Some possible problems .... if that tiny piece of spring was way up inside the cap, you just removed the center electrode contact. People seem to have problems pushing these caps on the plugs far enough before they try to make the 1/8 turn rotation. They will go on the plug but it is a hard push the first time or two. A very tiny bit of dielectric grease on the plug top electrode can aid pushing it on but it should go on w/o the grease. If it is not locked on the plug, some interference is almost guaranteed. At this point, you may have to replace the cap if that spring has been removed. Note is is an 1/8 turn and not 1/4. When locked in place you will not be able to pull the cap off unless your reverse the order and turn it the 1/8 turn to remove it.