Series 3 Hot Idle Speed Problem

Hi, I have a late series 3 4.2 which has a problem with the idle speed dropping when it is fully warmed (more than 30 minute drive). Gradually drops to around 600rpm or less in drive and then will stall if changed into reverse. It will start okay but will keep stalling when a gear is selected. After a few minutes it works again.

I had adjusted the idle speed using the usual allen key adjustment below the AFM to the max, but it was still not enough.

A local Jaguar /MG specialist has attempted to fix it but has not succeeded. The mechanic has suggested there is a problem with the torque converter locking up, and suggested pulling it apart to check. I have had no trouble with the transmission in any other respect. My gut feeling it is the too low idle speed causing the problem.

The mechanic has checked timing, cleaned the throttle body and made various adjustments. He did remark that the inlet manifold vacuum was slightly low at 16 hg? when it should be 19.

After their second attempt to fix it, I requested they drive the car to my house which was a 40 min drive.
When the driver attempted to reverse, the car stalled. He did manage to reverse it into my garage, presumably by keeping his foot on the throttle.

Any ideas what would cause the hot idle speed to keep dropping? or could it be a torque converter issue?

Any help appreciated.

Philip, 1986 Sov. S3, Auckland, N.Z.

Idle is set with the engine ‘warm’ (say 85C), Philip - what is the engine temps after your 30 minutes drive or as idle drops…? And what is your cold idle? And how long does it take, while driving, for the engine to reach ‘thermostat’ temps - some 82/85C? Is the coolant level to spec.?

Two lines of enquiry; is it temp related - or is it time related. In the latter case; a temp sensitive items takes some time to be affected - local heating. In the first case; high temp in itself is a cause - hence the question…

Idle rpms are set by three items; the throttle gap - did the mechanic check and set the throttle gap to the prescribed 0,002"? The Idle screw; does the idle change when the screw is adjusted - no/little response to idle adjustment is typical for an air leak? The AAV; have you verified that the it functioning properly - about half open when cold - and completely closed when hot…?

That said; even a low idle, 600 rpms, the engine should not stall under load. What happens with AC ‘on’? However, an air leak may lean out the mixture; causing a weak engine - which may stall under load.

Crude test; with the engine idling hot - remove air filter and gently push on the AFM flap. Even a minute motion of the flap should cause the engine to falter - indicating a ‘normal’ mixture. If rpms increase, even slightly; there is probably an air leak…

A different line of enquiry; the ign amplifier may misbehave when heating up - and may cause the engine to stall under load at low rpms…

The problem may be subtle and difficult to locate - but I doubt that it is related to the gearbox…

xj6 85 Sov Europe (UK/NZ)