I have a pair of Crusader/GM 5.7 L V8 engines (270 hp - fresh water cooled) in a 1988 Aquasport with 830 total hours. Both engines have been rebuilt. The engines have Quadra-Jet carbs and both have been rebuilt by National. The coils and resistors have been replaced. All fuel filters have been replaced. The points and condensers in the distributors were replaced several years ago with the Sierra Conversion "black boxes" and worked great until recently. (Note the new coils and resistors are Sierra and they are compatible with the black box conversion) Note the Sierra black boxes were replaced. The distributor caps and rotors (Mallory Distributor) have been replaced and the GM mechanical fuel pumps have been replaced. The fuel has been inspected and it's not the problem. Both engines have good compression on all cylinders.The problem / question: The boat runs great on a plane at the cruising RPM of 3200 to 3400 RPM for about 30 minutes. At that point one engine will slowly start to loose power. You will see the rpms start dropping. If you do nothing to the throttle the rpms will continue to drop and the engine will stall. It will not restart immediatly, however if you wait 10 to 15 minutes it will restart and run normally below 2500 rpms. If you try and put the boat on a plane (3200 rpms) it will loose power and stall however if you stay off a plane and under 2500 rpms it will run for ever. It is not a fuel problem. I set up a remote fuel tank with fresh fuel and the problem still occurs. The problems appears to be "heat related" but the engines are not running hot.Two differant marine mechanics have looked at the problem but they could not figure it out. One hooked up an electric meter and the coils are putting out 11 volts even as the engine rpms drop during the failure.What is causing this problem and what needs to be done to fix it?Thanks!
Make (of engine): Crusader V8 270 hp
Model (of engine): GM 5.7 L
Horsepower: 270 hp
See earlier explaination.
Thank you for the question,
After reading all the work that has been done and the issues at hand, I would start to believe we are having a classic example of Vapor Lock in the fuel system.
One method of testing for this would be to install the portable tank back on the engine and when they begin to loose power, have the mechanic squeeze the primer bulb on the portable tank (Really Hard). This will do two things, one will be to test the fuel delivery system and fuel pump, secondly it will increase the fuel pressure and likely keep it in liquid form between the fuel pump and the carburetor.
Should this not work for us, we will need to install a fuel pressure gauge between the fuel pump and the carburetor and go for yet another lake test. If the fuel is vaporising or the fuel system is not delivering liquid fuel for some reason, we will see the fuel pressure drop. The fuel pressure should be around 5 to 8 psi running and the needle should not even wiggle. This will test the entire fuel stream and I believe we will encounter our issue during this process and pressure test.
95% of the repairs we now do at the shop are fuel related. Fuels today are very susceptible to vapor issues and store very poorly. Fuel one year old "will" cause us all kinds of problems. Fuel 3 to 4 months old and any fuel with any alcohol in it will cause problems of this kind from the get go.
I did the remote fuel tank set up with the temporary fuel line installed at the intake side of the mechanical fuel pump which has been replaced. The fuel line from the fuel pump to the carburator is hard metal. Are you saying there is a vapor lock in the hard metal fuel line between the fuel pump and the carburator? If so how do you fix this? Have you ever seen this problem on a marine GM 5.7 L V8?
Yes, we do see this issue on a monthly basis at the shop.
Testing the fuel pressure in this hard steel line will confirm not only a fuel delivery issue, but will also confirm that we are on the right track and not wasting troubleshooting time on the wrong system.
Did you squeeze the primer bulb while running the engine on the portable tank? If not, we did not follow through with much needed information.
Knowing what the issue is will go a long way to preventing it in the future. Insulating the fuel lines and forced air flow through the engine compartment, new fuel and fuel additives will do wonders towards alleviating this issue.
Crusader and Mercruiser through the years did develop water cooled fuel lines to work with this issue. Generally we can ventilate the compartment more and certainly change the fuel new with additives to correct the issue.
25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified