502 MPI Problem...
 

Ran the boat last weekend, and it ran great both days. Motors have always started on the first turn of the key and run perfect.

We ran a good hour or two Sunday morning and parked for about 3-4 hours. Temps were in the low to mid 70's, so not too hot.

When we went to leave, both erngines started up as normal, but the port motor quit after about 3-4 minutes. It would restart, but not stay running. Acted like a fuel problem, as it never had a miss, just actd as though someone cut the key.

I am leaning towards fuel pump as it has a more quiet sound than the starboard motor upon the prime. These motors have the newer "cool fuel" system, and the water separator was full when I checked it the following day. Now it won't even fire. The other thing I am considering is the IAC? Thought?

Was going to put a fuel pressure gauge on the pump, and maybe swap the two between motors and see if that cures problem...

Thanks!

It is very unlikely the IAC would cause it not to fire at all. I think your fuel pump idea is probably correct. Does that engine have a fuel pump mounted on the water pump also, because that could be the problem also. A fuel pressure test at the rail and coming out of the mechanical pump would be the proper next step.

I don't believe there is a secondary pump at the raw water pump. Just the electric on the lower port side of the block with a line feeding in at the bottom, the coolant hose thru the middle and what looks to be a regulator/return line at the top. The return line feeds back into the water separator.

I'm not saying it is your problem, but others have been experiencing similar symptoms and it is related to vapor lock caused by e10 blended fuels.
Mercruiser makes a kit to help solve this issue.
I would hook a mechanical gauge to the schrader valve on the front of the fuel rail of each engine to verify pressures. You're looking for 42+ psi on that engine.

I had one that would lay down at 4200 rpm. turned out to be the fuel hose or anti siphon valve. removed the anti siphon valve and ran new fuel line and it ran better than new. try removing the fuel line from the filter and hook it to a jug with new hose and see if it starts. worth a try.

+2

Mine done exactly what yours is doing. It would vapor lock. Would not draw fuel even to the filter. At first I would bleed the shrader valve about 4 times and it would start right up and be fine all day. I suggest P/N 21-862271A 1. Its a check valve kit that installs on your water pump lines. Here is a TSB about it all.

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

Spray some Gumout down the intake while you crank. If it is fuel it will fire, if spark it will not.

Even the old basic stuff works on these engines.

On these later 502's with the "Cool Fuel" fuel pumps, is there just the single high pressure pump? It runs for about 2-3 seconds when you turn the key to on, and then runs continuosly after starting? Correct?

Not a low pressure pump at prime and then high upon startup is there?

If its vapor lock, it should fire or at least sputter and then stall. My 502mpi's will get some vaporlock if I run E10 fuel. You just have to keep turning the key in short 2-3 second bursts and it should fire after about 10 times of doing this.

Yes, at least on mine. I added the (low pressure) priming pump that is referenced in the Merc bulletin. Vapor locks woes never returned.