1991 V12 hunting at idle

hoping to get some help finding cause of hunting at idle

A common issue is an UNBALANCED throttle between the banks.

Thanks. would that just be throttle valve adjustment?

The bushings have a tendency to deteriorate and the throttles can open at different times.
The plates can get dirty and let different amounts of air flow.
Clean the plates, set them to the recommended open-from-sealed. (.003"?)

There should be a micro switch on one of the banks that can get damaged or faulty.
It’s been a while since I worked on these 5.3 liters everyday when I was at the dealer.

Hello Paul F. - welcome to the Jag-Lovers XJS forum - where are you located (if you place your country name in your profile information, a country flag should show up next to the icon with your name - this is good to know so that someone close to you may be able to help if needed) - I recently replaced the throttle shaft bushings on my 1991 XJS V12 - I have not had the chance yet to check the butterfly clearances (measured at the top and bottom of the plate) as “motorcarman” mentioned to check - at the present I do not have a “hunting at idle” condition, but I do have only approximately 550 RPMS indicated on the tachometer (rev counter) - I am wondering if you may have either a loose wire connection or loose vacuum hose that is moving periodically - if there is anything that you need checked for comparison, don’t hesitate to ask - there are a lot of knowledgeable persons on this site - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 11/14/2020 2136hrs. USA.

Does the hunting occur only shortly after start as the engine warms up, or even after the engine is fully warmed up? What is the range of RPMs when the hunting occurs?


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The engine hunts when fully warmed up

You might want to check your oxygen sensors. The earlier cars had 1-wire oxygen sensors that would get too cool at idle and quit working, and the resulting signal dropout would cause the idle to change, which in turn would warm the sensors a bit and start them working again, and the end result is a hunting idle. To fix that problem, the EFI system would shift into open-loop mode at idle.

Your '91 should have 3-wire sensors. The other two wires are for a heater so the sensor is always hot enough to work, hence no hunting at idle. Hence the circuit to shift into open-loop at idle was eliminated.

Note that the engine is always in open-loop until it warms up. Therefore no hunting when cold.

So, let’s say the heaters in your sensors aren’t working for some reason; they’ve gone bad or the circuit that powers them is faulty or whatnot. Or, possibly the sensors themselves are just old and weak and don’t work right. Whatever, something to look into.

It could be a vacuum leak. The injectors are cut off when the accelerator pedal is released. They go back on when the rpms drop. A vacuum leak will cause the engine to rev up, then the injectors are cut off. On and on.