Ive been stumped. Im working on a friend of mines 98 chevy blazer 4.3L 4x4 , and it just started not wanting to start on occasion. It has no codes and no check engine light. It also has great spark and fuel pressure, but the thing that has got me stumped is if we hook up a 200 amp quick start battery charger to it, it fires right up. I was thinking bad battery but we swapped my brand new yellowtop Optima into it and still just cranks and cranks no start. It will only start with the 200 amp jumper on it. Maybe the computer is going out, or weak sensor voltages. It also has no vacum leaks. Any ideas, it seems ive reached a dead end, any help will be greatly appreciated. THANK YOU.

 

Check for bad electrical connections. Loose or corroded grounds could cause that problem also.

 

whats the cap and rotor like,has it had a recent tune up.could also be a weak ignition coil.does the starter crank slower than normal, when you boost it, it will crank over quicker plus the extra boost will allow the engine to start.could be a bad ground as memtioned also.