hello. i have an 89 full size blazer with 5.7. i have replaced the fuel tank, pump, relay, filter, flushed the lines and after a while the pump starts to fail. or so it seems. i replace it and drive a few hundred miles or so and then it starts to lose power. i shut the key off and restart the truck. it runs good for a few miles. it has done this several times. i have replaced the pump 4 times any help would be greatly appreciated

 

Given your symptoms and the fact that the truck is over 30 years old, I am suspecting a wiring issue rather than a component issue. A poor ground or pinched wire can cause the symptoms you are experiencing.

I would inspect/test the wiring to the pump as well as the relay. You will be testing the +12V circuit integrity, relay signal circuit integrity, as well as the grounds for both the relay and the pump for proper conductivity to ground. You will need a wiring diagram to know what and where to test. I do not have a wiring diagram handy at the moment, but I will look later today for one if you need it.

*EDIT* - I found a link to a complete 1989 wiring manual for you --> 1989 Chevy R,V,G,P Model Wiring Diagrams

Along with the:
Service Manual --> 1989 Chevy R,V,G,P Model Service Manual
Unit Repair Manual --> 1989 Chevy Light Truck Unit Repair Manual

Grab them while you can!

 

I have no direct experience with your issue but thought I'd lend support to the wiring/electrical issue being a possible cause.

I discovered that my original 24 y.o. fuel pump relay had contacts that were pretty burned up. Since replacing it, my rare crank-but-no-run condition ceased.