US1 Fountain

iTrader: (1)

Currently doing the return line from fuel/water separator housing to gas tank modification on my stock '99 502's to rid the huge vapor lock issue.

I'm going to tee the return lines together so that I may run a single hose up to my gas fill hose and tee into it, vs running 2 separate hoses up. (No provisions at tank, nor sending unit return port option possible, gotta go to the fill hose)


I have no idea on how much fuel flows thru the return lines. Plan is to use 3/8" ID hoses. Will a single 3/8" hose handle the return flow of both motors for the 13' of hose? Don't want too small to where too much restriction effects the fuel regulators, nor any larger hose than needed.
Thanks

buck35

I would think if you had 2 3/8 you wouldn't want to less than half or 5/8 for the long run to the filler.

Young Performance

One single 1/2" hose will do just fine. You can tee your (2) 3/8" hoses into (1) 1/2" hose.
Eddie

Trash

Do you see any fuel/liquid percolating from the yellow VST lines that lead into the manifold or throttle body? Sometimes the VST diaphragm will decay and allow fuel to bleed through and into the manifold. The symptoms will be very similar to vapor lock. I recently had my diaphragm go bad and thought it was vapor lock. Instead it was bleeding fuel across the VST diaphragm and vented into the throttle body. Going to flood clear mode got the motor started.

I realize that heat induced vapor lock is likely the culprit for the 502 motors, but this is also another likely scenario that can be readily observed.

US1 Fountain

iTrader: (1)

Haven't noticed any fuel in the yellow lines. Will look closer.

I'll go with the 1/2" single line


Thanks

BUP

iTrader: (9)

Says he has stock 1999 year models - the death of that poorly design VST was at the end of 1996. A few 1997 boats still had it but most of 1997 models and up using the 454 and 502's did not have a VST anymore - they all went to Gen II cool fuel system that was mounted underside of the engine. That VST set up was junk.

Mercruiser had a couple of SB's how to combat vapor lock. One was if your fuel injection system did not have a boost fuel pump then there was a kit out to add it to help prevent VL.. I understand this alone by adding the BF Pump did not cure VL 100%.

buck35

Probably a dumb question, but what exactly do you use to maintain pressure in the fuel rail with the return line to tank.

US1 Fountain

iTrader: (1)

The return line just goes to the filter head and then the fuel is reintroduced into the system. All fuel from the tank, thru the filter and to the pump is being pulled by pump, so no pressure in the return line, if I understand it correctly.

Don't understand how running the return line back to the tank is any different than to the filter head, but 9 out of 10 dentist surveyed says it works.


Correct, no VST system.

buck35

Misunderstood that part sorry,
Happy vapor lock free boating

BUP

iTrader: (9)

The fuel pressure regulator persay regulates the fuel to the injectors that is needed any excess fuel the regulator bypasses it thru then thru the return line back to the fuel filter head.

I will say this the boats that have the electric fuel pump mounted inside the gas tank never have problems with vapor lock. One the fuel is cooler thru the system, the electric fuel pump is better pushing fuel rather than pulling it - so this alone helps with fuel cavitation hence really helps with vapor lock.

Also the electric fuel pump stays a whole lot cooler being mounted inside the gas tank especially after engine shut down. It does not heat soak being mounted inside the gas tank.

Mercruiser or Volvo has never mounted a electric fuel pump inside the gas tank. I believe Ilmor is or wants to. OEM engine manu's and boat builders have to work with the coast guard on the design of it. Mastercraft, Indmar does this on a lot of their boat installs.