I am in the process of getting to the root cause of my hot starting issue on my 2000 7.4 MPI motors.

What will happen on fuel pressure gauges (when hooked up to the Schrader valve on the motors) if it is vapor lock? Under a cold start condition, I can get ~40 PSI pressure when starting the motors. But I haven't had any gauges with me when its "suspected" to be vapor locking. Just wandering what the gauges do IF it actually is vapor locked, and how I can confirm.

Thanks Team!

Pressure is pressure weather its liquid or vapor. Gauge will verify pressure only
Have safety glasses, rag and small screw driver
Place rag near the schrader valve and depress the valve tip. You should see gas and air vapor.

So you are saying the gauges are not any help determining if I have vapor lock? Only way to tell is to see what is coming out of the valve? Fuel only good. Air and fuel bad?

Fuel pump builds pressure until regulator set point is exceeded and excess fuel returns to filter.
Motor is hot and when shut down, fuel rail is heat soaked and gas vaporizes, pressure increases and regulator sends liquid fuel to filter, leaving mostly vapor.

Try to restart and it may start and then die or not start at all. Pressure in rail is all vapor (air) and there is not enough fuel to create a bang in cylinder. If pressure is released at schrader valve, fuel pump can refill with liquid.

The fix is return fuel line to tank and not to filter

I had been fighting a seemingly random no start for almost 2 years. somedays it wouldn't start cold. wouldn't start hot, wouldn't start for hours. would stall after starting. would run all day at all RPMs just fine, for multiple trips! Then, wouldn't. Then, Would!

Bottom line: was the fuel pump.

I replaced the cooler and fuel pump and pressure regulator all at once. everything was 20 years and 700+ hours old, so I just went all new OEM and be done with it.

When I'd go to start it, there was mostly air in the fuel rails. Came in on a single engine too many times. Now it's been fine ever since the new parts.

yes, a tire pressure guage reads air ( a gas) pressure and in turn would read the fuel ( a liquid) pressure at the end of the fuel rail. Ideally when you release the pressure at the fuel rail the only thing that comes out is liquid fuel. Odds are the fuel is boiling and is filling the fuel rail with a pressurized gas vapor and is physically displacing gasoline in a liquid state.

Vapor lock is pretty easy to detect and pretty common on Merc engines.
What exactly happens with yours??

Are you running non-ethanol? My 1996's 7.4 LX Mpi's with VST (without cool fuel) do not vapor lock in S. Florida summers and I don't even have to lift the hatch. I run non-ethanol with Startron.