.