My 76 fuel injected XJ12C can idle poorly while in gear waiting at a traffic light. The indicated rpm is 650. If the a/c compressor is turning, the idle quality is worse. I do see the voltage gauge indicator drops to 12 or lower from 13.5. If I take car out of gear, the idle climbs to 900-1000 rpm and is very smooth.
The battery is recently new, and this characteristic did not change with the new battery.
My next step is to hook up my digital voltmeter on some extensions into the inside of the car for some real-world driving voltage, but I do wonder at what low voltage point the D-Jetronic system starts to misbehave?
Fold a $100 bill in half and hold it over an exhaust pipe tip. The fluttering of the bill will clearly indicate if you have a random misfire or an even misfire. Tell us what happens both at idle in neutral and at idle in gear with the A/C running.
I drove the XJ12C this weekend to a Jag and British car event about 25 minutes north. Car runs down the road excellent, especially with the 700r4 transmission. I motored along about 75 mph or 2,200 rpm or so. It has been a while since the car was on a long modest drive such as this.
When I arrived at the show, I had a friend sit in the car with the car in gear and idling. The indicated rpm was 600-650. The digital meter voltage at the battery after a few minutes was steady at 12.33 volts. When I turned the a/c on, it dropped to 12.21 volts.
The engine was idling very smoothly, but then every once in a while would miss and it felt electrical to me, as if for just a fraction of a second “turn off.” The same split second miss would repeat after a few seconds of idling smoothly, irregular time between misses.
I went to the exhaust pipes and the hundred dollar bill (no video) would blow smoothly away from the pipe on both sides (straight exhaust A & B banks). I never could figure out if the miss was on one bank vs the other or both.
This is not the rough idle I was experiencing the other day in traffic, but the indicated voltage on the gauge at idle does drop way off, but as verified at the battery, it is really above 12.
Next, I will try to figure out if it is a bank related miss.
I do have a Lumenition non-stock electronic ignition pickup on this engine.
Fuel pressure is a steady 30 psi (permanent gauge on system)
I guess an experiment you could do is to turn also the headlights, interior fan and rear screen defroster on and see if the problem gets worse?
Or the opposite, connect a charger or freshly charged battery and see if it goes away.
Once I drove my car with the warning cluster removed for some days, and a detail I didn’t know at the time, without the warning cluster the alternator does not get excited and therefore doesn’t charge the battery.
The third day there was a miss here and there, exactly as you described, after a while it was constant, and in the end the car died, luckily in front of my house…
Battery is new, replaced simply because older battery was 6 years old and I didn’t want trouble. The voltage at idle with transmission in “park” was 13.6 or so. I didn’t spend time looking at it as it was clearly charging off idle and I was thinking more about its behavior during the load of idle in gear. thanks for your ideas.