3.6 Base Idle Adjustment

[poprivet]

Solved my high idle problem by following the advice below.
Thanks and credit to Brett Gazdzinski from the JL XJ40 forum

From: Brett Gazdzinski
Subject: Re: base idle adjustment
This goes for 1988 to 1990 cars at least...

To set the base idle correctly is important. The Jaguar system uses an idle speed control valve to increase the idle from a base setting in response to loads and temp changes. If your car idle hunts up and down when its first started, the base idle is incorrect.

Ideally, the throttle plate should be cleaned and gapped to .002 inches, and the throttle position sensor checked for the correct calibration....check the haynes manual for correct voltages for your year car...mine is .52 volts at idle, and change smoothly up to about 4.7 volts at full throttle. You really should check and clean that stuff if the car has been in
someone else's care, as who knows who did what to the adjustments.

After the throttle plate gap is set correctly, you can set the base idle speed.................

Warm the engine to operating temperature.

Turn key off

Find the iscv..it has a fat hose going to the air intake tract, and a connector with 4 orange wires with an 'idle' tag on them. Its mounted to the top of the intake manifold. There is a hex head adjustment on the top.(recessed)

Turn the key on but don't start the engine.

Unplug the I.S.C.V. connector, and with it disconnected, turn the key off.
Wait 15 seconds, plug in the I.S.C.V. connector.

Turn key on, but don't start the engine.

Unplug the I.S.C.V. connector. This winds the I.S.C.V. closed..

With the connector unplugged, start the engine.

Adjust the hex adjustment on the top of the I.S.C.V. till the engine idles at about 750 rpm in park, or 600 rpm in gear. Clockwise slows idle, counterclockwise speeds it up. Normally its not opened much.

The adjustment effects the smoothness at idle and is quite critical. You can fine tune it a bit after you try the car a few days...

Plug in the I.S.C.V. connector, and you are done.

People screw up the idle speed by adjusting the stop that sets the throttle plate gap, with the iscv hooked up, so the CPU just corrects for the mis adjustment....the throttle plate and other idle affecting ports get plugged up with crankcase vent goo over time as well.

The adjustment should last for a couple of years if nobody "fixes" it.

To make adjustments later, wind the I.S.C.V. closed again, and adjust the hex head adjustment to get the idle speed that gives a smooth idle when cold. If the base idle speed is adjusted to low, the car will vibrate when cold, and the trans will clunk when downshifting...
If its set too high and the idle will hunt up and down.

[ptjs1]

Well done, Pop!

I'm sure that routine will help others eliminate that annoying idle hunting that many encounter.

Cheers

Paul

[poprivet]

The mass air flow body and throttle body were really clean but there was practically no gap on the butterfly so adjusted it to 2 thou.
I'm sure cleaning and resetting the ISCV helped a lot.

The hex adjuster needs a strange size Allen key. It was metric but not one of the usual ones, luckily I have a bin of all odds and ends and found the correct one. The adjuster was seized and needed an overnight soak in plus gas fluid although in the end it needed very little adjustment.