1988 XJSC vacuum issue

hey everyone. I Have recently acquired this Handsome car and am getting acquainted with engine operation. it had been sitting for 6 years before i picked it up. was owned by a wealthy man that took very good care of it. said when he parked it that it ran perfectly. When i started it up it would idle ok until it reached operating temp, then it would start to jump up and down 500rpm. Does not stall and will drive great. although at idle its a bit shaky.
injectors are all firing, spark to all wires (have not checked the plugs yet, that’s next).
exhaust pipe on passenger side is sputtering while drivers side is smooth. the weird thing that makes me wonder about how this engine works is the fact that i have zero vacuum on the right side! with the air cleaners on there is no suction through the intake, when they are off i can see there is no vacuum. drivers side is pulling like crazy…
valves stuck?? what do you think?

Have you tried to adjust idle speed on the AAV and check the throttle plate clearances?
I have noticed the vacuum difference side to side and always considered it normal and due at least in part to the setup of the balance pipe and the AAV and it’s connection to the L/H bank air cleaner.


As already pointed out the difference in the vacuum at the air cleaner intakes is normal at idle.

The idle cycling up and down is probably caused by a stuck piston in your Auxiliary Air Valve (AAV), located at the rear of the left bank intake manifold. You have too much air entering the engine due to AAV piston being stuck open and the fuel is being cut off due to the high idle speed, then when the RPMS drop the fuel is added again only to be cut off again. This is a common problem. If you search the Jag-Lovers archives for AAV you will find lots of information from others who had this problem and how they fixed it. It’s a vacuum issue so a properly operating AAV, good vacuum hoses and properly adjusted throttle butter flies should fix it. At least that made the difference for me on my wife’s 1990 XJ-S convertible.

I recommend that you download Kirby Palm’s excellent “Experience in a Book” from Jag-Lovers and read up on the AAV issue and many other problems that XJ-S owners have had and how they fixed them. I also recommend that you get familiar with the Jag-Lovers archives. At this point most of the issues you might find with your car will be new to you but not to this list. I search the archives regularly and find them a very efficient and effective way to do research and find solutions to problems with my Jaguars.


you were completely right, the intake system is Ported, it only pulls air when air is needed from the throttle body via the throttle plate. I warmed up to operating temp, the engine was shaky and hopping revs again. so i took off the air cleaner and felt the suction to the AAV, it was stuck full open! so i clogged it with some old rubber, connected the airflow sensor back to the trumpet and revs went back down and shakyness was gone. it is idling at around 850rpm warm, sounds better and responds good so far, haven’t driven yet.
I am going to dis assemble the AAV tomorrow if the baby sleeps :-/
thanks for the fast input fellas, the lack of information on this car seems daunting at first but it going to be quite an adventure.
one more question, for the AAV did you buy a gasket or make one out of gasket paper?
“No problems, only projects”
—some old guy----

There is a lot of good information in the Jaguar XJ-S Repair Operations Manual (ROM) and Kirby Palm’s book. I wouldn’t be able to keep my wife’s XJ-S convertible on the road without them. In my opinion repairs and servicing the XJ-S can not be done via emails alone or by guessing.

I believe in being creative when necessary, but the AAV gaskets (EBC8329) are readily available at numerous suppliers, including the dealerships and places like eBay. I think I paid about $10 the last time I got one. I am certain that there are those that use sealants, or make them out of other materials, but I prefer to use the correct parts when they are available at a reasonable price. That way I know that they will fit and work correctly.


I cant find Kirby palms download. Not sure where to look. I would appreciate the link;)


Go to the Jag-Lovers Home Page and search the archives for “Experience in a Book” and you will find it.

If you haven’t searched the archives before it is very easy. Just click on the magnifying glass icon at the top of the Forums Home Page, and type in your search string. Easy peasy.



DL’d . Thanks a ton.
The AAV is soaking in penetrating oil as we speak. I expect no change tomorrow. Do you know of any good places for parts? I am having a heck of a time finding anything here.
Specifically AAV and gasket.

The AAV is one of the items we seem to discuss frequently. There are at least 3 different part numbers, perhaps even more if one includes the pre-HE in the search.

Google some of the 3 Jaguar numbers in the thread provide above and you’ll find some vendors.
Be prepared for a shock – the part is ridiculously expensive.

Lots of info in the archives (the magnifying glass in the upper right corner of the window)

Where in the world do you live? There are lots of Jaguar parts vendors out there but knowing where in the world you live would help point you in useful diections.


Edmonton Alberta Canada. I am looking on Ebay. But I figured you may have a preferred spot you like.

I am in edmonton Alberta canada. I was just wondering where you would get your parts from. They combined knowledge of this forum is outstanding !

Check on boiling water to see if the penetrating oil did the job…
It should be fully closed at 80°C maximum. If not it’s not working well.

As you might have already red in “The Book” the AAV can be disassembled and fixed.
The biggest problem though is that the Thermal Bulb is unobtainioum…
Beware that if you buy a used one, besides the ridiculous cost, it will probably have as much life as yours, i.e. not…

I resolved in a bulb from a Marine engine that is rated at 50°C.
It’s a good compromise, albeit the problem is that it has a very short Temp Range compare to the original, so either you set it High and have low idle when very cold, either yo set it Low and you have high idle while warming up.
I chose the first option and added an extra Extra Air Valve controlled by a water heater thermostat and solved that problem!
Bulb: $20
extra Extra Air Valve: a leaking Fuel Change over Valve from the bin
Water Heater Thermostat: our local recycle bin
Air filter : $5 (plastic fuel filter)
As for the Gasket, I made my own from thick gasket paper, works fine several years later (make sure is good for water)

The principle is the same, it’s only the port opening that changes and maybe the temperature. But one can fiddle and modify…