Occasionally (it seems to be increasing in occurrence) the idle will go up and down pretty drastically until I give it some throttle. Once I do that it seems to go away. Yesterday when it happened I did note some fuel smell, I was in the garage that time. This is my first season with the car, so I'm still determining what was previously addressed. I do know the injectors were cleaned/tested and a new injector harness was put on. At this point I'm not sure if it is a fuel or electrical problem with the erratic idle. Any insight would be greatly appreciated. 1990 V12 XJS

Added info-Rough idle is between 1k-250 rpms.

 

Also, could it be related to the fuel I'm using? It seemed to start after using 93 octane (western NY), I know that these numbers don't always mean what they say. Today I filled up with 90 ethanol free and I haven't had the issue since. History has taught me with cars that it will come back as it's not usually that simple, but this is my first car that requires "premium".

 

Hi Stone

Hunting idle at Start Up, isn't at all uncommon in these Cars, with the Revs going up and down in the way that you have described and especially more so if the Car is not used for sometime, as the Piston in the AAV can get Stuck (AAV being Automatic Air Valve) down by the side of the Inlet Manifold

On the 'B' Bank Side on a UK Car

In some ways a bit of a non-event and nothing to panic about, as after a few long runs in the Car it often clears itself and gets back to normal, although if not a good squirt of WD can do the trick after first removing the Rubber Elbow, which has to be done quite Carefully to prevent it from Cracking

A New AAV can cost Megabucks! but even worse is the possibility of the Fixing Bolts snapping off, which has recently happened to one of our Forum Members, where as with other things in life, I find it's sometime better to leave things well alone

Rather that create a problem that was bigger than the one you started with

As for Petrol the XJS will always run a lot nicer on the 'good stuff' where as for myself I only use 99 Octane
Only the Best for my Baby @Greg in france

 

Essential to use the highest octane. The XJS HE engine USA version has an 11.5:1 compresion ratio. UK cars pre- Marelli have 12.5:1.
Jaguar ran test HE 5.3 HE engines at 14:1 with no problems, astonishing for the era.

 

The AAV, as mentioned, is a prime suspect here.

Or any other source of incoming air.

Or left-right imbalance of the throttle blade settings

When you open the throttles the amount of incoming air is large enough to mask the problem


Any fuel smell must be addressed, of course....but it may or may not be related to your idle problem


Hard to tell at times. Sometimes you just have to dig in and start eliminating possibilities.

Also, when you check the throttle gaps, make sure the throttle bores are clean.

Cheers
DD

 

Just an update, in the event someone has this in the future. I’m not 100% positive, but next day after my original post, the fuel pump started shutting off after 10 or so minutes of driving. Happened a few times on my way home from work and each time it took 30 minutes or so for the pump to work again. Since replacing the pump I’ve gone on a few drives and have not experienced the erratic idle. So in my mind erratic idle could be early signs of pump failure. Someone with more experience may correct that statement, but it seems correlated. Replacing the pump was an easy and relatively cheap fix too.

 

Pump failures are common enough and irregular fuel pressure could surely cause a problem.

There's no arguing with success...and I learned long ago that there are causes and fixes that don't always match the usual pattern.

Cheers
DD

 

whats surprising is the owners manual only calls for regular unleaded. Hard to believe with 11.5:1 compression(i will still only use 91 AKI/95RON)