Problems with my Merc 74 EFIs

"I have a 1996 Wellcraft Marti

"I have a 1996 Wellcraft Martinique with twin Merc 7.4 EFIs. It's been running great for 10 years, but recently I've had engine problems.

Whether idling or under power after about an hour one of the engines (always port first) dies. It cranks, but won't restart. An hour later the other engine dies and does the same thing. If you wait about an hour they both restart, no problem, but then we start the whole thing over after an hour.

The fuel pressure drops - seems to be related to heat. It has spark - we put a pressure gauge on after the fuel pump and the fuel pressure drops off. We've changed fuel pumps and regulators, fuel filters and pickups on teh tanks - fuel lines have been changed. We've had a mechanic attach the computer and no codes come up. We changed the module - still teh same problem. What could this be?"

Voltage drop to the fuel pumps

Voltage drop to the fuel pumps?

"Maybe vapor lock??? Does your

"Maybe vapor lock??? Does your motors have electric fuel pumps? If not connect an electric fuel pump inline and when it starts to lose power, throw the switch, and see if that helps to improve proforemance."

Check your fuel tank ventilati

Check your fuel tank ventilation. Good luck.

"One tank or two? Have you ch

"One tank or two? Have you checked the tank pick-up tubes. Some have screens on them which will clog if the tank starts to scale on the inside. You can easily check venting by running with the fuel fill caps loose. Also check the fuel cooler. Mercruiser uses a returnless fuel system which circulates unburned fuel within the fuel system rather than returning it to the tank. Heat builds up in this fuel which is dispursed by a fuel cooler. If he cooler gets damaged or scaled, it may not be cooling the fuel properly, which will cause the fuel fo boil and vapor lock.

I'm leaning toward venting problems."