Has anyone come up with a way to boost the idle speed when the AC compressor is engaged and the car is at idle? I'm thinking a solenoid operated valve bleeding extra air past the butterfly could work. It would need an adjustable metered orifice to set the increase and would be powered via a feed from the compressor clutch circuit via a micro-switch on the butterfly valve. An air source after the air filter is feasible but I'm not sure where to bleed it into the manifold.

I'm always a bit nervous sitting at a stoplight with the car in gear and the AC on when the idle drops to 500rpm. I'm also not comfortable with a normal warmed idle speed (AC off trans in neutral) above 850, which is what mine is set to. Put it in gear and it drops to 700, turn AC on and it goes to 500.

Any thoughts?

 

Jaguar offered a retro-fit, all-in-one kit for this exact purpose. I'll see if I still have the documents....which may be of some interest to you. But, anyway, the essence of the modification was just as you've described. The heart of the modification was an extra air valve to admit .....extra air. As I recall the valve was the same part number as used on many V12s for the same purpose.

More later

Cheers
DD

 

Thanks Doug, Any idea where the air was permitted to enter the manifold? I would have thought that this would be a more popular topic, unless others run at a very high normal idle.

 

I couldn't readily find my documents but....we got lucky!

I found this collection of Jag TSBs on line

Click the link. Scroll about 2/3 way down. Your looking for bulletin JD 08/84 Item 65


https://www.terrysjag.com/XJS%203.6%...e%20Engine.pdf


Cheers
DD

 

The amount of RPM drop you're getting with compressor engaged.....200 rpm.....is more than typical. I'd say 75-100 rpm should normally be expected.

The Series III 4.2 was slightly notorious for "Unstable/weak idle" issues; some worse than others for no apparent reason that I've ever determined....and others apparently not affected at all. Some just seem to have a problem sustaining life at idle speeds. Ever increasing age with corresponding slight and gradual degradation of all involved systems might play a part.

Many many years ago, when more of these cars were still in regular use, the topic came up more often.

I recall fattening the idle mixture on a couple cars and that seemed to help reduce the excessive rpm drop.....and give a smoother idle as well.

Cheers
DD

 

Just add an idle solenoid which are available from many vendors, inc Holley. You mount the unit in connection with your throttle linkage. wire it to the AC switch or add a new switch. when the AC comes on, the adjustable solenoid adds some throttle.

They work. good luck
geneo

 

Thanks again Doug,
Lots of late night reading in those TSB's. Just how exactly does a backyard tinkerer "Fatten the idle mixture a little". I don't have any kind of meter to analyze the exhaust.

 

There's an adjustment on the air flow meter. If you look at the top of the AFM you'll see a hole or well....capped off on some cars, not on others. Inside that hole is the adjustment screw. I can't remember offhand which way you'd turn the screw for "richer"....but I'll try to find out.

Or, just do some experimenting in both directions, maybe a quarter turn at a time....waiting a minute or so between adjustments to give the system a little time to stabilize. Ideally you'll feel the idle get a little faster and/or a little smoother....which means the engine likes what you are doing If the idle ends up being too high as a result of this you can then go back to the regular idle speed adjustment screw and back it down a bit

Cheers
DD