Hello All

My XJS has been running beautifully, yet not perfectly, and I've tried and failed to isolate exactly what is the problem.

When cold, there is a slight hesitation when you first press the accelerator. When warm, the idle comes down as it should, but it 'hunts' a little bit between 700-800 rpm, and sometimes it will stall when in reverse gear. Not sure why it only stalls in reverse, but not drive. I've been combing the archives, but I can't put my finger on the cause.

I've done the following recently:
Full cooling system
New plugs and leads
New dist. cap, rotor and 'Marelli fix'
Professional injector cleaning and fuel rail refurb
New fuel injection harness
Fixed the AAV
Adjusted the throttle plates
Checked throttle rods & bushings
Calibrated the TPS
Recent Italian tune up

Thanks for any advice or clues!


@Grant Francis @Greg in France@Doug

 

Lots of 'could be' scenarios.

Post #13 in this thread comes to mind as one possibility/fix

https://www.jaguarforums.com/forum/x...r-info-207647/

Cheers
DD

 

Italian tune-up last weekend (I'll edit post and add to list)

 

I think fuel pressure is worth investigating. Either fit a new B bank FPR, or put an FPR gauge on and see what it does when you get this hesitation.
Secondly, although you have calibrated the TPS, have you tested it for smooth voltage changes?

 

Thanks Greg,

Wouldn't a fuel pressure issue be persistent regardless of cold or warm engine? The hesitation is only noticeable when cold.

 

If there's a fuel pressure problem it would likely be present with the engine hot or cold. However, the engine's reaction to the incorrect pressure could differ according to temperature.

If there's a lack of fuel pressure, which would cause under-fueling, a warm engine might tolerate it without complaint where a cold engine wouldn't.
Conversely, too high fuel pressure would cause over-fueling....which a cold engine might love but a warm engine reject.

Cheers
DD

Cheers
DD

 

here's something to check:

The 'idle switch' on the LH throttle bell crank. The switch should be closed when the throttle is closed and open as soon..instantly... as the throttle is opened. This switch is on Marelli cars and does "something" with the ignition timing....not sure what exactly.

Easy enough to check. If faulty it might be another situation where a cold engine is more sensitive to the fault than a warm engine.

Cheers
DD

 

I took a little time to see if I could find anything that might explain something, and discovered that the microswitch located on the bracket at the outer end of the left throttle operating rod was not closing at idle. It's about 1mm off from closing. I think this microswitch changes timing or ECU signals at idle versus open throttle.

Should I adjust the throttle rod so the switch closes? Would this explain my symptoms?

Thanks!

 

Haha, great minds! See my last.

 

NOT the rods, that will reek havoc elsewhere.

There must be an adjustment for that switch, but I am not familiar with the Marelli enough to go further.

 

The idle switch adjusts ignition timing for closed throttle running in conjunction with engine speed and coolant temperature, this could explain why you have different symptoms for a cold and warm engine.

Here is a copy of the Marelli supplement page 6 has some info on cold and warm running.

Adjust the idle switch, then get the engine dialled in warm. Hopefully this will fix your issues.