The car:
1995 X300 XJ6, normally aspirated, 4.0 US version.


What I did so far:

1. Replaced spark plugs with the correct ones from Champion (previousowner installed autolites).
2. Welded cracked exhaust manifolds.
3. Cleaned throttle body
4. Replaced Throttle Potentiometer (Throttle Position Sensor) with brand new one from Jaguar
5. Replaced pipe with secondary catalytic converters (at least one of them was plugged)
6. Replaced pre-cat oxygen sensors (bank 1 and 2 sensor 1).
7. Checked compression and confirmed that it's good.
8. Ran a tank of fuel with injector cleaner.


What symptoms I am seeing:

1. During warmup the car seems to idle ok, although I once in while I sense an occasional miss. It's so faint, I am not even sure it's there.

2. When fully heated up, the ides settles to somewhere in the 650-750 RPM range.

3. Sometimes, either at a stop or when trying to accelerate after coasting the car "stumbles": The RPMs sink below 400 RPM, check engine light comes on for a second or so. It feels like the engine had a hiccup, no acceleration for a brief second. Sometimes the Geabox fault is illuminated, but not always.

The fault is intermittent. The car does not always stumble, seems to do it once in a while at random.

I have IDS at my disposal, but no VMM. How should I tackle further troubleshooting?

The only thing that comes to mind is checking waveforms for coils and crank sensor.

I am thinking the following possibilities:
- sticky injectors
- flaky coils
- flaky fuel pump
- intermittent electrical fault, either in ignition circuit or fuel pump.

I am open to ideas.

 

I'm pretty certain you are going to find it to be the coils.