1988 XJS V12
I have a very tiny oozing leak while engine is running, and it looks like it’s coming from this black rubber plug thing behind fan pulley.
?
Well, how about that- I actually know the answer. The rubber plug seals the access hole to the timing chain tensioner- a common leaker. It also can have an effect on the integrity of your PCV system. There are several options in the archives for its replacement. John in Lithuania makes very nice looking custom plugs. Easy job for you!
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Thanks Dave! Read the archives. What a dumb part to leak. You’re right, at least it’s easy.
I have ordered the part from John. Should outlast the car!
That’s likely not the end of the story, unfortunately. Even with a leaky plug, the PCV system should be applying a slight vacuum to the crankcase so air leaks in rather than oil out.
Hmmmm, I replaced pcv valve, and cleaned filter breather. Any other things to help pcv system?
I may look into your suggestions of improving pcv system. Don’t want all my tired seals starting to blow. Would leaving oil dipstick up a little help it breath in the meantime?
Although a bad seal at top of engine would still leak if pcv system was good. When giving throttle, there is no vacuum.
I have thought of piping it to the RH air filter
Hi Greg,
Plug is on the way!
Plug internals are pre-lubed with anti-seize and O-ring is made from silicone so should last for quite a while
Any instructions? Does the screw turn to expand plug?
Or merely tap it in! Examination of that clever plug should make it’s function clear.
Positive crankcase ventilation. The engine eats the vapours that were formerly vented to the atmosphere. Some include a crude valve that clogs after a time with crud. Old test. shake it. If clean and functional, it rattles. if no rattle, it is clogged up, usually in th4e closed position.
Yes, accel;eration = a momentary loss in vacuum. No big deal, the rest of the time, vacuum is plenty to keep the sump in a slightly - condition.
I recall a device decades ago. A small electric motor and pump that replaced the filler cap.
Racers plumbed the crankcase to a one way valve in the header. Vented and possibly a bit of extraction…
Butterfly valves in the intake “throttle” suffer from ingested cruds. some worse than others, dependent on the location of the connection.
You can slightly lube o-ring with engine oil to ease installation and when plug is in plce just tighten the bolt till it stops. This will expand the o-ring which will keep the plug in place. I think you will have more trouble taking your old plug out then inserting new plug in. They usually become rock solid with age. Here you can see how the plug looks like: Beware of aftermarket rubber chain tensioner plugs
Just checked tracking, 5 days and the little plug is still trying to get out of Lithuania!
Greg,
I have sold a lot of used Jaguar parts to international buyers through eBay over the past 10 years or so and it is not unusual for shipping to take 30 days or so, even for small packages.
Paul
Thanks John, got the part and installed it. I turned the screw with Allen wrench quite snug (like I would a fuel compression fitting), had to apply pressure inwards to part so it would stay flush as i tightened. I take it this is all good? Haven’t road tested, but no leak at idle 
Regarding getting old plug out, wow. Tried pulling, all outer edges broke off. For anybody else, I found my pick tool invaluable. Pierce in at angle, then pry out. Be careful, once all outer tabs were off, I came very close to pushing entire plug in! All in all, took me about 10 mins.
This is why I suggest that those going with an OEM plug rather than John’s excellent aluminum item should cut the ears off before installing. It’s rubbery when new, crumbly later when you’re trying to get it out. If you cut the ears off it’s easy to get out, even when dry and crumbly. To retain, simply fab a little tab under a nearby bolt head to sit down on the plug to prevent it from going anywhere.
And when I eventually got the plug out, one of the two inside ears was missing. Doh!
Hopefully it crumbles with heat and ends up in oil filter.
Normal for some of the crummy old plug to fall inside the engine. Don’t worry about it, won’t hurt anything.
Thanks Bernard, I feel better. BTW, it’s been educational reading your old write-ups about your XJS. Most of it is beyond my level, but I learn from it.
Should be good to go 
reg. Alllen bolt there is a limit how much it will compress the o-ring so there is no danger of crushing the o-ring. You should actually feel while tightening when the o-ring is fully compressed and you reached the end-stop.