I am about to replace the rear spark plugs (plenum off job) and was wondering whether to replace the spark plug tube seals. It is a 2002 TJ 3.5 that has done 190 000km, with regular oil changes. The rocker cover gasket does not leak. When do the tube seals normally start leaking??? I guess when i remove the plugs I will be able to see whether the tube seals are leaking or not. If the spark plugs are oil free and a look down the tubes reveals no/little oil, guess it might be wise to leave as is. I might buy the rocker cover gasket and 3 tube seals and ahve them on hand in case need. Comments, suggestions welcome and does anyone have a write up on seal replacement. MMAL manual had no info on spark plug tube seal replacement.

I like the idea of just buying the gasket and 3 seals because you may not need them now but it wouldnt hurt to replace them then you dont have to worry about it for probally well over another 190xxxk's.

Once you pull the insulators off the plugs, you'll know if there are leaks or not. Oil on the long insulators tends to have them swell and weaken. Getting the gasket and 3 seals would be a wise precaution, and if you don't need them for now, they will be on-hand for later. Can be used on either engine bank when the need arises, and if stored with simple care - flat/undistorted and somewhere cool-ish - should last well for use when you need em.

No real tricks for replacing the seals. The tin tubes poke out of the head and have a lip close to the top. The plug tube seal sits down on that lip and has a flared "donut" shape that sits horizontally against the underside of the rocker cover. The rubbery seals just lift off the plug tube, no clips or pressure springs like on valve stems or camshafts etc. Just a matter of remove old seals, give the tube and flange a wipe clean and place new tube seals on. Simple wipe of underside of rocker cover around the holes for plugs. Take care not to distort the two tin layers of the rocker cover (two skins form the channel the rocker cover gasket sits in), and don't overtighten the rocker cover bolts (risk of distorting the covers).