01 272 Boss

Formualfastech: To be honest, I'm not sure what the fuse is for yet. It's a 2 amp 250v fuse located near the battery. The fuse did not blow again, and the fuel pump primes with or without the fuse.

The engine does not quit instantly, it's kind of a slow process. Stutters, back fires, fires, stops. The tach needle completely drops each time it coughs.

CigDaze

This sounds very odd. At first, I would swear that it is fuel related, but now it seems more and more like some kind of ECM related problem. No way to diagnose that without a scanner.

cuda

I agree with BD, it sounds like a fuel related problem. Just as you thought the first time, it sounds like it is running out of fuel for whatever reason. I am unfamiliar with the fuel delivery systems of this engine and can't really make an informed diagnosis.

28SS

I beleive the problem is with the coil.
if eng will rev in neutral but not under load
had eng that I replaced fuel pumps high/low prressure
because of same problem same symptoms was coil
coil gets warm and voltage drops i.e sputter/die coil cools
off and is fine till warm.
hope this helps

Kentz

Cam Positon Sensor?

AZDominator

I agree with Kentz. In fact, I was out on the lake with a friend who had twin 496 Mags when his cam (or crank I'm not sure) position sensor was giving him problems on one side. It turned out to be faulty. His would start, backfire and run (if thats what you want to call it) up until the time he tried to plane then it would quit. He had some success in disconnecting and reconnecting it but ultimately it would die again crossing the lake. It's located on th top of the bell housing. It's a couple of wires terminating into a weatherpack connection.

Kentz

Your right!, I think it might be crank sensor, the cam sensor is what quite on my PSD. i

01 272 Boss

Gotta admit, you guys are pretty good!

I took the boat to a Merc repair facility this morning. He ran diagnostics on it, and called me with the results.

#1 The computer is throwing lots of codes which are associated with the problem.

#2 He said he's not 100% sure yet, but its either the crank position sensor, or the camshaft position sensor.

#3 He also said the computer shows 15hrs use on the engine, but the hour meter shows 47hrs???

He's calling merc tech support right now to see if they have any ways to confirm which it is. I'll post with the results.

Vyper340

I just thought of one other thing that has failed on these engines (including 496 HO). The crank position sensor fails and won't allow the boat to rev under load. Once it fails it won't restart for some time but when it does it will run at idle but not under load.

The sensor is about the size and dimension of a cigar and it's located on the rear of the engine where it attaches to the transom and drive. It's an easy and inexpensive fix for a Merc mechanic. Unfortunatley it does not show as a fault when you run a diagnostic. The part cost about $50 and it's less than an hour to install, adjust and test.

Good luck.

01 272 Boss

Just picked up my boat, and all is well. It turned out to be the crank position sensor. Imagine that, a little $60 plastic sensor causing all this trouble. lol Thank you all for your help and advice.

Now I finally get to take out my new boat!!