The 300SD has been a bit tough to start and a little rough idle for the first couple seconds on cold mornings. GP light comes on as usual. Except for this morning.

I went out to start and GP light came on but car would not start. Just cranked and cranked not even firing on one of the cylinders. After about 5 minutes and quite a few tries, the car started and idled very rough with a lot of smoke. I got home and this afternoon went to start the car again. No GP light at all.

I had a spare controller and swapped it to no avail. I installed new GP a few years ago. So now I need to troubleshoot but I cannot find anything in the DIY section. I also searched but came up with no straightforward answers. How do I test the GP and how do I test the controller? Is there a fuse for the system? Is my car different being an '85?

Thanks

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R