This is a US-Market 88 Hess XJS V12 for reference.

My XJS, ever since I got it running again, has had a bit of a rich idle and it's not perfectly smooth, can be a bit "lumpy" at times. I found that the CTS was bad, replaced it, helped massively. I also found that my original TPS was throwing pretty erratic readings anywhere other than at idle position and full throttle, so I replaced it with the Mustang sensor that many people on the forum have also done, also helped massively. The car up until today basically was running pretty good, but the idle wasn't as smooth as I think it should be and it still seemed to be running slightly rich.

Which brings me to today: I decided I should have a look at the throttle linkages and see if they've come out of adjustment since the car left the factory, since I assume/assumed they've never been touched before. I used the adjustment procedure PDF and properly gapped the butterflies and set the set screw where it should be. Before I adjusted them, the butterflies were pretty much adjusted all the way in to where I couldn't even drag my thinnest feeler gauge between the bore and butterfly. After adjusting both sides to the specified gap, double checking, and adjusting the throttle rods so there'd be less slop, I started the car and the idle is now super high... about 1950 when cold and drops to about 1500 after running for a couple of minutes.

Now, this could be a byproduct of the increased RPMs, but the car doesn't seem to idle nearly as rich and does seem smoother. This got me thinking, is it possible that the AAV is stuck and the previous owner just adjusted the set screws all the way in to lower idle RPMs? Or maybe I'm just adjusting it wrong somehow. Sidenote, would an idle TPS voltage of 0.44 cause such a high idle? That's what the replacement Mustang sensor reads at idle, which I know is slightly above what the factory sensor should be.

 

Yup

Remove the LH air filter cover. You'll see the air inlet to he AAV. About 1" diameter. Block it of with your thumb (or whatever). Does the idle drop down? If so, bingo.

(Don't be tempted to permanently block the inlet!)


Make sure the .002" and .003" blades of your feeler gauge are not stuck together. This happened to me. Unknowingly I had set the throttles to .005". This goof-up had be chasing my tail for the longest time

I've used the Mustang TPS and, yes, the idle reading is a bit high. This never caused a high idle or any other tangible problems. It ain't right to have the voltage that high (as you know) but the engine will perform OK as far as seat-of-the-pants goes.

Cheers
DD

 

Following on from Doug.

Someone has masked an AAV issue, closing the discs is a classic give away.

Make sure you have a NEW 90deg hose elbow for the top of that AVV before you start, they split without touching them, and thus ruin your day 100%.

These attachments should clear the mud a tad.

 

I think that 44 at idle is too high for the TPS. After the AAV is fixed, see what she runs like at tickover, if still not 100% satisfied, then an OEM TPS may be the answer.

 

Yup, seems the previous owner must have adjusted the linkages to cover up the AAV issue, idle dropped back to a reasonable number after blocking the inlet. I guess I'll go ahead and get a hose and gasket for the thing and try and rebuild it in the near future. My next question is, what are the adverse effects of blocking the AAV inlet or adjusting the throttle back to where it was, just for the time being? Not to drive on or anything, but just so I can move the car in and out of the garage and around the driveway without dealing with a 2k rpm idle.
Is the 90deg elbow on top of the AAV a generic part I could get locally, or do you have a link to one available here in the states? I figure if someone came up with the mustang sensor idea, wouldn't surprise me if someone found out that hose was the same as a 19XX Chevy Whatever. Thanks for the AAv Rebuild attachment, I'll get on that next weekend.
Yeah, 0.44v is above what it should read at idle, but drivability doesn't seem to be an issue. I'll fix the AAV situation and re-evaluate how it feels, the Mustang sensor was supposed to be a temporary fix since a friend had one on hand when I found that mine was bad, but ended up working so well that I've kept it.

 

No idea on Generic, never bothered.

EAC2655 is the number, and SNG etc would have them for peanuts, and you KNOW its the right part, no after thought issues.

I always had one on the shelf, up until I removed the AAV on all the V12 cars.

 

BuckleSpring asked for this.

Here ya go.

Many others have done this in different ways.
Mine was in 1994, and purely me, my brain, beer, and that Shade Tree. NO computers etc etc at that time.

 

Vacuum will suck engine oil up thru the oil seals of the distributor shaft, soaking the innards of the distributor with oil!

You could leave the inlet plugged-off if you found a different source for the distributor vent system


Sure. After all, that's how it was xxx-years before you made your discovery !



You might find something that'll work. The tricky part, though, is the nipple for the distributor vent


Cheers
DD

 

For what it's worth, and for anyone who might be interested.....

In my experience the V12 doesn't need an AAV at all.

My previous V12, years ago, had a dead AAV. Fortunately it died in the closed position so I didn't have a too-high hot idle. Anyhow, I had no cold running problems until or unless the ambient temps neared freezing. Start the engine cold and it idles perfectly at 600 RPM, drop the car in gear, drive away. If the conditions were really cold a bit of nursing with the throttle was needed for a few moments before the engine would idle on its own.

On my present V12 I ditched the AAV entirely and never looked back. Same result; no cold idle problems. If the temps get near freezing I use the foot operated AAV, the one hanging next to the brake pedal

I've rebuilt a couple AAVs. They never worked for long. Maybe I flubbed-up...always a distinct possibility . In any case they're a chronic failure point and, IMO, wayyyy more trouble than they're worth.

Later I'll take a pic of my idle adjust system and post it.

Cheers
DD

 

This is what I did:

 

Well I guess that won't be an issue, the previous owner broke the nipple off of the top of the distributor cap almost 2 decades ago... He left the remainder of the nipple in a ziploc bag in the trunk and I've been meaning to epoxy it back on but haven't gotten around to it.

As a temporary solution I'll just 3d print a cap for the AAV inlet and plug the line that went to the distributor. I'll order a new PCV and AAV elbow this weekend as well, since both are very cracked, maybe see about rebuilding the AAV before disabling it in some capacity. I sprayed some penetrating fluid into the inlet in the hopes it might free up, no luck on that yet.