Testing Fuel Pump

jzd · Post by **jzd** » 31 Dec 2023, 12:49

98 s70 GLT - Codes and symptoms are of a failing fuel pump. Fuel pump (purchased from FCP) was replaced around 5k miles ago. Picked up another one from FCP. Installed and not getting fuel pressure at rail. Battery, fuse and relay all test good. Replacement pump may be bad, but I am also concerned about getting voltage at the pump.
1. How do I test the fuel pump while it is outside the car?
2. Do I test the voltage at the pump with a voltmeter and by turning the key to position 2?

rrres · Post by **rrres** » 10 Jan 2024, 11:36

jzd wrote: ↑31 Dec 2023, 12:49 98 s70 GLT - Codes and symptoms are of a failing fuel pump. Fuel pump (purchased from FCP) was replaced around 5k miles ago. Picked up another one from FCP. Installed and not getting fuel pressure at rail. Battery, fuse and relay all test good. Replacement pump may be bad, but I am also concerned about getting voltage at the pump.
1. How do I test the fuel pump while it is outside the car?
2. Do I test the voltage at the pump with a voltmeter and by turning the key to position 2?

Mind you, this was on an AWD pump, I just jumped the pump's +- harness to the battery. Some say not to dip the pump in water but that's what I did to see if there was good flow. I just ran it for a few seconds, just enough to determine if there were obvious flow problems or leaking hoses or pipes.

Post by **abscate** » 10 Jan 2024, 18:30

The pump only runs for a second or two, so you need an assistant to turn the key. You can also jumper wire the relay at the front to test it

Post by **scot850** » 10 Jan 2024, 18:36

Can you hear the pump when you switch it on at the ignition to Pos II? Best to do this when the car has stood for a few hours to let some pressure bleed off. Get someone to turn the key for yo while you stand at the fuel filler with the cap off to try to hear the pump run, even for a few seconds.

I am not sure I am clear on your description, but are you saying you are getting no pressure at the fuel rail on pressing the Schraeder valve?

As Abscate says, you can also jumper the fuel pump relay.

Neil.

Post by **erikv11** » 11 Jan 2024, 09:39

jzd wrote: ↑31 Dec 2023, 12:49 98 s70 GLT - Codes and symptoms are of a failing fuel pump. Fuel pump (purchased from FCP) was replaced around 5k miles ago. Picked up another one from FCP. Installed and not getting fuel pressure at rail. Battery, fuse and relay all test good. Replacement pump may be bad, but I am also concerned about getting voltage at the pump.
1. How do I test the fuel pump while it is outside the car?
2. Do I test the voltage at the pump with a voltmeter and by turning the key to position 2?

I just echo what they said:

1. Just apply 12V across the two terminals, like from a car battery. The pump should vibrate and spin right up, kind if jump in your hand. Only do this briefly as the pump uses fuel to cool while it runs. Running a pump without fuel in the tank can cook and kill it.

2. Yes, but the pulse is brief as others describe. Use long leads from the voltmeter or have someone else turn the key while you are back at the pump.

The problem sounds like wiring so far but check it out. Best way to test the relay is to (a) take it out and jumper it or (b) swap in a known, working one.