I recently bought this 1985 XJS HE with 59,500 miles. It had been in storage for a while (I'm guessing at least a year). I have tackled most of the issues one would expect in this situation: LOF, New air filters, new fuel filter, brakes (Expensive!) are fantastic now, new shocks, new tires, fresh alignment, and then we moved on to engine tuning. They installed new plugs & wires and referred to jag-lovers.com/books/old-xj-s for detailed instructions of proper gaps and timing for the H.E. The distributor is advancing and frankly - after all that, it seemed to run like a good kitty. But good kitty randomly becomes bad kitty. It will be running and then suddenly it will be...shall we say....coughing. Usually it will simply die, sometimes during acceleration and sometimes just when idling. We can usually get it started again after, oh, 20 minutes or so, but not before it "half-way" starts. What I mean by that is it will idle extremely low and if i rapidly pump the gas it will sort-of half-way run, but never getting over about 1,500rpm and in an intermiten manner. So, we suspected ignition coil. We replaced the Aux Coil, but no change. I have a Primary coil on the way and should have it tomorrow. We are hopeful this will be the answer, but frankly, I'm afraid it's not. This is such a beautiful car with such potential and such low miles. It will be a shame if I am unable to get it to stop acting up! I'd like to be able to reasonably depend on it for the Fall when a nice drive through the Ozarks in this car could be such a treat. I am very hopeful someone with specific knowledge of this car and these symptoms could help meand this worthy specimenout.

 

Sounds like the "classic" ignition module failure. readily available for about $25 and will cause what you got without question. That is the module inside the "black box" bolted to your LH inlet manifold. You got Kirby's book, good info in there about it.

 

Replaced both coils - and the problem continues. I don't think it could be the module either because we've discovered one more thing: It's only ONE side of the engine that is having the trouble! The injectors aren't firing part of the time on the right side. Some of the time they are working and then suddenly they don't. We just can't figure it out! Anyone seen this before? When functioning, the injectors are working wonderfully. When they work, they work, but when that side dies, the other 6-cylinders can kinda keep it alive - sort of. You would have to pump the gas to keep it from dying. The problem seems to correct itself after sitting for a relatively long period (so, something that is sensitive to the heat possibly) but then does it again. Fuel pressure checks out fine they said. ECU? Ignition Amplifier? O2 Sensors?