Motor problems
 

I have a 96 502 mercruiser mag mpi and it is having fuel problems. It has a new mechanical fuel pump and also a new high pressure fuel pump in the vst chamber. The boat will run good for 45 minutes to an hour but when you kill it and let it set for a little wile it will start and idle for about 5 minutes and then starve for fuel and eventually die out. I'm not sure where to go from here.

Sounds like vapor lock

Odd how some (same fuel system) have the issue and others do not. Other than engine room venting (temp.) I can't put my finger on it.

Fuel pressure relief valve. Without engine running. Turn on key then turn off. system should hold close to 50 lbs. Had replace this on 1996 350 mag mpi gen plus. Your motor uses same one. The 350 had a Schrader valve like a valve stem on end of fuel rail. I used a cheap dial face air pressure gage to check it.. I found a couple on ebay and snapped them up. If you cant find one Believe it or not. A Honda civic uses the same one.

There is an internal diaphragm at the top of the VST chamber that will deteriorate and cause the issue you describe. You need to remove the VST top, and flip it over. You will see another cover with 3 screws that need to be removed and there is a diaphragm within that will likely be shot. Replace this and your problems should be solved. It is often confused with vapor lock but is not. Basically when this diaphragm ages and breaks down high pressure fuel will percolate through this vent into the manifold. It bleeds down rail pressure and floods the motor.

Genuine Mercury & Mercruiser parts. VAPOR SEPARATOR TANK, SN & Below (See Note)

http://www.mercruiserparts.com/Merc/...MMON/39670.gif

I just pulled them 3 screws out and the diaphragm looks the be in good condition no holes or tears.

replace it while you have it apart.

Interested to here what you found out?? Here is another pic of your vapor separator tank with fuel rail and pressure regulator. Got that from cp performance sight. The regulator drains back to vapor separator tank

Trey did you check fuel pressure as indicated above by Veloc?

I checked the fuel pressure on the fuel rail and I am getting about 35psi with it running.

I got 48 lbs on both of the engines after I replaced the regulator on port side. The port wngine would not hold pressure when it was turned off. When I pulled the return line off the fuel regulator I saw a lot of fuel getting past the regulator when the key was turned on. That fuel pressure should hold when key is off. Turn key on for a few second then off pressure should not drop. If you wait a minute then check pressure it should still be there

I know your fuel rail is different but same regulator. Its actually just a pressure relief valve. There is a brass tube in the end with O rings on it, don't loose it. The new one does not come with it and use grease on all o rings when you reassemble. You can see the cap for the shrader valve and the fuel rail. My pressure regulator is underneath the fuel rail.

I will check fuel pressure like you say and go from there, thank you for your help I really appreciate it.

No problem. That was on my last boat that someone I know bought from me. He had this issue in August and I helped him trouble shoot it

I just hooked up a fuel pressure guage and when the key is on the pressure goes up and when you turn the key off the pressure drops to 0.

Pull the return line off the pressure regulator , have someone turn key on again and see if fuel squirts out of regulator. Start looking for a new one. They are hard to find. You will most likely find it on line. I found the one I needed on ebay but like i said if you can' find it, look up the regulator by the raised letters on the casting. You might find it at an auto store. When I looked it up Honda civic came up. It's a Japanese part.

http://www.michiganmotorz.com/mercruiser-fuel-pressure-regulator-p-4612.html?gclid=CjwKCAiA0IXQBRA2EiwAMODilzqMJ3xmpK 4y-dKP-XaFf7gzAQ1ZTqgnvv4cmRFDALOuIwsMKNNlMxoCJkcQAvD_BwE

Any way I just punched in NR1 fuel pressure for mercruiser. Michigan motors came up .118 bucks. I think we found ours for 100

861126a1

Turn key on and fuel pressure on the rail bumps 40 and drops quickly to 10 and holds.

It should hold full fuel pressure not drop.