Series 3 fuel leak

Hello all. Long time member but it has been a while since I was on. Many post on the E Type forum. I have acquired a 1985 Vanden Plas from the widow of a good friend of mine. Sorting our maintenance that should have been done. I have a strong smell of gas when the car is shut down coming from the engine compartment. I have isolated the leak to a fitting into the fuel rail that has what appears to be a plastic fitting with 2 small fuel lines attached. One line goes forward to the fuel regulator and the other goes down to the intake manifold. I first thought it was the small fuel lines leaking. Replaced the lines and proper clamps so no leak
from the hoses. The plastic fitting must have a crack. It screws into the fuel rail toward the rear 1/3 of the rail. It appears to be a 5/8" fitting, though I have not wrenched on it. Could someone here in the brain trust let me know what that fitting is and where I might secure a replacement. I will try to figure how to post a picture. Thanks in advance, Rick

Attached is a picture of the component that I believe you are asking about circled in red.

That is the Thermal Vacuum Valve (EAC5086) that senses fuel rail temperature and increases fuel pressure when fuel temperatures get too high to prevent vapor lock in the fuel rail. Those hoses are just vacuum lines and should not have any fuel in them. They need no clamps, just the proper size vacuum hose to keep them tight on the plastic nipples. If you are having a fuel leak in that area most likely it is due to a leaky fuel pressure regulator diaphragm and you need a new fuel pressure regulator (EAC4864). You can test the fuel pressure regulator diaphragm to see if it leaks by applying vacuum (I would use my Mityvac vacuum tester) to the metal nipple at the front where the Thermal Vacuum Valve hose connects. It should hold vacuum. If it does not then it has a leaky diaphragm and fuel will leak through that hose to the Thermal Vacuum Valve and then to the intake manifold if the Thermal Vacuum Valve is open.
If you plan on keeping this car I highly recommend that you get a Series III XJ6 Parts Catalogue, a Series III Service Manual, an S57 Electrical Guide and of course the Owner’s Manuals if you don’t already have them. Together they will remove a lot of the mystery of working on these cars, but not all of the mystery. :wink:
Another document that you might find helpful is “JAGCARE III” written by Dr. Gregory Andrachuk. You can download it for free on Jag-Lovers. Gregory did a great job compiling a lot of helpful information about the Series III XJ6s and XJ12s and it is worth reading cover to cover.


Paul, thank you! That is the part. I do have the Jaguar Series3 service manual but could not find that part much less how to test it. I will order the parts manual and suspect your diagnosis is spot on. I will confirm with the Mighty Vac tomorrow. At shutdown the fuel will leak out the sensor. I have been chasing this problem for a few weeks!!
I will post my findings tomorrow.
Thank you again, Rick

A leaking membrane will leak continuously, Rick - not only at shut-down…

But while the engine is running, fuel is sucked into the manifold rather than show up as a leak. This cause the engine to run continuously fat - increased by lack of vacuum influence; the fuel pressure will be higher than necessary…

The pressure is regulated by a spring loaded valve, opening to the return line. And vacuum, which varies with manifold pressure, will help to open the valve, reducing l fuel rail pressure…

Simple test; disconnect vacuum hose at the regulator and run the pump. If fuel leaks out of the vacuum spigot; the membrane leak is confirmed - and the regulator must be changed, as Paul says…

Use a rag to catch leaking fuel. Reconnecting the vacuum hose; you can still run the car - but if the engine starts misbehaving, you should not…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

No need to check, just order the new FPR before you are left stranded in the midle of the road…!

The Thermal Vacuum Valve (EAV5086) is normally open and fuel pressure is regulated by intake manifold vacuum applied to the fuel pressure regulator. The valve closes at high fuel rail temperatures and the result is the fuel pressure at the fuel rail is elevated to prevent vapor lock.

I tested several of these Thermal Vacuum Valve before (they are on our two V12 Jaguars in addition to our XJ6s) only to find out they didn’t close properly. I found the spec temp somewhere but don’t recall it now.

The Thermal Vacuum Valve is merely screwed into the fuel rail and there is no way for it to leak fuel from.the fuel rail. If it is leaking fuel, the fuel is coming from a leaky fuel pressure regulator.


Diagnosis was correct! The replacement part is ordered. This helps explain hot start problem rich idle as the fuel flow from the leaking diaphragm was dumping into the manifold. The car will remained parked until the replacement is installed. No more gasoline smelling garage. The leak was not insignificant, fuel would flow as a constant fast drip.
Thank you again to all who responded. I had purchased a 1986 Vanden Plas new in the day and always regretted selling it. The replacement was a 95 XJS 4.2 convertible. Regretted selling it as well. My XKE remained throughout. Much simpler to service. Timing light, dwell meter and a few wrenches and good to go! I look forward to lots of cruising with the new old Jaguar.

That sounds great. I am glad to help. Please write back after your problem is resolved to leave a bread crumb trail for others encountering the same fuel smell leaky fuel pressure regulator problem when they search the archives.