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Full Version Single very clean spark plug....

Club XM Forum > Petrol Specific Issues
Mysteries... odd problems.. yep that's XMs.

The TCT won't idle properly on LPG, it does so on petrol just fine.
Pulling the plugs ( and you need to change them on LPG systems ) to replace them with new iridium LPG specific ones... I was stunned that one of the 4 plugs was utterly clean. The others had that nice 'running fine' colour, but one looked new. Almost a double take - did I confuse it with a new one - clean.

So, I found myself thinking head gasket or valve.
I asked Trev ( mechanic) to do a leak down and pressure test. He tells me that the compression is pretty much the same in all 4 cylinders, that the leak down didn't show anything amiss with the valves or rings.

A test of the coolant shows no contamination with exhaust gases. No telltale 'margarine' in the filler cap.

We're at a loss at present...
An air leak?
Faulty LPG injector or a wiring fault leading to petrol and LPG injectors working at same time ( not checked this )

Any ideas?


If the plug looks brand new with no combustion deposits it suggests that there's no combustion going on. This can either be because there's no gas being admitted or there is no spark to ignite it.

Can you smell the exhaust? Can you smell the odour of unburned gas? I suspect not. Therefore the most likely culprit is that the gas injector on that cylinder is not opening.

Please let us know how you get/got on.


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SO yeah it was a head gasket.

I suspect that the poor running wasn't helped by the failing alternator either.
You imply that the head gasket was at fault but a compression test was OK.

What was the cause of the initial problem.
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