Diesel heater

[sotheran57441]

2007 XJ6 tdvi 90000 miles.
The car is becoming increasingly reluctant to start, particularly in cold weather. The glow plug light on the dash illuminates only for a very few seconds before going out. It re-illuminates when the ignition is turned on again. How long should it be on for? Is it possibly not heating the system enough for it to allow the engine fire up and what may be the cause? There is no problem once the car has started.

[PaulGover]

If the glow plug warning light comes on before you turn the ignition on, it's a bulb test. It should come on when you turn the ignition on, and stay on for a few seconds, depending on the engine temperature. During this time, the car's passing something like 18A through each glow plug/heater to warm the cylinder (i.e. something like a 200W heater!). You should be able to start the engine immediately it goes out; I don't think there's any advantage waiting longer. If you switch the ignition off and then on again, I guess the plugs will heat up again, but for a shorter time.

To quote the Jaguar Workshop manual
"The glow plug control module (GPCM) is located in the engine bay on the right-hand side. It
acts as a switch for the current to the glow plugs and is controlled via the engine control
module (ECM). Above a given coolant temperature, glow plugs will not be switched on
before, during or after starting. Within a certain coolant temperature range, the glow plugs
will be active before cranking to increase combustion chamber temperature, during cranking
to assist engine starting, and after the engine has started in order to reduce the emissions of
white and possibility black smoke from the tail pipe immediately following a cold start."

I've had a problem with starting a warm engine - I think the glow heater decided it's not needed because the engine is warm, but it isn't warm enough; that doesn't seem to be your problem.

I'd guess you might have one or more failed glow plugs, wiring, fuse (there's one specifically for the glow plugs) or GPCM. If the module detects problems, it should store errors that an ODB-II reader can display. Most of the codes turn into tests to see if the wiring's OK, and if the plugs are OK (they should have a resistance under 10 Ohms). If the fuel is too cold, apparently the fuel system will keep cycling it until it warms up (I guess through conduction from the (heated) cylinder and fuel injector piping); it doesn't say how cold is too cold.