No Fuel Pressure

[aggiekid]

I rebuilt a mercruiser 5.0 MPI MAG engine on a 2008 Tahoe Q7i. Unfortunately, I couldn't test the engine before placing it in the boat. I believe I got all the wiring correct, and hoses hooked up to the correct spot....except one. Attached is the highlighted connection that I haven't been able to connect yet.

Also, unsure if lack of this connection would result in no fuel pressure. Any help is much appreciated.

 

[alldodge]

The hose goes to the regulator on the cool fuel module. Having it not connected will not stop the fuel pump from running

What is the motor serial number?

Does the motor crank (turn over)?

 

[aggiekid]

The serial number is -1a316544. The motor does crank. A few other things I did, and not sure if that impacts the fuel issue, is this boat sat for 10 years, so I emptied the fuel tank, and replaced with fresh fuel. Also, change the fuel water separator. There is fuel in the tank, but after trying to start the engine several times, I couldn't even find fuel in the fuel water separator.

 

[alldodge]

couldn't even find fuel in the fuel water separator.

There is a barbed fitting on the tank which the rubber fuel lines connects. This is an antisiphon valve. The valve s a USCG safety requirement but also over time and sitting they can get stuck. Remove it and clean the valve if it can be, otherwise it may need to be replaced


Can also get a can of gas and place the gas line into the can to get it running for now.

While its not a great thing to do because of the possibilities of getting dirt in the injectors, you need to prefill the fuel filter with gas

Place a rag around the TB fuel line connection and loosen just slightly. Were safety glasses and turn key ON/OFF a few times to see if you can get fuel up the line

 

[alldodge]

I don't want to clog the forum with my stupid questions ha ! I did prep the fuel water separator with fuel, plugged it, and still didn't get any fuel.

That's what the forum is for, so we can all learn for each other. Questions are best here

The clogged valve could explain the problem, so I need to remove the valve, and try it which I will do it today. I also wanted to ask if I should expect any pressure in the fuel line if the fuel pump is working but unable to pull any fuel. What i am trying to determine is if my fuel pump is even working. Hope my question makes sense.

If the pump is unable to pull fuel it will not build pressure. The valve if clogged will keep the pump from pulling fuel. Also it can also build air pressure in the line between the pump and throttle body (TB). This is why I mentioned loosening the fuel line on the TB just a bit to let air out

The easiest way to test is with a gas can. Use a length of gas line and just stick it inside the can

 

[aggiekid]

I removed the first line from the fuel water separator, and placed it in a container with gas. Tried to start the engine, and still nothing. I loosen the fuel line as well to let air out.

 

[alldodge]

You have relays 3 relays on top the motor next to the ECM. They are:
Main Power - MPR
Fuel pump - FP
Slave (start assist)

Next to the relays is also 3 fuses. Find the fuses and check they are not blown.

Find the relay with Pink/Green, Green, Pink and Pink/Yellow wires, this is the FP relay. Remove the relay, turn key ON and measure voltage at pin 30 of the relay. There should be 12V

If there is 12V, use a small jumper wire and connect pin 30 to pin 87. When this is done the pump should run full time, so long as jumper is in place and key ON

With pump running see if you can get fuel to the TB

 

[aggiekid]

Here is the image of the fuse, and the fuse connected to the voltmeter. I am testing the voltage at fuse 30, and I dont see anything. There is 12V at 87A/30A/40A/87 side.

 

[alldodge]

There is 12V at 87A/30A/40A/87 side.

If you had 12V on 87 the fuel pump would be running unless the ground was bad on the pump or the pump was bad. Pin 87 will only have power when the relay is OFF or if the relay is shorted internally

This is the reason I said if there was 12V on pin 30 to jump pins 30 and 87 (this directly powers the pump)

Pin 86 should go from 12V to zero for 2 seconds when the key is first turned ON. This is the ECM applying a ground to 86 to energize the coil and power the pump. After the ground is removed then 12V will be seen

I want to hot wire the fuel pump to see if it starts and runs. If it does then we know its either the relay, ECM or a bad connection.

 

[aggiekid]

I connected pin 30 to pin 87, and nothing happens. One Pin 86, it at zero with ignition off, and goes to 12v, and stays at 12v when the key is ON.

 

[alldodge]

To recap and to make sure we are talking about the fuel pump relay and not main power relay

When key is turned ON there is 12V on FP relay pin 30 (check)
When jumper is placed between pin 30 and 87 nothing happens (the FP does not run) (check)

Check that the FP ground connection is good.
If it is good, disconnect the plug to the pump and connect power to it to see if it will run direct.
If nothing works its the pump

 

[aggiekid]

Yes, this is for the fuel pump relay. When the key is turned on, I don't see 12V in FP relay PIN 30. I see 12V in PIN 87. When I connect PIN 87 and PIN 30, nothing happens.

How do I check FP ground connection is good. Also, can i just connect the fuel pump directly to the battery to check if it will run directly ?

 

[alldodge]

I don't see 12V in FP relay PIN 30. I see 12V in PIN 87.

What color wire is on pin 30?

 

[aggiekid]

Its Pink yellow.

 

[aggiekid]

also, when I was taking the old engine apart, a 15amp fuse fell from somewhere on the engine. I checked the only fuse box on the engine which has 3 20 amp fuse, and one 15amp. I tried to see if there are any other fuses, and I dont see it. I also hear a click around the fuel pump when I start the engine. Obviously dont see any fuel coming in the injectors yet.

 

[alldodge]

Its Pink yellow.

ok then the 30 and 87 where swapped at the factory making the harness. No issue it works the same

Unplug the fuel pump, jump the connections and see if you get power at the connector. Use both meter probes at the fuel pump connector. If power shows up then the pump is the issue.

If you don't get 12V at the connector, put meter negative probe on a good ground and see if 12V show up on one of the pins.
If it does then the ground side is bad, if it doesn't the hot side is bad

 

[aggiekid]

I ordered a fuel pump from Amazon, and connected it to the engine. As soon as I start the ignition, the pump starts running for a bit. I don't see the same happening with the old pump.

Seems like an issue with the pump ?

 

[alldodge]

As soon as I start the ignition, the pump starts running for a bit. I don't see the same happening with the old pump.

Seems like an issue with the pump ?

Agree, sounds like a pump issue