Just picked up a '99 Donzi 33ZX with 502's in it w/approx 100hrs;
A couple weeks ago, the starboard engine would start while out
after running awhile.....got home w/boat on trailer and engine fired
through perfectly. Yesterday, the port engine did the same thing
almost. It cranks through.....fired and idled for approx 4-5 seconds
and died....eventually it started fine. I have currently replaced
fuel/water separators and cleaned flame arrestor. Both engines
have approx 20 psi oil pressure @ idle and approx 40 psi running
up around 3-4000 rpm; Water temps are approx 170-180. Any ideas?

Sounds like a vapor lock problem. Never heard it talked about much on 502's, but several board members had similar problems with 500efi's.

Use the search function and search fro "vapor lock"

Any other suggestions?

Is this an EFI or a Carb engine? Based on the year I assume EFI but I wanted to be sure.

Roby

It's a EFI motor.....

All the 502's and 454 efi's can have this problem. It is a vapor lock issue that Merc has a tech bulletin out about. If I remember correctly you are supposed to let it idle a bit before shutting it down and always open the hatch to let the heat escape. Do a search here like Griff said and you will find it.

It's been running great now since I let the motors idle for approx 5 minutes
before shutting them down!!! Knock on wood....hope this was the fix.