No fuel pressure

[Elliot W]

I went to move my truck in the driveway last week and it was acting like it was out of gas (I don’t have a working fuel gage). I didn’t need to move it anymore, but I put some gas in it and left it. This weekend I went to start it and no luck. No noise coming from the fuel tank led me to a no fuel pressure. Fuel fuse is good. Ground from sending unit is good. I have continuity from the relay to the sending unit, and the relay puts out 11.7v when I turn the key. But, it seems like the fuel relay isn’t clicking. I took out the relay next to it to make sure I could feel the click (or lack there of) so I was thinking maybe a bad relay, even though it sends 12v to the pump. I swapped the relays around and still no noise on the fuel relay but it puts out 12v as needed. I replaced the in tank pump with a walbro 255 thinking the old one was bad but it still isn’t coming on (I don’t think). Supply and return fuel lines to the pressure regulator are squishy where I don’t think it’s anywhere near 58psi, so even if the pump were quietly running I really don’t think it’s coming on. I’m not quite sure what to troubleshoot next. Here are the questions I have though:
1. Is the relay possibly causing this even though it’s sending the 12v but not clicking?
2. Can I bench test my old pump even though it might have residual fuel in it (is there risk of ignition)?
3. Should I test the fuel pump installed with direct wires to the battery to see if it comes on and is that safe?
4. What else should I troubleshoot?

This is a 5.3 LS with PSI Performance harness.

 

[Ricko1966]

Have you checked for 12v at the pump? Check for 12v to the pump. Use something that takes a little current like a headlight bulb or a horn. Not an led test lamp or multimeter.

 

[Elliot W]

Have you checked for 12v at the pump? Check for 12v to the pump. Use something that takes a little current like a headlight bulb or a horn. Not an led test or multimeter.

I tried checking voltage at the pump while the wife keyed the ignition but couldn’t get a good read. That’s why I did a continuity check of the wire from the relay to the pump. I’ll check voltage there today. Any idea how I could have a good wire but lose 12v from the relay to the pump?

Also, I took ground readings from the pump power wire. Is it normal for that to be grounded? I assume that’s how the pump motor works but could not fully sure.

Thank you for the response.

 

[Elliot W]

I'm getting 12V to the pump, confirmed this morning. So I'm thinking its either a bad power connection, a bad ground, or a bad ground. If its none of those, then this is the most quiet pump invented (along with its relay) and the lack of ignition is another problem all together.

 

[Hunter79764]

It is possible for a relay to pass 12V but no current to speak of, but if that happens, you wouldn't see 12v when connected to a load like the pump. When you say the pump has 12v, do you mean the connector when you unplug it, or can you probe the terminals with it plugged in and confirm that you keep 12v to it then? (That's what Rick is referring to with a test light that produces a load/current flow).
Bench testing the old pump is fine, just don't have a cigarette in your mouth when you do it. Extra step of caution would be to connect wires to the pump, then connect those wires to your power source a few feet away so that any spark is away from the fuel residue. That and/or putting the pump in a bucket of water, which will actually make the test more accurate since there will be a fluid load on it.

I'd try to get a fuel pressure gauge, that will take a fair amount of guesswork out of your troubleshooting. Doesn't need to be fancy, just something to see about what pressure you're making, just able to tell the difference between 50-something and Zero-something psi.

 

[Elliot W]

Huge thanks for the write up. When I just checked it this morning, I disconnected the pump and got the 12V. When I tried to get the reading on the wire while it was connected to the pump, I couldn't get a reading. Figured it was just a bad connection. I'll go check that again. Then bench test the old pump in water. What would make there be no current to the pump?

It is possible for a relay to pass 12V but no current to speak of, but if that happens, you wouldn't see 12v when connected to a load like the pump. When you say the pump has 12v, do you mean the connector when you unplug it, or can you probe the terminals with it plugged in and confirm that you keep 12v to it then? (That's what Rick is referring to with a test light that produces a load/current flow).
Bench testing the old pump is fine, just don't have a cigarette in your mouth when you do it. Extra step of caution would be to connect wires to the pump, then connect those wires to your power source a few feet away so that any spark is away from the fuel residue. That and/or putting the pump in a bucket of water, which will actually make the test more accurate since there will be a fluid load on it.

I'd try to get a fuel pressure gauge, that will take a fair amount of guesswork out of your troubleshooting. Doesn't need to be fancy, just something to see about what pressure you're making, just able to tell the difference between 50-something and Zero-something psi.

 

[Elliot W]

Put a multimeter on the power wire, not on the pump yet and its reading between 1-20 microA.

 

[Hunter79764]

Think of electricity like hydraulics. If you have a (mostly) plugged hose, you will see 1000 psi on both sides of it, until you try to move any fluid then pressure drops to nothing on the downstream side. With the pump unplugged, you have "pressure"/12v, but with the pump connected as a flow path you might see no voltage anymore (or so low it doesn't matter).
As to what causes it, your relay might be corroded or generally gunked up. When closed, you have a couple of tiny contact points that let voltage pass through, but as soon as you try to pass current (pump is plugged in), the voltage drops to basically nothing because there's too much resistance in that little contact spot. Corroded or broken wire can do similar. Try popping the case off the relay and look at the terminals, or just swap it with a known good one and see if you get clicking.

By the way, careful trying to get amperage readings with a multimeter. It is very easy to blow it up if you aren't familiar with the equipment. If you know how, awesome, but even then most meters can't do more than 10 amps and your pump can easily pull more than that.

 

[Ricko1966]

When I said check for 12v at the pump use a headlight as your test light. Did you do this? There is a reason I said to do it this way. Ever get in a car and the dash lights would come on but as soon as you turned the key to start you lost power.You didn't have a good enough connection to run more than the lights. That's why I said use a headlight as your test light,a lightning bug fart will light an LED or a multimeter. Put a load on the wires to the pump. Disconnect both wires to the pump connect one wire to each side of a headlight bulb,post your results. There are more technical ways of checking volts/amps to the pump,but I'm trying to make it simple..

 

[Elliot W]

Think of electricity like hydraulics. If you have a (mostly) plugged hose, you will see 1000 psi on both sides of it, until you try to move any fluid then pressure drops to nothing on the downstream side. With the pump unplugged, you have "pressure"/12v, but with the pump connected as a flow path you might see no voltage anymore (or so low it doesn't matter).
As to what causes it, your relay might be corroded or generally gunked up. When closed, you have a couple of tiny contact points that let voltage pass through, but as soon as you try to pass current (pump is plugged in), the voltage drops to basically nothing because there's too much resistance in that little contact spot. Corroded or broken wire can do similar. Try popping the case off the relay and look at the terminals, or just swap it with a known good one and see if you get clicking.

By the way, careful trying to get amperage readings with a multimeter. It is very easy to blow it up if you aren't familiar with the equipment. If you know how, awesome, but even then most meters can't do more than 10 amps and your pump can easily pull more than that.

Are you saying my cheapo Harbor Freight multimeter can't sustain a little current lol. Thanks for explaining the electrical differences with load/no load. That could easily be the problem...voltage at the terminal but none when it tries to start the pump. I swapped the 2 relays on the fuse block. The old fuel relay is clicking in the new spot, and the old, good relay is not clicking in the (new to it) fuel spot. Maybe something to do with the signal to the relay.

 

[Hunter79764]

Sounds like you have a signal issue. Backtrack from there and see what you find.

And yep, current checks with multimeter get scary sometimes. Easier to just look at voltage as an indicator of current, either voltmeter or just wiring in a headlight instead like Rick suggested. Same idea, different route.

 

[Ricko1966]

You can bypass a relay entirely to take that out of the equation. Safest way crimp 2 x 1/4 female connectors on 2 x 4 inch l9ng pieces of 16g wire. Push your 2 female connectors onto each end of a 20 Amp fuse. Strip 1/4 inch off the bare ends and stick them in the relay socket power and load terminals,usually 30 and 87 now that circuit is live until you remove the lead. Now if you have power at the circuit you are testing look at the relay and relay wiring as the problem,if not make sure you have power to terminal 30 and a good wire from 87 to your end load. For ground at your end load use the end load ground if you don't see power ground to frame that will ididentify a problem on the ground side.

 

[Elliot W]

Fixed! Thank you to everyone that chimed in with help! Turned out to be connectors at the relay, and possibly a burned connector to the fuse block. The continuous 12v to the relay was only getting power at ignition too, so I put that on a true constant 12v. When I tried to run it with only the fuel pump relay in, it wasn't getting a signal since the PCM needs the other relay to be powered too. Sorry for the confusion to everyone....just my faulty crimping skills.

 

[Hunter79764]

Good deal! Yep, those connections inside the fuse block can be tricky. You can probe them and they seem fine, but you install the relay and it just isn't making contact. A guy on another forum went through similar hassles with his fuel pump that actually ended up being the same thing, bad relay connection in the block.

Glad you got it sorted, now crank the AC and enjoy driving it again!

 

[Elliot W]

Looks like I'm back to square one on this issue as my truck won't start and I can't hear the fuel pump turn on. All fuses, relays, and wiring are good and making good contact. I measured voltage and amps at the fuel pump fuse and on the wire right before the pump, with both spots getting 12V and about 3A. I checked my batt to chassis ground, and my pump to chassis ground, both good. Then I took and old car battery to the pump leads and got it running. Are my amps too low? does anyone have an idea of something I haven't tried yet?