1990 vw jetta deisel hard to start when cold?

my vw jetta deisel is hard to start when cold,if i plug it in fo an hour before i try starting it in the morning it fires right up but if i dont plug lt in it takes about 5 minutes of cranking to get it to start . has all new glow plugs,could it be a relay fuse that is weak causing the glow plugs not to preheat... show more my vw jetta deisel is hard to start when cold,if i plug it in fo an hour before i try starting it in the morning it fires right up but if i dont plug lt in it takes about 5 minutes of cranking to get it to start . has all new glow plugs,could it be a relay fuse that is weak causing the glow plugs not to preheat long enough or does anyone know what else could cause this problem as once it has been started it starts right up when engine is warm
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