1997 502 MPI Issues - Vapor Lock?
 

I was out today in a 1997 Checkmate w/ twin 502 MPI engines. After burning a few gallons of fuel we stopped to swim for a couple hours. The boat restarted normally but then after 3-4 minutes of low speed (1500 RPM) cruising both engines died. Both would restart and die and finnaly the starboard engine stayed running and the port engine would no longer start. I have only owned this boat a short time so I have little experience with the MPI engines (at this point some primative carbs sound pretty good). Do they have a tendancy to have vapor lock issues with the fuel system? I plan to try and restart both engine cold tomorrow and go from there. Any other suggestions???
Thanks.

It sure sounds like vapor lock. What type of fuel system do they have? Do they have a VST tank on top of the engine toward the back behind the manifold or are they the cool fuel system? I had a 96 with the VST and they would vapor lock on the hottest days after running in no-wake zones. If you do have the VST tank you may want to pull the VST tank and take it apart (be sure to have a new rubber gasket to replace the old). There is a very fine screen on the bottom of the tank before the high pressure pump and it may be clogged or partially blocked causing fuel delivery issues. I had both issues and cleaning did help. Let me know if you'd like more info based on your EFI system. :)

Do they have a sea pump mounted fuel pump?

Mercruiser eliminated this pump in a certain serial # range and they are suseptable to vapor lock.

There is a electric booster pump update available to correct the problem.

I have done several here at the shop and all corrected the problem.

Thanks for the responses. The fuel system on my boat does have a black tank on the rear port side of each engine (the fuel pump vent line goes into it as well as in and outlet lines) and it looks like someone has been into the one on the port engine as the philips screw heads are buggered up. Also, there are fuel pumps on the sea pumps (strange design). Since there are fuel pumps on the sea pumps does this mean the electric booster pump should not be required or should the mechanical pumps be replaced witht the electric ones?

Thanks.

It bothers me that both engines died. It speaks for something that is common to both engines. How is the fuel comming out of the fueltank? Is it a common hose with a common anti siphon valve?

Tha tank itself is common to both engines. Water in the gas tank?

Hey guy's there was a service bullitin on VST tanks in 97 that fixed this same problem. The only reason I know this, is I had to apply the fix on my boat when I bought it used. I got a good deal on my boat because it had this problem and no one could fix it. Of course the seller denied any knowledge of the problem.
Anyway, Mercruiser changed VST tank top in 97 and the air passage lines were different, so the spring was installed on the wrong side of the vent diaphram. All I had to do is get a o-ring seal kit and pull the top off, then pull the little plate holding the vent diaphram on the upper housing. After inspection, the spring was holding the diaphram closed all the time, so it would never vent, and in effect vapor lock. Put the spring on the opposite side of the diaphram and reassemble, and never had a problem since then.
The spring must be on the side of the diaphram, that has the vacuum hose orifice. When the motor starts, the vacuum opens the vent port in the VST tank, to release the pressure buildup due to heat soak in the tank.
If your boat has the incorrect setup from the factory and nobody has fixed it, then it could happen to both motors at the exact same time. I eventually found the fix in the VST overhaul section of my mercruiser manual.

BTW mine did the exact same thing as yours. I figured it out to begin with, by rigging up a fuel pressure gauge on the inlet fuel line. As the motor sat, the fuel pressure started increasing to about 10 psi inside the VST tank. Then after starting the motor the pressure stayed there and after the electric fuel pump sucked the VST tank dry then the motor died. The mechanical pump only pumps like 5 psi, so it can't pump against the 10 psi in the tank.
Sorry for the long post, but I'm pretty sure this is what your problem is. I spent countless hours figuring this out.
Hopefully you get it figured out, because the hotter it gets outside, the more this will happen.
OH, a simple test would be to let it set after running to build up pressure and then take out the inspection plug on the top of one of VST tanks to release the pressure and then put it back on. Then drive it and that motor will not die, but the other one will.

I have the same problem with mine, did it on saturday. Mine is a gen 6 block, but has the 1996 vst and mechanical pump. Would this be the same with mine?

The manual doesn't really say when they changed the design, but if you pull the inspection plug after it heats up and release the pressure it will run if that is your problem, but it might still vapor lock after you drive it longer, because the vent isn't working and the pressure will start to build as it heats up. You could pull the plug all together just to test in calm water, but make sure your blower fan is on to keep the fuel vapors at a minimum in the compartment.
But it sure sounds like the same problem!

Both VST tanks at the same time??