Hi ! 

I resently dropped my 2002 GU 4.2 in a bug hole and got the cold start timer wet. The pre heat light kept going on and off on my way home. Then hard to start for the next 4 days. Got a new timer fitted. (After market). Car was ok for two days but this morning it struggled to start, ran rough and blow white smoke until it warmed up. 

Is this a possible glow plug fail ?? 

G'day,

It sounds like glow system problem for sure, but you need tyo identify which part. 

Firstly check if the control system actually powers up the glow plugs, then depending on the results measure each plug's resistance or continuity individually. Resistance should be low,  around 0.6 Ohms, if open circuit or high resistance means failed plug. You'll need multimeter to do those checks.

If the system doesn't power up, check fuse or relay for power supply first.

TD42 control system (timer) uses positive signal to actuate the glow relay.

Honestly I wouldn't even bother with timer personally I'd just wire up momentary push ON switch to control the glow relay myself for those 5-10 secs before cold start. 

Regards