Was driving to the office yesterday and while at a light, noticed smoke seemed to be wafting all around from under the hood and the smell of oil burning. As soon as i got to the office I popped the hood and examined the valve cover gasket closely - the same one I replaced just last month - and was stunned to see that the rear of it had blown out, just like with the (old?) one I had replaced.
What in the world is going on? Just as with the old one, both half-moon seals appear to be firmly in place and not leaking (when I put my gasket in I used anaerobic gasket sealer on them, per Kirbyās suggestion). Thereās just a length of gasket (approx 3") between the half-moons that is missing, with the torn ends sticking out. As indicated in my earlier thread on the job, I made sure to torque the valve cover bolts (and shouldered washers + studs that the PO used for some of them) down in the new sequence indicated for the '94 4.0s in the factory workshop lit.
I know sometime back there was talk of defective aftermarket v.c. gaskets for the 4.0 ā¦ Did I maybe have one (this one was Fel-Pro, btw,)? Did I maybe tighten things down TOO tight (I didnāt use a torque wrench), or should I have gone back a week or two later and re-torqued them? Could I have a cylinder head leak that is causing very high pressure inside the valve cover?
I guess Iāll try again, but this time with a Mahle gasket, which is supposedly what Jag used. Iāll also try to use a āclickā torque wrench this time, and will do that āglove testā someone recommended earlier to see if the PCV is not working as it should. I didnāt do it the first time b/c the car otherwise ran perfectly fine. Also, I just donāt see pressure (normal) building up inside the valve cover strong enough to blow out a new rubber gasket like that, but I guess itās possible (?)
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
2
I gather this is a 4.0 with rubber cam cover gaskets and metal half moon seals? If so, Iād conclude there evidently is a venting issue in the crankcase on this engine. One possibility is that the PCV system is somehow compromised allowing pressures to build up, but frankly I doubt if thatās the problem. Somehow it seems more likely that fumes are being allowed to collect in the crankcase and then are being ignited, producing an explosion that blows out the gaskets.
With rubber valve covers, usually the bolts involved have shoulders to prevent over-squishing of the gasket. Is it possible this gasket is being under-squished in this area? Whatās with the āshouldered washers + studs that the PO used for some of themā? Is it possible the gasket is being over-squished in this area?
I think rubber gaskets are supposed to be installed dry because any dressing or sealant may act as a lubricant and allow the rubber gasket to sorta ooze itās way out of position. If this assembly truly involves a rubber gasket and aluminum half moon seals, Iād recommend anaerobic sealant only on the curved underside of the half moon, not on top where the rubber gasket sits.
Yes, thatās it, but the half moons are made of rubber too, not metal. Also, until '95 MY, the v.c. gaskets like mine had the half moons separate from the main seal ā¦ For '95-'96 it appears Jag decided to make them integral with the main seal. Possibly it was for this very reason. (?) Anyway, Iāll try again, but I think this time Iāll consider using Permatex Ultra Rubber gasket dressing, which they indicate specifically for v.c. gaskets w. rubber seals. After I get the cover off if in fact the half moons are still tight, then Iāll use P.U.R. on the back end of the main where the blow out occurred(s). Itās non-hardening, which supposedly makes it seal better in that application. In fact, I wonder now if the old black stuff I scrapped off the mating surfaces of the valve cover and the head flange when I r/red it is that stuff. Looks like it was liberally applied at the back end by the PO.
btw, my goof - I meant āshouldered boltsā ā¦ As I mentioned in my original thread, I discovered when I went to r/r the cover gasket there were only 4 of the original (torx-head) bolts holding it. The other 8 bolts consisted of 3 or 4 āstudsā with shouldered nuts on top and 3 or 4 bolts with hex heads. Apparently the PO twisted off the heads of some of the original torx heads and just left them in the head as āstudsā, then tightened them down from the top using shouldered nuts. I guess the proper fix would be to use penetrating oil and vice grips (or tach weld on some āfinsā) for removal and replace them with OEM bolts, but as someone pointed out they are very hard to come by and $$$$ when you can find them. Also, they seem to tighten things snuggly down anyway. I used metal washers on all 12 of them, btw.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
4
OK, rubber on rubber. I guess I donāt see any inherent problem with that. Not like the paper or Gortex over rubber on the V12, anyway. It certainly would be better as one piece, though. Does the later gasket fit the earlier engine?
Yes, Permatex Ultra Rubber sounds like a better idea for rubber gaskets.
As I said above, you donāt want to be torquing down hard on rubber gaskets. Theyāre supposed to involve shoulders that the fasteners hit when torqued down, compressing the rubber a specified amount and no more. Are you saying the shouldered nuts serve the same function that the original shouldered bolts did? And compress the rubber gasket the same amount? And whatās with the ā3 or 4 bolts with hex headsā? Are they free to just squish the #$@$% out of the rubber gasket? Because thatād be NFG.
We can all appreciate the difficulties with finding OEM parts, but if the original shouldered bolts are unavailable the substitution should be a stud with a sleeve around it, and that sleeve needs to be the same length as the original shoulder. Either that or we need to con one of the machinists on this forum into making a batch of cam cover bolts for the 4.0.
The guys are absolutely correct. They can get filthy. I used brake clean initially and then put it in ultrasonic cleaner for 2 days. Here are before pics
Donāt know if anybody has talked about this, but that metal line going from that filthy vent filter to the PCV, has anybody tried running an oil catch can in between? Iāve done this on my Turbos to keep oil from going back into the intake, and it does a good job, although it needs emptying quite a bit during the winter due to mainly catching water vapor.
Worth doing on a V12? Does it need an extra large catch can?
The problem though is that the filter gets blocked, an oil separator will not help much in that.
I check mine every now and then as a regular maintenance.
In my 13 years of ownership and 50.000 Km I had to clean it only a couple of times.
Not that bad.
I just take it outside, burn it with my blowtorch and in five minutes itās like new.
The OEM bolts are basically 8-mm. torx-head machine bolts (no shoulders), using a metal (OEM are copper, but apparently aftermarket steel ones work as well) washer. They are also that gold-tinted color which, per my understanding, are for high heat applications.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
11
So what keeps the rubber gasket from getting squished?
Iām confused what you mean by āsquishingā ā¦ The gasket is supposed to be sandwiched hard in between the valve cover flange and the head flange (correct/?). The bolts, w. washers on them, are torqued down Unlike the V-12s (?), the pre-'95 facelift 4.0s did not use āisolatorsā (i.e. small rubber bushings) for each of the screws. And maybe that was yet another gafe when designing the valve cover assembly for the '92-;94 4.0s., corrected in MY '95 for the 4.0s. And no, unfortunately, the main gasket for the '95-'96 MYs will not fit the '92-94ās due to a difference in shape with those magnesium valve covers. Again, I guess itās possible I over-torqued the bolts/fasteners (thinking āthe tighter the betterā?), so this time I will use a torque wrench (assuming I can find a socket attachment that will fit it for 8-mm torx machine screws and 8- and 10-mm hex-head bolts.) btw, I do like your idea of someone machining up a batch of the 8-mm. torx machine screws. They could probably sell a ton of them
Looks like you used the anaerobic stuff on the main gasket surfaces as well, James. As to the PCV valve, I have found new ones for sale online for the V-12s, but none for the 4.0 facelifts. I guess I could try cleaning it like you did, but I first have to ID it. For those with a 4.0, is that the little gold āflying saucerā (or possibly silver) attached to the top of the engine (intake valve cover?).
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
14
Iām guessing weāre not talking about the same type of gasket. When you say ārubberā, Iām thinking the kind of gaskets that are on a Honda Civic valve cover. They are rubber. If you tightened bolts down on the cover, youād probably cut the gasket in half. The assembly involves shouldered studs that the nuts tighten down on, only compressing the rubber a set amount and no more so as not to damage the gasket.
If this isnāt what you have, then Iāve been misled by the description of the gasket as being ārubberā.
Are you smearing RTV on the half moons as suggested, if not required, by the service manual and TSBs?
Are you using aftermarket gaskets? I have found that the OEM gaskets are the only ones that fit properly. My history with the aftermarket gaskets are that they are about 1/2ā too long, causing challenges with installations, increasing the probability that the gasket isnāt perfectly in place, especially in difficult to see areasā¦
Yes, I used anaerobic gasket sealer on them, but again the issue is not the seals (nor was it on the original leak). They are firmly in place and not leaking. The area that āblew outā is the small strip between them.
Yes, used Fel-Pro the first time, and have ordered Mahle for this second time. I have not been able to find any actual Jag-branded VC gaskets. Mahle claims that they are the original gasket supplier for these cars, i.e. Jag contracted that part out to them.
Talking to some local techs here, they have advised that (assuming there is not a PCV issue) the reason for the blow out is likely the fact that I did not go back and re-torque the cover bolts after a couple of weeks or so. I never knew that needed to be done on VCGs, but it makes sense. The rubber (and yes, they are rubber, not cork or polyurethane) initially compresses at the time of install, but the exposure to heat (up to 200-400 deg. f.) and the oil causes them to lose some of their initial compression, thus requiring the re-torque. This might also explain why Harlem, my 2000 XJ8, developed leaky VC gasket(s) not too long after I had hers r/red a few years ago. The tech at that shop apparently should have told me to bring her back to be re-torqued later on, but didnāt. Now sheās actually blowing oil smoke through the a.c. vents from time to time until I re-tighten them (or r/r them if blown).
Felpro is a solid aftermarket maker. Guess there are bigger differences between the gaskets needed for the AJ6 versus AJ16 engines.
AJ16 has two choicesā¦mystery maker or OEM. The OEM is almost $100 while the aftermarket is about $30. URO makes aftermarket gaskets for the spark plug wells, but not the valve cover.
Iād just roll into the dealer and place an order for this part. If itās available for the AJ16, Iām sure they still sell the AJ6 gasket as well.
The blowout is curious. You should unplug the vent hose from the valve cover and blow through it. See if itās easy to blow through. It should be, because as soon as it hits the intake elbow, the air will blow out through the MAF and air filter.
Then do the same thing, but into the valve cover. Youāll need to remover the valve cover oil cap. I wonder if that metal mesh thatās there to filter out the air is gunked up?
Valve cover gaskets have perplexed the auto world from the onset.
Shouldāve kept the valves in the block where they need no extra cover and no leaks. Humour !!!
Those cam covers are long and skinny. Just pining to twist and deny a flat surface for sealing. Iād say check it with a known flat surface or a framing square.
Torx are slick, but need a whole flock of new tools!! Iāve gathered some. My Jeep is loaded with 'em.
Carefully measure the length of the required fastener, Torx or whateverā¦
Check the ARP site for available fasteners. Great quality. Possible tech help as well.
I learned from my first Corvair, do not over torque fasteners.
ARP, Carl? Do you have a link to that ā¦ I just checked with three of our usual āJag suspectsā and they either have none of the OEM screws in stock or only a few of them ā¦ Those things must be scarce ā¦ They all reported though that they could get them from a source in āabout a weekā after order. I hate waiting almost 2 weeks to do the job (again) ā¦ Iāll also try the tests mentioned ā¦ btw, according to the classic parts site, it doesnāt look like the AJ6 has a PCV valve ā¦ And when I check under those letters in the factory workshop literature, it doesnāt hit on anything. They do mention something called a āheated engine breatherā, but that doesnāt seem to be the same type of thing ā¦
As soon as i got to the office I popped the hood and examined the valve cover gasket closely - the same one I replaced just last month - and was stunned to see that the rear of it had blown out, just like with the (old?) one I had replaced. 
I guess the proper fix would be to use penetrating oil and vice grips (or tach weld on some āfinsā) for removal and replace them with OEM bolts, but as someone pointed out they are very hard to come by and $$$$ when you can find them. Also, they seem to tighten things snuggly down anyway. I used metal washers on all 12 of them, btw.
The gasket is supposed to be sandwiched hard in between the valve cover flange and the head flange (correct/?). The bolts, w. washers on them, are torqued down Unlike the V-12s (?), the pre-'95 facelift 4.0s did not use āisolatorsā (i.e. small rubber bushings) for each of the screws. And maybe that was yet another gafe when designing the valve cover assembly for the '92-;94 4.0s., corrected in MY '95 for the 4.0s. And no, unfortunately, the main gasket for the '95-'96 MYs will not fit the '92-94ās due to a difference in shape with those magnesium valve covers. 
Again, I guess itās possible I over-torqued the bolts/fasteners (thinking āthe tighter the betterā?), so this time I will use a torque wrench (assuming I can find a socket attachment that will fit it for 8-mm torx machine screws and 8- and 10-mm hex-head bolts.) btw, I do like your idea of someone machining up a batch of the 8-mm. torx machine screws. They could probably sell a ton of them 
Now sheās actually blowing oil smoke through the a.c. vents from time to time until I re-tighten them (or r/r them if blown). 