I had same problem during recent weeks
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Having New long blocks , twin 4.3 Volvo Penta - fuel injected
New long blocks are basically new engines using old external parts ,but on mine installed new alternator , distributor , starter . using older working items is prone too issues but was lot cheaper then new crate
I am in shake down with 10 hours since install and expecting issues
2004 Regal Commodore 2860
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Fuel pumps replaced 2 years ago out of warranty, ( recent weeks pumps not quiet, hearing wining sound )
Fuel pump wining is an indication of weak fuel pump cell , high or low ?
My Issue is port engine .
Engine is sensitive comparted too starboard, any water hitting engine , becomes hard starting
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Few weeks ago cold start , port would not start ? Not smelling gas after hitting throttle many times ,.
Starboard fired
My set up ( 1 ) 100 gallon fuel tank feeding 2 engines . Each engine own external GREAT VOLVO PENTA
electric pumps
Cranked a few times, no go.
Sprayed engine start down throttle body .
Fired up , fine all day ( just like U )
My thinking , loosing pressure and fuel is returning into gas filter on that engine
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Returning too boat few days later, Port engine , would not start .
Tried something different
I primed pumps , turned key not starting , hearing fuel pumps . Did this 5 times, on 6th time, cranked almost started .
On # 7 it started
I sprayed a AMZOIL product , coats distributor and wiring with a film too stop moisture, .
What ever I did worked . Now starts first turn . Both engines cold stall unless gas is given few minutes then idles fine
Don't believe this AMZOIL product corrected anything , just pump are holding pressure then loosing . Filter tight , lines tight and clamped
If I have issue again will put a meter on fuel pump . There is a fitting like a bike air fill on pump and another I believe near distributor cap , check psi , I believe it should be around 50 ( not 100 % have too check on psi )
GOOD LUCK