Fuel pump not keeping up

[sam60]

454 with new fuel pump and rebuilt Holley 750 CFM is running fine until you get it over 1500 or so RPM. The fuel pump voltage should be 12V +/- .1 per the service manual and I am only getting 8.5-9.0V. The fuel pump relay is powered by the "light" terminal on the alternator and the "S" terminal on the assist solenoid. There are diodes in each of these leads to the fuel pump relay hidden in the harness.

Here's the voltages that I'm getting during cranking-

8.5 volts at fuel pump -- should be 12V

2.3 volts at light terminal from alternator -- should be 13.9 to 14.7V

Am I correct in assuming that I need an alternator?

I have also ordered a diode kit before they become obsolete, as I can't find the microfarad or whatever those are rated at.

Thanks for your advice.

 

[sam60]

Re: Fuel pump not keeping up

I went and had the alternator rebuilt today as it tested at 35 amp output @ 3,000 rpm and should be 51 amps. This did not make any difference in the pump voltage. I checked the fuel pick-up screen, tank vent and all that is left in the factory service manual is the diodes, relay and relay base. Those will all be here Thursday.

Has anyone else had a problem with the electric fuel pump dyeing above idle? I Don't get it as the pump, engine and carb are all fresh.

 

[jjacobs007]

Re: Fuel pump not keeping up

I feel your pain brother,your info is confusing to me,start at the beginning please,and ill try to help.

 

[Don S]

Re: Fuel pump not keeping up

8.5 volts at fuel pump -- should be 12V

Are you still getting this 8.5 volts since the alternator was fixed?

 

[NetDoc]

Re: Fuel pump not keeping up

Are you still getting this 8.5 volts since the alternator was fixed?

That's the crucial question. If so, start there and work backwards. Find the connection or device responsible for the voltage drop by using sequential isolation.

FWIW, did you fully charge BOTH batteries before you re-installed and started your boat? Did you check to see what resting voltage your batteries have (test them independently). Running a perfectly good alternator with sub par batteries can give you erroneous results all over the place.

 

[Don S]

Re: Fuel pump not keeping up

Let's do some troubleshooting, in the picture below you see your fuel pump wiring. First, remove the fuel pump relay so you can get to the socket.
1. Ignition switch OFF With your DVM, check the battery voltage (write it down) so we have a baseline number to start with.
2. With your meters black lead on the battery negative terminal, check the voltage at the 30 terminal of the socket. what is it. Should be very close to battery voltage.
3. If voltage in item 2 is ok, move the black test lead to the 86 terminal with the red test lead on the 30 terminal. Voltage should be the same as in 2.
4. Disconnect the fuel pump connector from the pump. Make a jumper wire with 2 male spade terminals on it so you can jump across the terminals of the fuel pump relay socket. With the pump disconnected, with your jumper you just made, jump across the 30 to the 87 terminal. Now, with your meter you should show the voltage you had in step 2 across the 2 terminals of the fuel pump connector. Do you?

 

[sam60]

Re: Fuel pump not keeping up

Are you still getting this 8.5 volts since the alternator was fixed?

Yes, still 8.5 volts after. But I found another manual for the same that says the voltage during cranking is supplied solely by the assist solenoid and switches to the alternator power during run. I'm going to trace the voltage point to point back down the harness next and also try a dedicated ground wire to see if the ground is the issue. Thanks

 

[sam60]

Re: Fuel pump not keeping up

Let's do some troubleshooting,

Thanks Don. I will do this tonight when I get home!

 

[sam60]

Re: Fuel pump not keeping up

FWIW, did you fully charge BOTH batteries before you re-installed and started your boat?

There are charged and with the cables off, 12.7V each.

 

[Don S]

Re: Fuel pump not keeping up

If you are having problems with it running over 1500 rpm, then why are you worrying about voltages while cranking? They will always be lower.

If the engine is starting, and you have no problems getting it started, then forget the cranking voltage tests. Until the engine is running, the voltage test on the green wire at the alternator is a meaningless test. The alternator is not working while cranking.
You need to be doing fuel pump pressure tests, not voltage tests.

 

[NetDoc]

Re: Fuel pump not keeping up

This is the most basic of relays out there. Its quite common for heavy current to degrade the connectors. As you are removing the relay inspect those terminal connectors for corrosion and the wiring for swelling. These could be easy visual clues to what's gone south.

 

[Don S]

Re: Fuel pump not keeping up

This is the most basic of relays out there. Its quite common for heavy current to degrade the connectors. As you are removing the relay inspect those terminal connectors for corrosion and the wiring for swelling. These could be easy visual clues to what's gone south.

He doesn't have heavy current, 3-4 amps max, on a 30A relay. He's checking the voltage while cranking and expecting that to be what is happening when the engine is running, and it's not even close.

 

[NetDoc]

Re: Fuel pump not keeping up

He doesn't have heavy current, 3-4 amps max, on a 30A relay. He's checking the voltage while cranking and expecting that to be what is happening when the engine is running, and it's not even close.

I fully agree with your assertion that he's chasing rabbits looking at cranking volts. Still, the voltage drop has to come from somewhere and dirty/corroded terminals are pretty common culprits. It only makes sense to visually inspect them as he removes the relay in order to test for voltage. As simple as that set up is, it's probably going to be the relay or one of the connectors.

 

[Don S]

Re: Fuel pump not keeping up

The Low voltage he measured is not his over 1500 rpm running problem.
Watch what happens with some troubleshooting instead of just random searching.

 

[NetDoc]

Re: Fuel pump not keeping up

The Low voltage he measured is not his over 1500 rpm running problem.
Watch what happens with some troubleshooting instead of just random searching.

Hmnnnn, in spite of the low running voltage? Or, did I miss something here? I have seen bad hoses before the pump cause a number of issues, and boats have a huge deal with venting. I am just now beginning to understand the relationship.

 

[sam60]

Re: Fuel pump not keeping up

Let's do some troubleshooting, in the picture below you see your fuel pump wiring. First, remove the fuel pump relay so you can get to the socket.

Thanks all for the help. Don, here are the results:

1. 12.7V
2. (cranking) 8.7V
3. & 4. same of course.

Should I make a pigtail from the + side of the fuel pump connector to see what the voltage from the alt is when running?

Thanks,

Mike

 

[Don S]

Re: Fuel pump not keeping up

2. (cranking) 8.7V
3. & 4. same of course.

Where did I ask for voltages while cranking. All the tests I asked for were with the Key OFF

Should I make a pigtail from the + side of the fuel pump connector to see what the voltage from the alt is when running?

Re-read item 4 on the troubleshooting reply.

 

[sam60]

Re: Fuel pump not keeping up

Key off + 0 volts

 

[sam60]

Re: Fuel pump not keeping up

I am borrowing a nice fuel pressure tester tomorrow that will allow me to test the pressure while running. If the pressure drops off as it runs, I'm draining the the tank again with the trailer positioned up high at the tongue and tilted toward the pickup side so I can get a suction hose into the corner of the tank. The fuel lines and filter are new, the vent checks fine and the pickup tube screen is clean.

If the pressure stays constant, it's tear into the carb again.

The alternator output was 14.7 volts to the pump when running tonight.

Not sure what else to do, and I appreciate all of the help from everyone one.

 

[sam60]

Re: Fuel pump not keeping up

If you are having problems with it running over 1500 rpm, then why are you worrying about voltages while cranking? They will always be lower.

If the engine is starting, and you have no problems getting it started, then forget the cranking voltage tests. Until the engine is running, the voltage test on the green wire at the alternator is a meaningless test. The alternator is not working while cranking.
You need to be doing fuel pump pressure tests, not voltage tests.

You are absolutely correct. The cranking voltage from the alt. doesn't mean a thing. The factory service manual tells me to test the that while cranking when it should say when running. The low voltage while cranking at the fuel pump connector is also mute as it it starts as you mentioned. Fuel pressure tests in order. Thank you sir.