a. Fuel temperature (at the engine’s fuel inlet fitting) and the amount of vacuum
required by the fuel pump to draw the fuel from the boat’s fuel tank can contribute
to vapor locking.
Mercury MerCruiser’s maximum engine fuel temperature specification that became
effective January 1, 1996 is:
Under the hottest outside air temperature condition that the boat will be operated
in, the temperature of the fuel being supplied to the engine shall not exceed 110o F
(43o C) at any location between the fuel tank and the engine’s fuel pump.
Mercury MerCruiser’s specification for the maximum vacuum measured at the fuel
inlet of any MerCruiser engine is:
2 in. Hg (7 kPa) maximum at idle rpm, 3000, full throttle and back at idle rpm.
Use an accurate digital vacuum gauge that reads in either in. Hg (inches of mercury)
or (kPa) to check this specification. Common vacuum gauges to check an engine
intake manifold vacuum are not accurate enough to make this type of measurement.
b. Reducing the temperature and maximum vacuum of the fuel being supplied to the
engine will help reduce vapor locking problems.
NOTE: Carbureted and EFI/MPI with VST models only: The Water Separating Fuel Filter
can be removed from the engine to a lower, cooler location. Use a Coast Guard approved
fuel line between the filter and the fuel pump

If you are saying this is only a preliminary starting issue then what i would do is hook up a psi gauge on the fuel rail...prime the system up and see if it hold the psi...if its dropping off then you either have a leaky injector/s or possibly a bad fuel pressure regulator.....

Yes this seems to only be a starting issue. I did fuel pressure measurements with the appropriate injector gauge and all seemed relatively normal. The thing im not sure of is that i was getting a lot of air in the bleed line when i drew fuel off of the gauge at the fuel rail. That seems like a problem. Seems like no matter how many times i cycled the fuel pump to move air out of the system i was still getting a good amount of large air bubbles off of that bleed line.

Thanks SB for the tech information. I already measured the vacuum and it was about 2in Hg. so that seems normal

Has anyone relocated their Fuel Filter lower in their bilge? How did you do it? Any special brackets you used? Did it help with vapor lock?

I added a Procharger to mine. Just did a complete Aeromotive system. Burnt up two pumps in two weeks. After that I learned the right way to plumb an Aeromotive pump and had many trouble free hours. I did the same thing on a customers 500's that was having fuel pump issues. Changed thwm over to Aeromotive pumps and regulators with returns. Also put larger pickups in the tanks. Setup was trouble free for years. And what Mercury gets for a new pump I would just convert them...

Im not talking about fuel pressure while running....when you first energize the system the pump should turn on for a few seconds and pressurize the fuel rail to a set amount for starting...although im not 100% sure if the 500EFI's do this as they do have a mechanical pump as well.....either way fuel pressure should hold steady before starting...and should be holding after running too....if its dropping then you have a leak in the system and its bleeding down.....my guess is the regulator....but it could just as easily be an injector dripping.
I would take the injectors out and have them cleaned and balanced....Talk to BUP about that ;)
And since the manifold would be apart at this point just replace the regulator while in there. :)

Look here at part#19
http://www.mercruiserparts.com/bam/s...31335/4889/110

Also just out of curiosity.....your not getting an "fuel sheen" on the water behind the boat as it runs do you?? That to me would signal that the fuel cooler has a leak and dumping fuel into the raw water system and could be your leaking point...

Here in texas, in the summer , the carbed direct drive ski boat guys seem to get the vapor lock blues first.
Dumping ice water on the intake and fuel lines to get it started again really kills thier mojo.
The local marina fuel at double the cost of road side pump gas is much better and swaps to winter fuel better. Todays ethanol gas is worse than ever. Fuel under vacuum near hot exhaust power steering pumps ect will start to boil almost at ambient if its hot enough outside. Next time it happens hose down the fuel system with ice water all the way to where the lines go to the tank or undsr the floor and see if it makes a instant difference

Merc recommends 2 hg MAX for Vacuum testing if you are higher than that - you possible can have a restriction(s) or air leaking into the fuel system or even tired fuel pumps.

If your fuel pressure will not stay steady - you have a restriction or a voltage problem or a fuel regulator problem

if you have fuel pressure that drops - that can be weak fuel pumps or leaks or o ring gasket seal at the injectors or the injector(s).

And yes I do test and flow test injectors and clean them professionally. , 8 years coming up on and only for marine apps.

It is becoming recommended maintenance by the marine OEMs to have the injectors looked at every 4 to 7 years or sooner if fuel issues can not be traced down. And if you have water in your fuel and or rust especially rusted internal fuel lines./ fuel rails.. Actually on 496's that is common more than you think (internal corrosion in the fuel rails) . Believe me when I say this. I see it almost weekly during boating months.

Also another big cause for Vapor Lock is RVP ( Reid Vapor Pressure ) of the fuel. This also is a big problem as well. Search for it up here as I have made many posts about. Years ago all the OEM marine schools would talk about this all the time --- RVP.

Instead of writing this out read the SB from Mercruiser. Vapor lock has been a problem in boats way before 1999 per Service Bulletin aka SB.

http://www.boatfix.com/merc/Bullet/99/99_07.pdf

And basically the same info

https://www.perfprotech.com/blog/art...ser-vapor-lock

Engine compartment temps matter --- incoming air and venting especially venting heat soak after engine shut down. Running the blower can help as well and proper cooling of your engine(s)

Have tried idling, keeping the hatch open, running blower at all times. None have helped. Just ordered the merc boost pump kit. Every post i have seen where someone installed, that seemed to have solved all of their issues.

I will repost here with my results so that others have another data point to go from.

I assume that installing the booster pump increases the pressure on the supply side to the high pressure pump (through the water seperator) to prevent the fuel from boiling At a minimum it should put a pump in a spot where non-boiled fuel can be picked up and pumped to prime the high pressure pump in the case that vapor lock does occur.

I am keeping my fingers crossed... I hope the kit comes with instructions on how to install. I have not been able to find any instructions anywhere online.