I have a 1991 Jaguar XJ6. I realize that the best advice to fix this car involves a cliff and/or a can of gasoline and a match, but suffice it to say, I have an attachment to this car that prevents me from making such rational corrective decisions. I have 2 problems with this car. 1) in wet weather, after driving a bit, the engine bogs down for no apparent reason, and occasionally will backfire. To keep going, I have to floor it, and hope it works itselff out of the bog, and usually it does. But it continues this problem as I drive normally, but only in damp weather. I looked under the hood after one of these episodes expecting to see a wet distributor and wires, but the engine was dry. 2) Despite replacing the head gasket and rebuilding the head, the car idles rough, and occasionally will hunt for the right idle speed when cold. At driving speed, all is well. However, when stopping at a stoplight, it can lose idle speed, and act like it is about to die. There is no idle adjustment on this car, except for the stepper valve, which is correctly set for an odle at 580 - 620 rpm. What’s my problem and what’s the fix? The catalytic converters are clear, the injectors are now, as are the plugs, wires and cap, and all fluids are correct. The car has 144k miles.
The backfire and running problems in damp weather are ignition system issues. On a dark night fire up the car and open the hood and let your eyes adjust to the darkness. If you are seeing a “light show” of arching and such then your plug wires are suspect. There are many types of plug wires and some are pretty poor quality. The wires don’t have to be “wet”, dampness and humid air is enough to make some ignition components arch to ground.
Sometimes you can locate the main problem when you see some really bright blue arching. A another new cap and rotor are relatively cheap. Search out the very best quality wires, cap and rotor you can find put them on and see if there is a difference.
I think the idling problem is unrelated to the dampness issues and can’t help you with that.
I am right there with you on the ignition troubleshooting. Unfortunately, I’ve done just what you suggest with no resolve. I have replaced the wires, distributor cap & rotor, and plugs - 2 times with ridiculously high quality parts, hoping it was just a product quality issue. I even got new spacers for the wires to hold them apart and off of the valve cover. No arching or anything to indicate an electrical issue with the wires and/or cap & rotor.
Sounds almost like your idle control system shuts down after driving a while. Not familiar with the Jag system, but it must surely be under the control of the engine management or fuel management controller and idle air control valve. I would start there, and with any relay on the idle control circuit.
Is this a 3.2,3.6 or 4.0L AJ6 engine ?
I’m assuming that no error codes are being reported.
It is the 4.0l. No codes are coming up.