You may have paint fouling your fuel pumps, search this forum and the internet for Volvo fuel pump paint problems or check the video below. This is a common problem with your fuel pump assembly.
Just take a reading from the low pressure pump and then the high pressure pump, do nothing else. Low is 5 psi high is 50 something..If those readings are good time to down stream a bit. Now with that said do check the tach terminals a ground can drive drive the system batty so to speak.
It does not have a water separator just a fuel filter that i just replaced yesterday. The reason i changed the rotor cap was it had corrosion inside it so i just put new plugs and wire on it because it is easy and cheap. I took it to a shop last week he thought it was fuel pressure regulator so he changed it. he also said the pressure was 29. I thought carb eng was to be 30 and fuel injected was to be higher like 45. Is that correct? I just filled the 60 gallon tank up with non ethanol gas so i don't have anyway to clean it. I was going to try and find a little fishing boat tank and try that. I'll try to get engine number today
Opps theres the problem low is 6 high is 50...get your fuel pressure regulator back the guy who put it in is well not so in touch. Marine fuel injection is very different from auto very basic stuff with no epa guidelines.
If you give it more throttle, holding throttle there and it starts, runs 20 sec, dies and can do it again and each time its about 20 sec. Then I would check fuel pump relay and pump trying to find out if pressure is going away or is it spark. That said, if it will only start with more throttle then IAC issue also
Had the same problem with my boat and it was the fuel pump relay... They are designed to run off battery power for about 20 sec. then circuit switches over to Alternator power...
Is the fuel rail that feeds the injectors made of plain steel, you can check it with a magnet. If it is steel it is probably rusted on the inside and the rust flakes could be plugging the small screens on the top of the injectors. Mercruiser uses stainless steel . One of my friends has a shop and ran into this on a Volvo recently.