Port side, pump is on the outside, probably the least amount of air circulation, next time you stop after running put an ice pack on the cool fuel

I think the pump is under the front of the engine with the fuel cooler. But, the thing to keep in mind is that this problem never occurred before the engine was rebuilt last August, so I think it is unlikely that its being caused by lack of air circulation. It has to be something that was caused in the process of R&R'ing the engine last August. That same engine was hauled out again over the winter for when I ripped out my Silent Choice exhaust. It's just not that easy to diagnose because the engine has to be run for a considerable amount of time, sit for an hour or two and then try to start it. Not really easy to just give to my mechanics and say figure it out..

What does R&R say about the issue, ya its wierd that it happered after the rebuild, did R&R rig the whole engine or just do the long block

Yes, they reassembled the whole engine, re-rigged and put back in the boat. They're great guys and have been trying to help, its just not easy to find because it only occurs when its been run for a while and sits for an hour or two and those times are generally not when the guys are around. I just spoke to the guys at Merc. Racing and they suggested that the next time it happens, take the fuel/water separator filter off and see if the filter is full. If not, then there is a problem occuring in front of the engine. If full, the problem is on the engine.

again never happen before and even with vapor lock that just comes out of the blue and never has before - then what changed - if anything.

Vapor lock that just happens and or fuel boiling off that never happened before - something causing it - mechanical, electrical, a failure - issue , and so on. Now if this was a new boat and started from day one then of course different story. You have to find the problem that is causing the problem but beforehand narrow it down to what changed. if any.

In his case motor starts just by advancing the throttle as he reported - this is not vapor lock. Advancing the throttle only and the motor starts RIGHT UP and runs great all day afterwards - again this is not vapor lock nor logically should not be a fuel pump problem.

It reminds me of a heat related resistance problem that applies to the starting circuit - sensor(s) connections and or wiring to them or even possible ECM or bad or loose ground(s).

Like I said with an MPI engine that starts by advancing the throttle 8 out of 10 times it has something to do with the IAC and or coolant temp sensor. I would look at your MAP reading as well especially after this heat soak takes place. Look at it before you start the engine and then reading while the engine is running.

problem IMO possible with any one of these, including the connections , and wiring circuits -

MAP
IAC
Coolant TEMP
MAT
Also would nt rule out TPS

Find someone that really understands the scan and the parameters

What about the heat transfer compound bup mentioned, may have been cleaned off and not reapplied during parts chachangeover.
Edit Oops.

This is why all the cool fuel stuff has been removed from my engines and replaced with Aeromotive pumps and bipass regulator's and -8 fittings 1/2 inch hose

Want to be very clear - boat sits hours later - you advance the throttle the engine STARTS RIGHT UP - LIKE NORMAL and runs fine and all day - is this CORRECT.

If so that is not a fuel problem nor a vapor lock problem, Can not believe if you told MERC all of this and the exact words they did not tell you to scan the engine.

Yes checking the fuel level in the fuel water sep filter is good way to see if you have a boost fuel pump problem that can not keep up pumping fuel that the engine needs and or keeping up with the high pressure fuel pump. it is a good laymans test out on the water after shut down.

To add is - Engine heat has to pretty darn hot to boil half the fuel out of the fuel water sep filter. No other engine is experiencing this hmmmm.

the heat transfer compound dries up over time and or flakes off over time. The reason its there is to pull heat away from the fuel pump. Funny how people all of sudden vapor lock shows up and never think about this nor the fuel that can boil out. Again if a problem just shows up that never happened before what changed ? in that case the heat transfer compound is not doing its job or hotter running engine temps or an electric fuel pump resistance higher than it should be .

Just saying this as an example not pin pointing to anyones problem. Unless your description is not what I keep asking you about to be 100 % clear. (advance throttle engine starts right up)

Call me and I will run your rig while you are at lunch. Problem solved