PCV valve in a vintage XK?

I’ve a fairly early Mk2, predating the advent of the rope seal and, therefore using a scroll to sort-of keep the oil in the rear of the engine. Now that the car is pretty well sorted otherwise, I’ve been trying to road it a bit more, and I’m finding more mess from rear non-seal oil leakage than I like. As for blow-by, this is operationally the best of several XK engines I own, and I seriously doubt that this engine has major wear. It starts instantly, blows no smoke in any circumstances, doesn’t seem to burn oil (sometimes hard to tell on low driven mileage with oil leakage), has great power, and is easy to tune - probably could idle it below 500rpms if I wanted. Running 20-50W Castrol with ZDDP additive. It hasn’t been very long since I had the breather screen off, and I haven’t rechecked it. I’m running some other engines with rope seals that I have rebuilt and I can’t say that they have no leakage, but they are better than this one - it’s my only XK with a scroll and the only one I did not personally rebuild.

I’ve read a lot of posts here about PCV systems although mostly pertaining to V12s, and I’m pretty familiar with it after struggling to get it right on my Morris truck with 1275 BMC engine - these will have serious leakage issues if you don’t.

Question - has anyone actually added vacuum to a Mk2 or other vintage XK crankcase through PCV, a vacuum pump or other means? I’d like to benefit from your experience. Before I take on the significant job of adding a rear seal or going through this engine, I’d like to try a simpler solution. If so, how and where to pull vacuum. I’m concerned about the wisdom of a T-fitting off the vacuum line to the brake booster. Maybe tap the manifold for another fitting? Thanks

If you put enough vacuum on the engine to prevent oil leaks, you may suck in a lot more than clean air , like water grit etc. Do you really get that much blow-by from your vent pipe to think you are building much internal pressure?

The leak is at the rear, i.e. the scroll, not the vent pipe. The pipe doesn’t have more drainage than I would expect. The pipe would be plumbed to the pvc valve, then to the manifold in any conventional style system, so it’s closed. I’m not aware that any of the systems could apply enough vacuum to suck outside material into the scroll. What I’m asking is whether anyone has actually done this on one of the earlier engines. I’m not familiar with the 60s XJ6s, but I’m betting they had something.

Hi Ron, I’ve experimented with a full PCV system on an XK engine in my 72 triple SU XJ6. None of them have ever had a real PCV system that pulls clean air in through a vent and then sucks blow-by into the manifold through a pcv valve. They evolved from draft tubes to tubes into the air cleaner housing to tubes into the inlet manifold, either through the carbs in the pre-injection days or through a fitting in the inlet in the later cars.

All of them vent from the big hole in the front of the head. The problem is that there is nowhere to draw in clean air into the crankcase. The head doesn’t have any openings to the crankcase near the back. All the oil galleries just drain forward to right near the big vent hole which is too close to the vent… If you put a breather inlet in the back of the cam cover it would just suck clean air over the cam, ignoring the crankcase.

So what I tried was using the only intrusion into the crankcase that exists for the clean air inlet -which is the dip-stick tube. Basically my system had a PCV valve in the vent hole in the front which went to a sealed, baffled catch can to avoid sucking in oil then from there to a vac fitting on my Mk 10 Triple SU manifold. As I said, fresh air was provided by a 3/4" tube that was sealed around the dipstick tube with a filtered vent at the top. The system worked but it was messy. I had to carry around the dipstick and I did get some oil in the dipstick tube inlet filter which was telling me it was probably not in a good position.

In the end I got rid of it and now pull the blowby through the vent, into the catch can and then into a manifold I made that supplies each of the 3 K&N air cleaners the same amount of blowby. It works but it isn’t a pcv system per se. It’s just a version of the old air cleaner system like the Mk10 had originally. I doubt there is any significant vacuum being generated to scavenge the gasses and help keep the oil from being forced out the seals.

If you want to use a pcv type system without a fresh air inlet it might be too much vacuum. But if you try it would definitely need a catch can or you’d suck alot of oil into the manifold. There is a company that makes an adjustable pcv valve… That’s the way to go for something like this but the problem is always no good fresh air inlet.


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Thanks, Mike. I appreciate your real-world advice. Let me share what I learned from my BMC motor, which suggest the possibility of a slightly different approach. When I initial built the motor, I used a timing cover that was missing the catch can in front that can connect to the manifold via pvc or through a Y fitting at the carb bases. I also used an early valve cover with a vent tube to the air cleaner. My new engine poured oil out the scroll. By putting a piece of paper over the oil fill with the engine running, I could see it was being pushed away from pressure. When I added a proper timing cover with can connected to the Y-carbs, as well as using a later valve cover with no vent pipe to the breather, I had a slight suction on the oiled paper, and my leak went away. Key to this was a new filler cap on the valve cover.

What I think might work is to use a very small hole in the filler cap. The plastic caps for the BMC motor are specifically built to allow in only a small, graded amount of air. They are supposed to be replaced periodically to maintain the proper relationship of pressures. Any extra holes would simply cancel any vacuum that was developed. By my way of thinking, the goal is not to ventilate per se, but to relieve any crankcase pressure and create a scosh of vacuum which helps keep the oil from going out the scroll. I think your idea of an adjustable pvc valve in combination might work. I’m also interested that you were apparently happy with your vent tube going to the air filter cans. Obviously, you don’t have a scroll, but did you have leaks that improved?

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This is above my head, but that never inhibits me from posting. :slight_smile:

I now have a setup similar to Mike’s, basically the factory Mark X carbies, air box, and breather. A 7/8 i.d. pipe connects a single breather port to the air box, just as that same diameter hose was used as a road draft tube on earlier cars.

But this is a modification; the original configuration of my S1 XJ6 consisted of a breather with two ports–one about 1/2 inch that went to the air box, and another about 3/16 inch that went to a thin tube running along the top of the intake manifold and plumbed into a port associated with each carb–in the carb base I think, but maybe the manifold. I believe that that arrangement may have provided metered suction–similar to a conventional PCV ball valve but metered by the i.d. of the thin “capillary” tube (as is the term when a similar thin tube replaces the metering expansion valve in AC systems).

I don’t understand how the manifold vacuum would evacuate the crankcase rather than draw air from the larger port going to the air box. But both are under vacuum (slight in the case of the air box), so perhaps the small port chooses instead to evacuate the crankcase. I also don’t know if this was intended as some sort of PCV or if it was associated with the closed fuel vapour system that was introduced about the same time. Fuel vapour is supposed to be “stored” in the crankcase and burned the next time the engine is fired up.

I’d appreciate comments.

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Ron, the engine I am experimenting with is newly rebuilt and although the Jag crank still has the scroll it is sealed with a rope seal as you say. At first when I started this I thought I was losing oil at the crank but, long-story-short, it turned out that one of my modified, old-style cam covers had an imperfection in a weld that was leaking like a sieve so I couldn’t tell exactly what was leaking. But I thought it was the crank seal so I started fooling around with pulling a vacuum to the vent, the PCV system etc… I actually played around with a pcv valve in the big vent going straight to the intake. This actually pulled so hard that it started to crush the rubber cap that XJ’s have on the vent. Needless to say, none of this helped my oil leak because it wasn’t what I thought it was so I can’t really tell you if it worked in that respect. The hard part is metering the amount of vacuum - just a little is enough.

Unlike an early Mk2, the Mk10 intake has a vacuum take-off because there is some vacuum-activated stuff in the heating and ventilation system. That was the take-off I used. You had mentioned Teeing into the takeoff for the power brake servo - this is frowned upon and there is alot of chatter online about why. As @Robert_Wilkinson mentioned, the original XJ Federal spec setup had the breather vent sucking the blowby directly into the twin Strombergs. This sounds alot like what you describe in your other car and this would probably provide enough vacuum to help your leaking scroll issue. The problem with that is that my HD8 SU’s don’t have that provision. I actually had thought about cnc-ing a spacer that fits between the carb and the manifold with a nipple tapped into the bottom of each so I could basically mimic the stromberg system. This would be a problem in our XJs because it would push the carbs a little too far from the intake which would cause interference with a cross brace. That might not be a problem with a Mk2. My other thought was to do the same thing with the spacer but place it between the carb and the air-cleaner. Probably not ideal but I think it would work.

With anything you try though, I would suggest putting an aftermarket sealed, baffled catch-can inline to keep extra oil, fuel, moisture etc… out of the inlet tract. In scouring the internet about all of this I’ve read alot of good things about how pulling a slight vacuum on the crankcase can really help a scroll leak - it just seems like it’s a little more complicated to implement on a Jag of course.


Again, helpful comments. Mike, I think your first system would work great if you metered the amount of vacuum, which you could do with that adjustable PVC, but you might also have to play around with the fresh air supply through say the oil filler cap by gradually enlarging the diameter. This PVC is interesting, and they recommend adjusting it with a vacuum gauge.
I like the idea of a somewhat scientific approach, and this would allow control of pressure (negative) and volume. I have an extra manifold and I’m going to think about whether I could add a port. As for the spacer with tube, I believe that it would still see manifold vacuum as it’s below the throttle plates on an HD8 setup. It’s been awhile since I looked at the Stromberg setup, but I think they have secondary plates within the manifold, so it might be ported in that scenario.

I could live with quarter-sized spots on my garage floor, but this has become coffee cup to megagulp size. I’d not be surprised about leaks if it was an obviously tired engine, but it isn’t.

That’s the valve I was talking about Ron. I didn’t want to invest until I was sure I was gonna keep the system. It did work the way I had it setup but it was an off-the-shelf unadjustable PCV valve that I found through my research that others have used successfully with similar engines of similar displacement.

When I tried it without a fresh air inlet I did try to meter the vacuum coming off the manifold by using smaller and smaller restrictions in the 3/8" tube in front of the PCV valve but it always pulled too hard. I’m not sure you could mitigate that even with an adjustable PCV valve if you’re coming off full manifold vacuum. Plus you still need to have enough flow to evacuate the blowby gasses. IIRC I investigated the possibility of an adjustable vacuum valve but didn’t try it - I believe they are also quite expensive The addition of the filtered fresh air inlet on the dipstick tube sorted the problem but it was of course a vacuum leak - all of the systems like this are pretty much controlled vacuum leaks.

Definitely would see full vac if it’s behind the carb for sure but it’s a way to add vac takeoffs without modifying anything so I considered it.

That system has a secondary manifold behind the carbs but independent of any venting. The vents lead straight into the carb in front of the throttle butterfly.

BTW blowby is a serious issue for turbocharged engines as you can imagine. They actually make electrically operated vacuum motors to evacuate the system without creating a vacuum leak. I’ve even heard of people re-purposing the air-injection pollution pump to act as a vacuum pump to do the same thing. Probably not necessary for what you’re trying to do :slight_smile:


I had also through of adding a vacuum pump to my Mk2 XK engine but finally decided against it for the following:

  1. A vacuum pump is noisy. The two of them I looked at gave a high pitched shrill which almost equaled the volume noise of the engine.
  2. There are the vacuum management issues - that the pump draws a lot of power and you do not need the pump all the time, i.e., low idle. There was some suggestions that running the pump constantly caused early pump failure
  3. So how do you have the pump kick on and off? There are some rev counter switches (sold by MSD as a speed activated switch) that are used for timing shifting but most will not engage a relay at less than 2000 RPM. With overdrive I would rarely exceed 1900 RPM
  4. I decided to build an aluminum catch below the rear seal as suggested by Mike Eck in June 2012 on this list. As my sump is aluminum, I couldn’t weld the catch to the sump. .Mine was one-half of a round aluminum ashtray with welded sides. It drains to a AN4 hose that runs to the front of the engine. There is just enough clearance on the flywheel to allow the AN hose to pass to the side of the sump. In all costs were about $50. Not perfect but with a Jag, I learned to accept an imperfect would.

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Just getting back on this topic. Based on Mike’s experience, I decided I should do a bit more investigation of the source of the leak. I invested 25$ in an endoscope (borescope) from Amazon that interfaces with my phone. It’s quite an interesting tool. As you probably know, the rear cylinder head area of a Mk2 is very difficult to evaluate, but I got a clear view with the scope, and it was a bit wet below the tach drive, as well as behind the starter. If you’ve never pulled a tach drive in place, let me tell you, it’s a MISERABLE job. If you do decide to do it, I suggest you remove the battery and move any cables/ wires that you can, drain the radiator and remove the lower rear heater hose, cut back the heater pipe if you have to (mine were aftermarket stainless left long, and gather every 3/16" allen wrench that you can find, which you will then modify to various lengths. Patience and about 1 hour for each of the 3 bolts will get them out, and you can put a new o-ring on the drive. Replacing the copper washers (get the correct thin ones!) on the oil feed pipe is not too bad, and the endoscope is great for examining the mating surfaces after the cleanup. Then back together - all in all, a days’ work. My leakage seems significantly reduced, and I’m back to the usual messy rear engine and tranny with a few drips on the garage floor - watching it for now.

I do still want to try an adjustable pvc system, but it’s back-burnered for the moment. I’ll report back if I do try it. Thanks for the helpful comments.

Glad to hear it was just a regular leak Ron. BTW I just experienced that exact nightmare with a 61 Mk2 tach drive I was working on. I feel your pain :slight_smile: I wound up losing so many allen wrenches to the top of the transmission area. I found a wobble allen wrench bit on amazon that eventually wound up saving the day. Luckily I didn’t have to remove any heater hoses or the oil lines but definitely a battery out job.

A job that is not fun if you have any sort of back issues… but if you don’t, pulling that tach drive will definitely give you some!

Got one of those amazon phone-scopes as well. It is a great investment. I’ve jammed it in so many difficult places on various jags.