• I have a 1996 Blazer with 4.3 motor.
• When the key is turned on, the fuel pump runs for 2 seconds, then off for 4 seconds then continues to cycle on/off.
• If key is turned to Acc it will cycle on/off
• You can also hear the pump cycle when the Truck is running.
• There are no codes.
• I need to know what could cause it to cycle like this. It did it about 2 years ago and quit by itself. Thanks Mark

 

Well as far as the pump being on when the truck is running....its supposed to.
With the key in the acc position its not supposed to run so no issues there.
But key on engine off I'd say its interesting that it cycles on and off.
How does the truck run? Smooth and even idle? Any hesitation during acceleration?
I would do a leak down and preasure test just to ensure the system operates normally.
Your looking for 60-60psi key on engine off. And no less than 55psi after ten mins. Idle pressure should be between 55 and 59psi.
Good luck and report back.
One more thing.........
If it aint broke, don't fix it!

 

I misread your post....
You are correct. The pump should not run when the key is in the acc position.
check and or replace the relays in the glove box. And then check the harness for shorts.

 

Relay was replaced and the problem is still there.
When Key turned to either On position or Acc position - Pump runs 2 seconds on, 4 seconds off, 2 seconds on, 4 seconds off, 2 seconds on, 4 seconds off, 2 seconds on, 4 seconds off, till the key is turned off. Runs the same way when engine is running.

 

Do you mean you hear the relay in glove box cycle with the motor running? If all the pressures are correct I would start looking for fuel pump wiring diagram that includes the relay. My guess would be ignition switch stating to fail but believe me I am not very good at this.

 

You can hear the pump itself cycling On 2 seconds - Off 4 seconds while the engine is running.
The ignition switch was replaced about a year ago for another problem.

 

check signal to pump relay and see if it to oscillates

 

Pulled the relay to check for power problems. Unplugged ECM connections, reconnected. Now pump works like it was suppose to. Until my problem returns - Thanks for your help Mark