Oil dripping out of both throttle bodies, necessarily PCV related?

After my 86 jaguar XJS is driven over a very short distance, opening the throttle plate by hand yields a steady amount of oil streaming out of the throttle body housing. Clearly, the PCV breather system is the number one culprit. However, I have installed an oil separator between the pig snout and the metal why tube that connects to both throttle bodies.
Short of oil being drawn in via this breather system, is there any other conceivable way that a healthy amount of oil can find its way into the throttle body?


I would like to see exactly how you plumbed the oil separator "between the pig snout and metal “Y” tube.
The “Y” tube should be attached directly to the PCV valve behind “B” bank air cleaner. The pig snout metal tube should also be attached to “B” bank air cleaner. More photos, please!
What mileage does the engine have?
Is there any oil inside the “B” bank air cleaner housing?

So when I first got my XJS running (@48,000 miles), which had been sitting for the last 7 years before I bought it, i would get oil like that in both throttle bodies. But over the years, running synthetic oil and getting the engine tuned up, it seemed to be less and less. I guess the piston rings slowly got back to sealing better?

Even after a few years, i would still get oil residue in the throttle bodies. Not drips like before, just hints of oil after a month of driving. I installed an oil catch can, and now I get pretty much no oil in throttle bodies.

I assume this means your PCV system IS working well, as it’s pulling out the oil vapors from blow by. But you may have excessive blowby? Do you run full synthetic oil?

Here is how much my oil catch can catches in about 3 months of driving, currently with what I believe is a well running engine (64,000 miles with excellent compression readings) This is quite typical water condensation/oil. My guess would be 3-4 oz? (ignore the solid bits, the bowl was dirty when I drained the catch can)

Thank you so much for taking the time to compose that message. I am not running synthetic oil, just regular 20/50. Incidentally, my vacuum is about 17 inches at idle. With my oil separator connected to the pig snout and y tube leading to the manifolds,I can measure 1 inch of vacuum at the dipstick orifice. So basically, I have a slight negative crank case pressure. Perhaps my oil separator is not as effective as yours. Incidentally, I would love to see a picture of your catch can set up.

I also have about 17inHg of vacuum at idle.

My catch can/pcv setup is quite unique and different than the Jaguar oem setup. I may post a write up at a later date, but it’s such a big change, i don’t want to influence anybody changing their pcv system so differently.

You should try synthetic oil, i think its really good for these engines. I am running 15W50 Mobil.

Would you have time for a quick telephone call? Kevin 4044417793

Based on your reply to Greg, I have just one comment. You have connected the catch can between the pig snout and the “Y” metal tube going to both throttle bodies. The “B” bank air cleaner connection has been bypassed. Now the engine is applying full manifold vacuum to the crankcase, as there is no way for air to enter the crankcase. If I understand correctly the way this is plumbed, you should have a much higher vacuum reading at the oil dipstick than 1inch Hg. More like 7in Hg. I think all of the above is the problem.

You are correct, that is an accurate assessment of my current set up. I have circumvented the Bank air cleaner completely. With respect to the vacuum At the dipstick, 1 inch vacuum is the most I can achieve. So, if I were to introduce outside air from the B air cleaner I would no longer have a slight negative vacuum.

Ok, Here is what is happening. The running engine is applying a lot of vacuum to the crankcase, and it is sucking in all the blowby it can, plus whatever leaks in around worn seals and gaskets. This why you have oil in both throttle bodies.
You are almost there. Ignore the “B” bank air cleaner housing. You need to introduce fresh air into the crankcase, but not from that air cleaner.
After some experimenting about a year ago, I made this change to my PCV, and Greg’s approach is similar to mine. At the time some interest was generated, some positive, and some not so much.
Probably the easiest access point for fresh air is through the timing chain cover plug-see photo.

The next photo shows my vacuum line from the “Y” tube to the PCV valve, which is plugged in directly to the pig-snout.

The whole idea of the PCV is VENTILATION, not evacuation. As you are removing blow-by gases, something has to fill that void . You may not achieve much vacuum at the oil dipstick tube, actually 1-3 in Hg is perfect.
I have several hundred miles on my revised PCV, with really clean oil and no leaks as a result. Vacuum at the oil filler cap runs around 2inchesHg.

I hope all this helps, and makes sense!

Failed or filing intake valve seals. High vacumn on deceleration sucks it in.


Thank you so much for taking the time to provide such a comprehensive view of your PCV set up. It’s absolutely gorgeous, I am envious.

I am in full agreement that the desired and result is ventilation as opposed to evacuation. However, in my particular set up I am applying 17 inches of vacuum at idle and achieve only 1 inches of vacuum at the dipstick orifice. Clearly, fresh air is being drawn in past the crank seal, rear main seal, etc., for effectively “filling the void” created by the 17 inches of vacuum. If I introduce, as you suggest, additional fresh air by way of the timing cover inspection plug/orifice, I will no longer observe the slight 1 inch vacuum at the dipstick that I enjoy presently.

Incidentally, as Carl indicated below, oil can also be drawn in from the valve train assembly by way of the stem seals especially upon the acceleration with the throttle plate closing where vacuum can approach 25 inches. However, there is ABSOLUTELY no oil present in the exhaust.

Now I’m confused … in my earlier thread, members were saying we don’t HAVE a PCV valve … Is that something missing in just the AJ6 engines, then? :confused:

Again, just another comment. Your 17 in Hg. is in the intake manifolds The vacuum of 1inch Hg. is in the crankcase. As long as the PCV valve is in place it will limit the vacuum level attained in the crankcase.If you removed the valve, then the full 17inchesHg would be applied to the crankcase. Not good.
What you do to your car is of course, your choice. All I can do is make suggestions. I wish you success .
I think it would help if you read Kirby Palm’s book covering the cars we have- particularly the section on PCV. He explains things probably better than my attempt.
Edit. Perhaps your solution would be to go back to Jaguar’s original plumbing. Then you would only have to clean “B” bank air cleaner housing once in a while. Or you could re-ring the engine and reduce the blow-by.

Hi Paul, don’t get confused. The poster’s oil problem is on a v12 engine, which has a (largely ineffective) PCV system. Your engine is quite different, and I don’t know how Jaguar dealt with any blowby that might occur.

The V12 is a very strange set up. Yes, 17inHG is sucking on the PCV valve, which limits that vaccum (let’s say by 50%?) which then sources from both the crankcase AND the B airbox. So no crankcase ventilation, and no negative pressure on crankcase. It’s basically sucking a bit of the oil vapors out at vacuum, and allowing a relief (B air box) during positive pressure/0 vacuum (better than your seals, I guess!)

During positive crankcase pressure, such as flooring it, now any blowby crankcase pressure from the pistons goes out the pig snout and into the PCV valve, where some goes into intake manifolds via the y tubes, and some goes into B airbox. So instead of sucking out the oil vapors, they are now being forced out. Like I said earlier, better here than your seals.

We still aren’t certain what your setup is. If you have the OEM setup with a catch can simply between the pigsnout and the Y tubes, and have eliminated the pcv valve, like Dave said, that’s wrong. If it’s between PCV valve and Y tubes, I guess that should be working?

My setup in a nutshell (very similar to Dave’s) - I stuck my PCV valve INTO the pig snout. I then ran a hose from it to a catch can, and then that catch can ran a hose to the Y tubes. I then used the front timing cover plug as an external air supply, which gets filtered air from the A airbox. I get from 0 to 1inHG at dipstick at idle. I’m not too concerned with maintaining a negative crankcase pressure at idle, but I like that the crankcase is well ventilated now like they used to do on old V8s. My guess is that while cruising, where you can get 20inHg vacuum, i am hopefully getting some negative crankcase pressure. I get pretty much no oil in throttle bodies, and of course 0 oil in B airbox now that I’ve eliminated that section. I’m sure the engine is happier too. I’ve tested the A airbox supply hose, and see no oil in that. So during positive crankcase pressure, I have two large exits for all of that pressure to go. The catch can plumbing I think serves as a great relief, due to it’s volume. And hopefully my rings are not allowing too much to get past (Last I checked, I was at 210s/220s cylinder compression on a warm engine) That’s where I think using synthetic oil has helped those rings get cleaned and seal better.

On a side note, the oem setup is mediocre, but ok. But i found out, make sure to use Jaguar PCV valve! A few years ago, I went with cheap $5 valve as suggested in the book. Started getting some oil leaks at various seals. Discovered that the Jag valve allows much more volume than standard pcv valves (a good 50-100 rpm at idle). With the strange oem setup, i guess you need a bit more vacuum pulling to get any pull on the crankcase.

1 Like

Dave, thank you so much for the response. Just to clarify, currently I am drawing 17 inches of vacuum on the crankcase without the use of a valve and observe only 1 inch vacuum at the dipstick. By adding the valve, I will reduce the vacuum substantially and no longer have a slight negative pressure measured of the dipstick.

When i was testing my new pcv system, i plugged my crankcase air intake that i made for A airbox. With a pcv valve still limiting vacuum, i was getting a good 10 inHg at dipstick. (Too much imo)

If you are pulling 17inHg on your crankcase with no pcv valve limiting that, and you’re only getting 1 inHg at dipstick, air is getting in somehow. Hopefully its just a leak in dipstick, and not an engine seal.

Without question, it’s leaking at the crank seal and likely the rear main seal. However, no oil leaks when the crankcase is under a slight neagtive pressure.

Kevin, your last response explains everything. No wonder there is oil around the throttle bodies.
The slight vacuum in the crankcase has to be achieved using a PCV valve, and a fresh air intake, which the v12 does not have. Your “fresh air” is being pulled in through leaking seals, and excessively high vacuum being applied to the engine’s crankcase… We still don’t know how many miles are on this car, nor any of it’s history, but I suspect a compression test is in your future.
The negative pressure that Greg and I have on both our cars is a result of simply duplicating General Motors approach, which has been used for more than fifty years on millions of vehicles- but it has to be plumbed the right way. There are no short cuts. Best of luck.


Out of curiosity and outside of the social logic - why would you need catch can in V12?

There is no gobal option for longer cam shafts unless going up to V16. This makes these prone to twist and wear already.
V12 is like a tank engine that need plenty of servicing, lubrication and grease. I assume there should be oil residue in the inlet to help with cam lubrication, seats seal/eaqual wear, all the old-fashion benefits (oil mist). As I’m not V12 fan - please, help me to finally get it, in my struggle…
Going back to vacuum. Why not using old fasion vacuum gauge and record a movie for us? This waa we can say if everything is perfectly fine or some things went wrong.