Why do i have sludge build up on my spark plugs

Hello, If the sludge is on the spark plug where the wire connects, you will want to replace the plugs and wires, and likely replace the valve cover gasket (if leaking into the spark plug wells). However, if the sludge is on the portion of the spark plug that threads into the cylinder head, you can try replacing the spark plugs, but the issue is likely to come back very soon. I recommend a compression test on that engine if this is the case.