I have a mystery vacuum leak on my 1994 XJ6 daily driver.
I tried replacing a badly cracked crankcase breather hose but even after that I think the mystery vacuum leak is still there.
So I made a video about it to hopefully illustrate the problem.
https://www.youtube.com/watch?v=ISeV9AZ1l9Q - Searching for vacuum leak in a 1994 Jaguar XJ6
I hope the sound of the hissing noise is audible over the sound of the running engine.
Steven …
Usually an engine vacuum leak can be tricky to track down, but If you can actually hear it hissing, maybe not so much.
Carefully spray some brake cleaner, carb cleaner etc over all the parts of the engine …
If that doesn’t work spend about $30 and build yourself a smoke machine. When you’re done you can use it for Halloween 
Make a stethoscope from a suitable length of 10 or 12 mm PVC hose. Put one end in your ear and move the other end around the engine close to all the points of possible leak. The hissing will be obvious when you are on the spot. Another way is to dribble a small amount of water slowly over all leak points one by one and listen to a change in sound. The ideal dispenser is a squeeze bottle with a tube and a small nozzle, the type used in laboratories. If the leak is on the underside of the manifold, precise tracking is more difficult.
Thanks for the suggestions folks. I will try all of this.
Just curious, has anyone ever heard of an intake manifold gasket blowing out due to backfiring of the motor? Just a theory I have had running around in my head for a couple days.
OK, not the intake manifold gasket. Using a can of carb cleaner and a “hose stethoscope” I think I have isolated the leak to the lower sealing oring on number four fuel injector. So I guess I have to pull the fuel rail off so I can get the injectors out and take a closer look at the sealing orings.
Steven …
After almost 30 years this would be an excellent time to send away your 6 injectors to be professionally cleaned with new filters installed and they normally include new O rings. Depending on who you use the service is very reasonably with quick turnaround times.
Found the vacuum leak problem. Two of the lower injector o-rings were pretty torn up.
When I went to put the fuel rail with the injectors all stuck into the cups on the fuel rail back into place, I carefully put some silicone grease on the lower o-rings before seating them into the holes on the intake.
This made the process of getting them properly seated WAY easier. I recall struggling with this before so I would recommend anyone do this if you have to pull your fuel rail for any reason.
If you don’t have a tub of silicone grease like I do, you can buy little packets of the stuff at the autoparts store since I know O’Reilly’s has them for keeping ignition wire boots lubricated.
Did you ever find that leak
