Intermittent cut-out with 1986 XJ6

The ignition coil and module are more prone to failing when hot but that's not carved in granite.

I'd take a couple hours to inspect-clean-tighten all temrinals/connectors related to ignition and fuel injection, including the main ground at the rear of the water rail as mentioned by JAM. Remove the driver's side under-dash panel and, with the engine running, gently jiggle the wiring at the rear of the ignition switch. If the engine reacts, you've hit pay dirt. Take a really close look at the connector for the coolant tmep sensor.

In a perfect world we'd have only "hard failures" with a clear path to diagnosis and repair. Mystery problems such as this, though, will often defy logic. Sometimes you just gotta dig in and start eliminating possibilities.

Cheers
DD

Thanks guys for your suggestions,and I will explore them, but since the fault is so clearly temperature dependent, are ignition problems a likely culprit? The symptom only happens during a very specific time window starting about a minute after cold start-up and ending once the engine is warm.

I left it as a garage when it was doing it to the point where it was undriveable, but by the time he tested it, the heat soak had warmed up the engine so that it drove perfectly! He said he was too busy to wait till morning (I guess I won't be taking back to him!)

I agree that the coil and ignition module are good candidates.

The Ser III 4.2 doesn't have a throttle position sensor, at least not in the conventional sense. It does have a open/close throttle *switch* but it recognizes only two things: closed throttle and full throttle. Not sure if that would play in here but you never know. They are not known to be failure prone---quite unlike the coil and module.

Cheers
DD

and another idea straight from the ones that know:

The Throtle Position Sensor.

Quote:

"Throttle position sensors eventually wear out as they age and are a common cause of vague and erratic drivability problems, such as hesitation and flat-spots during acceleration".

ideas are ideas and troubleshooting an engine online is like troubleshooting internet problems by phone, so here are three: running on 1 cylinder: bad ignition cap or wires or bad wire connections or bad rotor or bad ignition amplifier or even a bad coil.

I go for the ignition amplifier first.

Hot the Grounds: remove the 2 bolts and clean the ground looms located at the rear top of the cylinder head, near the battery. Use a brass toothbrush to leave the ring terminals shiny, the bolts shiny, the washers and the contact surfaces very shiny. If nothing changes, look at it this way: that's one less 22 year old car maintenance job you need to do for the next 22 years..

I'm not a mechanic or a comedian, I have owned my 1984 XJ-6 for 17 years, and proudly starting the 18th. I''ve had the symptom you are having. It cured itself..

I now have more data. The right turn was a red herring (ther happens to be a right turn when the engine gets to a certain temperature). I have driven the car extensively, and it is a matter of engine temperature. The car starts fine from cold, and runs perfectly for about 2 minutes. If you were brutal enough to floor it within the first minute and keep it there (if you lived on a racetrack) you would never know anything is wrong, but if you come to a junction and slow below 2500 rpm, the engine doesn't want to idle - feels like it is runniong on 1 cylinder at idle. If you put it into neutral and get it to 3000 rpm, it runs fine. After 5 minutes once it is warmed up the car runs perfectly.

The problem is thus ONLY during warm-up. Weird.

I suspected a faulty temperature sensor (not the thermolyne sensor) so I replaced it, and the first drive was perfect. Symptom returned one day later. . . . . and remains to this day.

The last time I drove it, I tried to restart it about 45 minutes after I parked it, and it was almost impossible to start (i.e. when it had cooled down to the troublesome temperature range) - took me 10 minutes of spluttering and dying before I could get it revved up to 3000 rpm and did a most un-jaguar like exit from the work parking lot!

A dirty throttle body, I'm thinking would not be a function of temperature, besides which it was cleaned last year, and I ran 2 cans of injector cleaner through (one per tank).

Ideas anyone?

Pretty strange.....

I'll assume that the turn is being made while coasting down under closed throttle. If so I suspect a 'weak idle" condition.

I'd clean the throttle body and make sure the throttle blade gap is set at the correct .002" clearance. Then check your idle speed. My experience with these cars is that the a/c compressor and torque converter really pull down the idle, so I like about 900 rpm in "P" which should give you about 650-700 in "D".

Next I'd check wiring and connectors at the coil, fuel injector ballast, and firewall relays. A loose connector might be aggravated by the movement of a right turn. Its a stretch, but I'd check anyway

Although not always possible, obviously, the next time this happens and the car won't start, try to check for spark. Knowing if we do or don't have spark would really help narrow down the search

Post back if you need help with anything I've suggested.

Cheers
DD