PD150 Rough starter

Evening all.

I've had my leon mk1 for a year now, done 20k miles in it and it always starts within a few cranks.

Theres just a couple of bits that are niggling a bit.

As i say, it starts first time, but most of the time its rough for a few seconds like its running on 3 and smokey until it clears.

The EML comes on once in a while for a glow plug, its always either 1 or 3 but this afternoon i changed all 4 and it still did it so im thinking it may be the wiring loom. Where does the loom run from and to?

The fuel filter is new, i did that a month ago along with a full service and i have run a diesel purge fuel treatment which definitely perked things up a bit.

I have checked the cam for wear and its pristine.

Any advice would be great.

Thank you

Eventually il get around to yanking out the inlet manifold and cleaning it too its just motivation

Any idea where the point of failure is usually? The reason i ask is because i can make a new loom no problem using new connectors ect

I havent but i have found what looks to be the cause. the loom itself is fine but the clips that go on the top of the spark plugs were corroded. ive cleaned them up and the EML hasnt come on since