HELP!! 7.4L/330HP with Issues!
 

HELP!! A friend of mine just sent this to me. Its a 7.4L 330 Fuel Injected motor.


When I run the boat at full speed (4200 RPM's) it could take anywhere between 15 and 45 minutes before the engine cuts out. I can watch the tach go from 4200 rpm's to 0 and then spike back up to 4200. It will keep doing this sporadically. If it is not doing that, while at full throttle I can watch the tach drop from 4200 rpm's to 3800 and remains there. If I keep running it, it will drop to 3200 RPM and stay there. During both of these occurrences the boat will no longer idle it will stall. If I let the boat sit for 15 minutes or so the problem goes away and I have to wait another 15 minutes or so to see it happen again. I can run the boat at 3200 RPM's and never see the problem. The marina has run a fuel pressure check and that was OK , replaced the coil and replaced the computer I think he called it the ECM. The error codes the computer originally gave him was a knock sensor and the ECM. Any Ideas???????

As a test diconnect the tach, I have heard of two boats with the identical problem you described and it ended up being the tack causing problems with the ignition system. It might be worth a try. The fact that it runs fine after it cools sounds like a ignition module problem.

When the motor died on you, did you check for spark and fuel? You have to determine which is droppping out on you before you change parts.