I agree Ragjag.......Cybercg has totally let the side down with no pictorial add ons into this post!

 

I have attached a pic of the Idle Air Bypass Screw. It is just to the right of the Idle Air Control Valve on the driver side of the engine bay (Left hand drive). If your bypass screw has never been adjusted since manufacture, it may be covered with a soft metal or plastic plug.


I want to reiterate that the adjustment of my bypass screw was not the cause of my original problem. Its adjustment was the final piece of the puzzle that returned all the component players to values that the ECM could use to manage the idle and air/fuel map. No silver bullet here. Just the final piece.


Thanks for all your help.

 

My engine does not seem to have this adjustment screw!

It think it's fascinating that your '95 has a different part than my '96!

Check out the pic!

 

I think in theory no AJ16 should have it since the idle is all controler by the ECU, but my does have it and it has been adjusted to lower the idle a bit by a mechanic a few years ago....And I think my idle is way too low. It always idles at 500 on drive and 650 on park. Shouldn't it be a little higher? How do I know what the correct possition is for it?

 

I can check my manual, but I swear that it should idle between 550 and 600rpm at warm idle.

You have no strange idle issues?

I happen to have an idle issue every 30 seconds when stopped. It drops the idle and voltmeter for a second and then runs fine, until 30 seconds later.

I was hoping that maybe I was on the low end of the idle spectrum.

 

No strange idle issues.

As far as your drop, it's a coil Vee. Know that drop in idle by heart.

 

I just replaced them all! How can that be?

It only happens when the engine is warm. Cold idle is fine.

 

Sure enough. Your '96 doesn't have the bypass screw. Maybe Jaguar eliminated it because too many owners were messing with it. ;-)

 

Damn... do you have a horizontal set screw that affects the rest point of the throttle plate? Its a screw that requires a 3mm allen wrench to adjust.

I will attach a pic another time.

In the meantime...

Engine idle speeds should be set as follows:

Cold in Neutral - can be up to 900rpm at -20C
Hot in Neutral - 700rpm
Cold in Drive - 780rpm
Hot in Drive - 580rpm

 

That's so odd about the screw, because all 95 and 96 cars have the same part number for the throttle body.....

 

Agreed. When I looked on eBay, I did find one that had a perfect picture taken of it...no screw.

http://m.ebay.com/itm/310900940107?nav=SEARCH

 

I have looked multiple times for a throttle plate adjustment. I do not believe that I have one. The throttle plate on my '95 closes completely at idle position. The ECM programming may be different on the '96 to compensate for the lack of an air bypass adjustment. The '96 may still have an air bypass passage in the throttle body, it just isn't adjustable any longer and the ECM programming has changed to compensate.

 

Cybercg, do you think that your throttle body may be from a pre-facelift XJS?


I have a 1996 and it has the 3mm allen wrench for the throttle plate located 180 deg (front side) from where your air screw is, and no air screw.


Idle air is controlled by the IAV and the IAV reset procedure will not work for this style.


When I was having idle problems, I disconected it and set it manually by pulling the plunger to different lengths.


Regards,


Craig.

 

I was just thinking that maybe Jaguar installed the old AJ6 throttle bodies on the AJ16 cars until they ran out! It's not unheard of for them to do that sort of thing...

 

On my 1996 the correct idle is 600 rpm. Cold, it may idle at 750 for a few seconds and than drop to a smooth 600. When it is in neutral it is still 600. Putting it into drive doesn't drop the rpm at all. I believe the computer compensates for any changes. Installing a new CPS, IAC and EGR valve fixed any idle problems. I have a new TPS that I have not installed. I did install new coils and spark plugs early on. My old lady is docile as a mouse and is quite a pleasure to drive now. That is today, of course. lol
RagJag.

 

Mark,

Any chance that this shudder at idle (every 32 seconds) is caused by a failing harmonic balancer?

For what it's worth, unplugging the IACV reduces the drop of RPMs and voltage, but its still there, just less noticeable. Every 32 seconds. Strange that it does indeed have an affect....

 

Vee, is it a "drop" or "blip" or is it a shudder? Because a drop is a coil. Specially when hot since the coil is probably failing when it's getting warm. What brand of coils did you get? New old stock? Made in Japan?

 

Drop or shudder. I see a voltage and RPM dip.

I have 6 coils from Japan.

Two I bought back in 2010 from a british vendor. The other four I bought more recently. All six were installed this past August.

I just ordered another one so I can play musical chairs with the coils. I figured I will replace one coil for my morning commute, and if there is no change, I will put the original coil back and replace the next coil with the new coil for the PM commute.

Worst case is that it takes me three days to figure it out, right?

You are making sense. Doesn't happen with a cold engine...

 

I looked again and have found that I DO have the throttle plate set screw AND the air bypass screw. Sorry. I don't know why I could not find it before.


I have only owned the car for 2+ years so it is very possible that the throttle body has been replaced with a prefacelift body at some point in its life.


My throttle plate appears to/does close completely at idle and maybe it should not. Or maybe that is one of the purposes of the air bypass screw.


Do you or anyone know what the specs are for the throttle plate closure gap? I am willing to experiment a bit as it could improve my idle even more.

 

Throttle plate should be 0.002" or 0.05mm between the butterfly valve and the throttle housing.