95 merc 7.4 mag mpi vst problem need help.
 

Need some help. Before I sink this dam thing. Thanks for the help in advance and sorry I’m keep coming here for help.


1995 Mercruiser 7.4 mag mpi

Does anyone know how the vst tank fuel system works?



My problem is im looking fuel pressure. When I turn key pump primes but pressure drops soon as pump stops priming. Is there a check valve after the pump, or something.



Things I’ve tryed.

Swapped injectors( both sets fuel pressure drops)



Swapped fuel regulators- still drops



Pulled off the return line and blocked it and fuel pressure dropped.



I replaced all the seals in the vst tank last summer because the top cover seal started to leak. So I swapped it all.



Only thing I can think of left is fuel pump being bad? I have no idea. The diagram shows all kinds of parts but only has 12 part numbers and none if witch helps me with info.

Do you have a spin on fuel filter? If so is it tight?

The only fuel filter that is there is before the vst tank. It’s the fuel separator that’s it. And that is new and it’s tight. From vst tank to the fuel rail there is no filter. Return line goes right back to the tank

Are you having a starting or running problem ?

Sounds like the pump. The pump builds pressure and should keep fuel from bleeding back. Fuel can bleed back but it should happen slowly

Yes I’ve had run issues. I’m going to keep it short

so last summer motor ran good all day then at end of the day it started to pop and wouldn’t get on plane. So after messing with it and I came down to the injectors were bad. So swapped them over winter time.(btw last summer I checked fuel pressure and it was good. But I never looked for it when primed just when running. If I did I forget)

this year first start up, started right up no problem ran good. Next day I started it to check all my gauges and give everything a run down. It started but shut off right away after 5 secs. Did it a few times in a row that’s why I came down to the fuel pressure issue. So i got regulator cause it was a cheaper route.


now to the seconds thing. I was thinking it was the fuel pump. After the regulator didn’t work. I just wasn’t sure if there was a check valve or something wrong. I’ll order a pump and go with it. No choice