‘88 XJS V12... once warmed up, in park or Neutral in hits the magic fuel cutoff and drops and repeat. AAV was my main thinking but I had just taken it off and cleaned and tested and it worked. Could not find any other vacuum leaks. So today I decided to unplug the tps and idle shot to 1900rpms and sat steady. Does this indicate a bad or poorly set tps? I know I tested it a couple years ago and possibly adjusted it(poorly). If the tps is set wrong will the ecu act as it’s getting a load and idle too high? The screw on the aav is all the way in and if I block the hole in the intake to the aav nothing happens. No drop in idle. I’ve never had a more annoying issue. Car basically wants to drive itself. Lift off gas and it bucks and then won’t engine brake under 35mph. Help!

 

1) Plugging that AAV as you describe should stall the engine, FACT. If not, there is a Vac leak, and maybe a lot of small ones, FACT again.

2) TPS out of range, 100% needs to be confirmed as SPOT ON.

3) Throttle rods are aet incorrectly, possibly done in hast, without the spacer on the capstan ear.

4) Timing is way off.

5) Distributor innards are seized, or seizing.

6) Plug gaps are too wide.

NOT all the above will cause "hunting" on their own, but they all manifest, and multiply.

Your emissions equipment will be involved, and I have NO idea whatyou have or have not, we got zero, so very simple.

 

Two or three years ago I had an idle problem as seen in the video below. Not sure if it's quite the same as yours, as it's more like 'going berserk' than 'hunting' . But, anyhow, a new TPS fixed it. It wasn't a matter of TPS adjustment in this case but, simply, TPS malfunction

Cheers
DD

 

it definitely seems like there should be a vacuum leak. I think I need to do a smoke test.
Timing is off a bit. That’s been a whole big pain in the *** but not terribly off. I gutted the cats and cleaned the o2s when I did. New throttle cable. Could the vacuum modulator on the transmission cause this? I’m going to smoke test it and check the tps range for sure

 

yeah, that’s exactly what mine is doing. That’s very annoying but then on top of it the surging off throttle... like riding a bull

 

well, smoke tested it today. Lol it’s unbelieve how bad the vacuum leak was and how I never looked before... Whoever had it before me, completely removed the vacuum screw on the right side and nothing is connected to the nipple on the bottom. Plugged those holes and did it again. Then the hose going from underneath the intake to the right airbox(runoff valve?) was split in half. Haha I’ve never felt this car as it should be! It now idles perfect if not too low and can coast to a stop. It usually wanted to run about 35mph with foot off the gas. I’m feeling really dumb.

 

I wouldn't feel 'dumb' or bad about anything at all. If anything these cars are great teachers.

I've been learning that listening (can be hard when the idle is high) is a good way to check for leaks. THAT and a giant can of brake cleaner. Spray it around the manifold and air intake areas and when the gaseous fluid is sucked in, the car will sputter, if not stall completely... That second photo is a supplementary air valve. It kicks open (when everything is working correctly) when the AC comes on to bump the idle up just a lil lil bit for added load. Works good when working... The hose runs kinda close to the RH manifold under the intake and gets heat, drys and cracks or, worst case, melts.

Had to be a air leak you were dealing with.
Good job!

 

the rubber turned into plastic from the heat

 

Yup!

By the way, do you have a 944?

Don't cringe... Well ya can if ya want, I did.
My early 928 attempts at body work. Luckily a body guy screamed,,, drop the paper and back away. He's an enthusiast and is gunna take over for me for a small fee...

Ol BooBerry...



She can be purrrty...

 

I had a 944 for a couple years and loved it. If I were to get another it would be an S2 or turbo. The standard 2.5 was a little buzzy. The 928 is one of my favorite cars! I’ve driven a couple and still look all the time for one that needs a little tlc. The prices have gotten out of hand though.

my 944

 

Selling my 633csi 5-speed at the moment. Awesome car as well

 

Good find.

NOW

You are about half way done.

Once ALL the wires, ALL the connectors, ALL the vac hoses, are attended to properly, you will enjoy the REAL XJS moment.

You think its good now, just wait until those items are sorted.
So very few V12's are sorted to that "next" level, and they are awesome to those owners as they are.
Get them right, and the sensation is "priceless", and sooooo easy to keep that way.

 

I’ve owned the car like 4 years and just today found a leak the size of your pinky. Simply due to the fact the previous owner somehow thought it was a good idea to unbolt a vacuum screw! Never owning another jag I didn’t know if that was just the way they ran... like a runaway freight train. Ha

 

I had the same issue after I swapped in a lower mile 85’ V12 motor. Tried all the suggestions below, even bought and installed a new air regulator and rebuilding the distributor. However, it ended getting resolved when I increasing the idle on one of the banks. I am told that the issue stemmed from each bank not getting the same fuel as the other, thus creating the disparity's. Finally resolved the up and down idle.