1991 GMC 1500 Pickup Won't Start

Purchase a good repair manual and use the diagnostic information to diagnose the problem. There are good online manuals (alldatadiy.com, chiltondiy.com) that you can get easily and quickly, read up on how to diagnose this type of problem.

Injectors will only pulse when the engine is spinning. You need to test for this pulse at the injector and check the resistance of the injector itself.