502 MAG Mercruiser
 

I just got my 2001 Velocity 502mag in the lake today. Just bought it. Has 28 hours.The engine starts fine first thing. Then I ran about 30 minutes and stopped to eat. When I came back it took 30 or so trys to start.Would run about 1 or 2 seconds then quit. Just barely start. Then started and ran fine. Stopped and went out in a friends boat for about an hour. And same thing only was on the last turn of the battery when it started. Then ran fine. Got back to my dock. Shut it off for about 5 minutes and started and it hit first try. Any ideas ???:::confused:

Sounds like it could be a vaper lock. Some 502Mag EFIs have this problem. How fresh is your gas? Might have some water in there. I had a similar problem with a 4.3 and had to install a water separator.

From my understanding the 502 and the 500 EFI are the worst for vapor lock. Yours should have cool fuel and that may help some. The only fix I know is to let it idle and cool down for a few minutes (like 5) and also crack the hatch while parked to keep it cooler in there.

Check with a merc. dealer there is a fix for that.

Try This
 

This may sound stupid but be sure to check all of your fluids. my buddy had the same problem and come to find out the motor was like a quart low on oil. we talked to some people and they said that those motors have sensors that will prevent them from running if they are low on oil. try it and see what happens. good luck!

Directly from the Mercury Marine Website:

Q: On a hot day after I run the engine for a while then shut down, the engine won't restart or it starts and idles fine, but when the throttle is advanced the engine bogs down or quits running and will not restart. Why?

A: It sounds like your engine may be vapor locking. One factor that contributes to vapor lock is fuel. Fuels produced for use during the winter and early spring months around the country are designed to vaporize at a lower temperature. This lower vaporization point can cause the fuel in the boat supply lines and engine fuel system to vaporize prior to being used. Another contributing factor to vapor lock is a restriction in airflow into the engine compartment. Make sure that lifejackets, fluid containers or other equipment have not restricted the airflow.

Other items that will help to reduce vapor lock are:

Letting the engine idle for 3-5 minutes before shutting it off.
Opening the engine cover to let the hot air escape. Note: Always reinstall the engine cover before starting the engine.
Operating the bilge blower to remove the hot air.
If these items are OK, an authorized MerCruiser dealer can perform additional tests on the fuel and ventilation systems.

Sorry, but I don't buy the vapor lock theory regardless of what Merc said. FI engines operate at very high fuel pressures negating the chance for vapor lock. Low fluids soind more feasible since some of Mercs engines have a Hobbs switch that engage the ignition only after oil pressure closes the switch.

Vapor lock
 

The second day I ran it..after reading these posts. I opened the hatch and ran the engine a couple of minutes before shutting it down. It started right up...was a little hard one time but several other starts it went fine. It was real hot out both days.I'm still running the first tank of fuel and it had about 18 gallons when I filled it. So could be fuel also. But looks like it was vapor locking.:)