Jaguar XJ6 Series 2 Misfire, Rough Running
On a Series 2 a misfire that comes on only once the engine is hot is usually ignition, not mixture. I'd start with the points and condenser, then the HT leads and coil, then the distributor itself. If it cuts out rather than misfires when you lift off, look at the warm idle and the AED.
How bad is it · Garage, carefully
Drivable with care, but don't rely on it
A misfire pulling away or a cut-out on the overrun can leave you stuck at a junction. Keep journeys short until the cause is found.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
Points and condenser that break down with heat
On the points distributor, a condenser that's going, or points that are pitted or badly gapped, fail as the engine bay heats up. That's why it can run perfectly for half an hour or more and then start to hesitate. Even a new condenser can be faulty.
Fine for the first 35 minutes to two hours, then hesitation and misfire pulling away from a standstill, getting worse; normal at road speed.
Look at the contact faces for pitting and check the gap and dwell. When it plays up, freeze-spray the condenser, then the coil — if it clears, you've found it. Watch the tachometer during an episode.
Fit fresh points and a new condenser, and gap them carefully — the gap matters as much as the parts. Some prefer to convert to electronic ignition afterwards.
HT leads, coil and weak spark under load
A plug needs more voltage under acceleration, so a weak ignition shows up as a stumble pulling away and looks like a mixture fault. Carbon-core HT leads and oil-filled coils weaken with heat. A coil of the wrong resistance can also overload an electronic ignition.
Stumbles or misfires under load once warm; new plugs and leads help for a while and then it comes back hot.
Measure available voltage with a spark-gap tester — around 30 kV. Run the engine in the dark and look for sparks tracking from the leads to earth. On electronic ignition, check the coil is the 1 ohm type, not a 3 ohm points coil.
Worth trying: solid-core HT leads and a new coil of the correct type.
A worn distributor: wobbling shaft, vacuum unit and timing
When the distributor shaft wobbles in worn bushes, the vacuum unit can't hold the timing where it should be, and the engine stumbles as you pull away. Wrong plugs or plug gaps give a similar stumble.
Hesitates and stumbles pulling away when warm, about three times, then clears; new pump, filters, plugs and coil changed nothing.
Check the ignition timing, then grip the distributor shaft and feel for side play. Check the vacuum unit moves the plate. Make sure the plugs are the right type and gap.
Worth trying: rebush or replace the distributor and reset the timing.
Cuts out when coasting once warm: warm idle, the AED and the fuel cut-off switch
On the carburettor six, a warm idle set too low, carburettors out of balance or an AED that keeps richening when hot let the engine die as you lift off. A faulty fuel cut-off switch on the carburettor side of the block, or kinked float-bowl vent pipes, can do the same.
Happy from cold; once hot it dies as you lift off or slow down, then restarts straight away. With your foot holding the throttle it keeps going.
Check the warm idle speed and the carburettor balance. With the engine hot, see whether the AED is still over-fuelling. Check the fuel cut-off switch and that the float-bowl vent pipes aren't crimped.
Worth trying: set the warm idle and balance the carburettors; renew a faulty cut-off switch; free the vent pipes. A failing AED can be converted to a manual cable choke.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Note when it happens
Only once hot, only pulling away, or only when lifting off. Heat and load point to ignition; lifting off points to idle and fuel.
freeRead the plugs
All the same colour and burning evenly rules out mixture and carburettor tuning.
freePoints gap, dwell and the condenser
Look for pitting and check the gap. When it misfires, freeze-spray the condenser, then the coil.
freeLook for HT leaks in the dark
Engine running, lights off: sparks tracking to earth mean leads or cap. A spark-gap tester shows how much voltage is available.
freeTiming and distributor play
Check the timing, the vacuum unit and side play in the shaft.
freeWarm idle, carburettor balance and the AED
For a cut-out on the overrun.
freeCompression test
Throttle held open. All six within a few psi of each other — for example 145 to 150 — rules out the engine.
cheapWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 Series 2 Won't StartIf it won't start at all, or dies once the key leaves Start, start there.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.