I recently had my xjs in for service.It was running sort of rough.My mechanic replaced the spark plugs,new spark plug wires,distributor cap and rotor and some throttle bushings.Well, after picking it up, I noticed it was idling high and then really erratic. Since it was the weekend, I looked at the bill and noticed on the second page that there was a note, "car has broken idle switch that is no longer available, adjusted idle as best as possible". Needless to say, it would have been nice to have been told that before any work was done.That being said, would a broken idle switch cause the erratic idle? I've seen some other posts that peg an erratic idle as being a vacuum problem. Any suggestions would be appreciated. Thanks, Don

Submitted by darelmatthews@… on Tue, 10/02/2012 - 22:07

Don't waste your time on the AAV. It will cause idle problems, but by its' very nature it can't cause a wandering idle like you have.

Start from the beginning and fix everything to start ruling stuff out. Set throttle plates and linkages, adjust your TPS correctly, fix the broken switch (my '85 doesn't have it so not sure if thats the problem), check for any and all vacuum leaks. Set your base timing and then start looking for other problems.

Submitted by DVerr94100@com… on Sun, 09/09/2012 - 20:53

Thanks for the photo Steve, that really make it easy to see. is it difficult to replace? Seems like it wouldn't be so hard. Also the AAV seems (key word!) easy enough to get to and remove. Is it difficult to remove?

Doug, William, Steve, thanks so much for your input. I never fail to get some good ideas and answers to my questions from JCNA forums. Please feel free to add any more words of wisdom. Final question: Are these parts replaceable by a very novice type of person. My expertise is pretty limited. But I'd like to give it a try before bringing it back to a shop.
Thanks again!
Don

Submitted by stevejag@sbcgl… on Sun, 09/09/2012 - 19:41

Donald,

The idle switch is shown in the attached pic, red circle partially hidden under the brace. The Auxiliary Air Valve, or Extra Air Valve is in the green circle. Very easy to get to.

The idle switch closes at idle to change the ignition timing to the idle timing mapping and also makes it ignore other load inputs, like the MAP sensor in the fueling ECU. It can surely cause an idle hunting problem, but like Doug said, what you describe seems a bit too much for that. I'd expect 700-1500 maybe, if the idle switch was not working. Usually get a bit of a tip in hesitation in parking lot maneuvers without it, too.
Problems with the hose and tubing that goes off the balance tube, and all the way back to the ECU in the boot can cause issues like hunting and surging also.

Good luck!

Submitted by bonnettoboot@e… on Sun, 09/09/2012 - 19:19

Usually this would refer to the Auxillary air Valve on the left rear of the engine. these routinely freeze up and they are not available. They can sometimes be freed off by removing them and soaking them in wd 40 etc. as for the high idle I would check the valves at the front of each manifold. Then clean the throttle bodies and synchronisethe air intakes.

Submitted by dougdwyer1@com… on Sun, 09/09/2012 - 18:28

There's a throttle postition sensor under the central throttle capstan....the turntable thingy where the throttle rods attach. Not the same thing as the idle switch which is on the left side of the engine near the bellcrank at the end of the inlet manifold

Cheers
DD

Submitted by dougdwyer1@com… on Sun, 09/09/2012 - 18:25

The idle switch retards the timing at idle. An inoperative switch could cause a high idle, as the timing remains advanced. However, I don't think it would be as high as 2000 rpm.

An idle the fluctuates as you say is more commonly caused by unwanted air....like a vacuum leak....entering in. The extra air increases the idle speed until the ECU cuts off fuel....at which point the idle drops down. After dropping, the fuel is reinstated and the idle goes back up .....and the cycle repeats. Often it's the AAV (aux Air Valve) being stuck that causes this problem

Cheers
DD

Submitted by DVerr94100@com… on Sun, 09/09/2012 - 18:20

I did some digging on the internet and saw something called a throttle switch, that seemed to be underneath the bell crank?? Throttle switch or idle switch....two different switches. Also seems that they are available.

Submitted by dougdwyer1@com… on Sun, 09/09/2012 - 18:18

Couldn't find a picture (yet) but the DAC6049 idle switch still comes up on some vendor pages. Of course you'd want to verify stock-on-hand before clicking the "pay" button.

Better yet just call Coventry West and ask them :-) 800 331 2193

Cheers
DD

Submitted by DVerr94100@com… on Sun, 09/09/2012 - 18:15

Hi Doug,
erratic being going from 500 rpm to 2500 rpm....leveling off to maybe 2000 rpm for a couple of minutes. Sometimes it has been varied, like someone is pushing down on the gas pedal, sort of revving the engine. So it's not very subtle......

Submitted by dougdwyer1@com… on Sun, 09/09/2012 - 17:58

Edited on 2012-09-09 17:59:17

Can you define "erratic" for us?

Now, the idle switch....

I'm going from memory a bit but others will help fill in or clarify.

There IS an idle switch, left side near the throttle bellcrank at the end of the intake manifold, that opens/closes just as the throttles open/close from the idle position. As I recall it changes the ignition timing at idle. If your "erractic" idle is fairly subtle then I'd say the switch might be the problem.

Again going from memory I recall that the idle switch isn't especially exotic. I don't think it would take much imagination or effort to adapt a typical snap-action micro-switch to work. I'll try to find some pics to refresh my memeory.

Cheers
DD