1987 Ranger

I have a 87 Ranger that has a bad fuel pump and was wondering if I could install an after market one and leave the old one in the tank?

That would depend on what is bad about the old pump. If you hook up an inline pump on a fuel supply line and the one in the tank is clogged or valves wont open you may still have to go after it anyway. I'm not talking from experience though.

Jai

jai is right but why in the hell would you want to half a## it like that in the first place