I recently purchased a 88 XJS V12 with the Lucas ignition. Car was sitting for over 5 years. Had some issues getting it to start but worked them out. Car starts and runs good till warm, then the idle drops as it should but keeps surging. Surge cycle is about every 3 seconds.

89 XJS

Submitted by peddlarbob@sym… on Mon, 06/13/2005 - 18:00

It has been my experience that vacuum leaks normally cause a constant rise of the idle RPM not a surging situation. I happen to agree with Tom that should one or both the rubber bushings that support the throttle rods become worn or are completely missing which is more then a possibility. The V-12 engine starts to hunt. Another thing that could be causing this situation is if one throttle body is set at a different setting than the other. This could be caused by dirt in the throttle body or just badly or unequal length adjustment of the throttle rods. Both these items can cause and imbalance within the engine resulting in the engine searching for its correct setting.

Bob.

92 Series III V-12 VDP Canadian Limited Edition.
87 Series III V-12 VDP
86 Series III AJ6 Sovereign.
85 Sreies III XJ6 VDP.

Submitted by mr442nt@wideop… on Mon, 06/13/2005 - 11:54

Are we talking the same bushings here? The ones I am talking about are the little round donut bushings on the bracket that connects the rod to the butterflys to the pivot arms off the turret. Now, my connector to the butterflys were slopping around, but those bushings are on an external bracket. Pardon my ignorance, but I don't see how that causes a vacuum leak, unless its related to the rod slopping around in the housing.

I do agree, replacing, or at a minimum checking all the vacuum lines is a good idea. I just know that the AAV is mentioned a lot, and that part isn't cheap, and unless the problem is at startup, which this doesn't seem to be, is not usually the culprit.

I've had experience with petrified vacuum lines, trust me. I had replaced all of mine (I could find)last year so I guess that's why I jumped to the other possible problem.

All I know is I had two surging XJS's, and new throttle bushing on the bracket and some tweaking solved the problem.

Tom Kipp
89 XJ-S Coupe

Submitted by NE52-32043 on Mon, 06/13/2005 - 10:17

Tom,

Hate to burst your bubble, but rotted throttle bushing cause a vacuum leak. Along with checking the various spaghetti of vacuum lines, one of the first things that you check for a vacuum leak is -- The Throttle Bushings. On the old E-type V-12's, that is a notorious source of vacuum leak and plays havock with the engine. Same for the XJS. Effectively, having that leak around the bushings is like having the throttle plates part way open when the system is telling itself that they are or should be closed.

I agree with Doug. Look for vacuum leaks and check the throttle bushings. Remember, those old V-12's build up a huge amount of heat under the bonnet and literally cook all of the rubber components. Even though the lines might look good, they can become brittle and not fit tightly anymore. It's worth going through the engine compartment and changing all those little vacuum lines if they've never been updated, as well as doing the bushings.

Steve Weinstein, JTC-NJ
'70 XKE FHC; '69 XKE OTS
Former '72 E-type and 89 XJS owner

Submitted by mr442nt@wideop… on Mon, 06/13/2005 - 07:32

My car is a Marelli, so this may be apples and oranges, but a few months back it did the same thing. Everyone mentioned AAV or vacuum leak (reasonable places to look) but mine was neither. It was a combination of the throttle linkage bushings being rotted away and the two throttles way out of synch with one another. New bushings, and the two rods from the turret tweaked, and the surging went away. Very cheap fix too.

A lady friend of mine with an 88 with Lucas had the same problem with surging, and new bushings and adjustment fixed hers too. I've been told that a bad AAV will only cause issues until the car warms up. Once the car is warm, the AAV is out of the equation. If this happens after warmup, I personally don't think it's the AAV.

Tom Kipp
89 XJ-S Coupe

Submitted by dougdwyer@adel… on Sat, 06/11/2005 - 11:28

There are many possibilities but a vacuum leak is the most likely. Or a seized Auxilary Air Valve. Either way---too much air.

How high does it surge up to ? A common scenario is that the engine will rev to 1500 rpm at which point the fuel is cut off by the ECU. Then the speed drops back down to idle. Fuel is resuplied and the speed increases up to 1500 rpm again, and then.......well, you get the idea.

Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe