5.7 hard start

[Chrisw87]

5.7 mercruiser tbi hard to start. Motor and everything on motor is new. I put a clear line from tank to fuel filter and seems like when you turn key on the pump runs the prime cycle and once it stops the fuel goes back in the tank is this correct? Once it’s started it runs fine

 

[stresspoint]

fuel should go to the injectors first then what is not used goes back to the tank ,
in short >the pressure regulator defines how much fuel goes to the return. if injectors are not spurting fuel then all the fuel gets returned to the tank.

 

[alldodge]

5.7 mercruiser tbi hard to start. Motor and everything on motor is new. I put a clear line from tank to fuel filter and seems like when you turn key on the pump runs the prime cycle and once it stops the fuel goes back in the tank is this correct? Once it’s started it runs fine

No and only because your tank fitting where the clear line is attached should be a antisiphon valve. The valve is a safety requirement and also helps keep fuel in the line

 

[Chrisw87]

injectors are new and all getting power just seems to me it’s not getting anough fuel on the start up prime. Read on another forum that some have a anti siphon valve at the tank

 

[Chrisw87]

No and only because your tank fitting where the clear line is attached should be a antisiphon valve. The valve is a safety requirement and also helps keep fuel in the line

Thanks that’s what I was thinking cause once it primes you can see the fuel in the line all run back in tank.

 

[alldodge]

EFI and MPI all are controlled the same by the ECM.
Key on and 12V applied to one side of the injectors. When motor starts cranking ECM looks for signal from the distributor that plugs are firing. Then ECM apply's a ground to other side of injectors so they fire

When cranking see if you can see the Tach moving up around 300 rpm

 

[Bondo]

Thanks that’s what I was thinking cause once it primes you can see the fuel in the line all run back in tank.

Ayuh,..... The hose barb at the tank is supposed to be an anti-siphon valve, aka: a check- valve,......

 

[dubs283]

Would also add that on mercruiser fuel systems the regulator does not allow fuel to return to the tank. The line returns fuel to the water separator housing

Boats that have a fuel pump in the tank (mainly ski/surf/wake boats) return fuel to the tank. Mercruiser has never used an in tank pump on their efi setups

+3 in regards to the anti siphon valve being in place and working proper. Not having an anti siphon valve can cause issues just as easily as having a faulty one in place

 

[Chrisw87]

Would also add that on mercruiser fuel systems the regulator does not allow fuel to return to the tank. The line returns fuel to the water separator housing

Boats that have a fuel pump in the tank (mainly ski/surf/wake boats) return fuel to the tank. Mercruiser has never used an in tank pump on their efi setups

+3 in regards to the anti siphon valve being in place and working proper. Not having an anti siphon valve can cause issues just as easily as having a faulty one in place

Yes fuel does return to water separator. Took the pickup hose out the tank and the brass elbow is hollow. I’m assuming if it was a anti siphon valve it would have something in it to keep the gas from freely flowing back to tank

 

[alldodge]

The AS valve is a barbed fitting connected to the tank pickup

 

[Chrisw87]

The AS valve is a barbed fitting connected to the tank pickup

What’s the inside look like

 

[Scott06]

.

What’s the inside look like

Spring with a ball

 

[Chrisw87]

Thanks I have one but there’s nothing in it

.
Spring with a ball

so I’m sure this is my problem

 

[Scott06]

Thanks I have one but there’s nothing in it

so I’m sure this is my problem

Takes a little suction to open prevents fuel from siphoning out of tank into bilge if their is a leak …

Cheap part to replace and rule out

 

[alldodge]

Post 10 has pic