Guys - I have been through a number of pages on Offshore looking for an answer - this is my last resort before parsing the job out to someone else. I'm not big on having someone else touch my boat - but here's my problem.

I have an 03 Cobalt 360 with twin 496's. Put a new port motor in 5 weeks ago. Motor ran great for the first trip out - on the way back it sputter a bit but corrected and was fine the rest of the day. Since that day the motor has not passed 3000 RPM. The motor appears to be lacking on fuel, or have a timing issue. Here's what I know:

I am gettiing a code for Cam AND crank sensor "inactive" while running. Its coming up in the history, they pop up within a couple RPM of each other. I replaced both sensors with no change.

I swapped ECU's from one motor to the other - first run out - the boat ran perfect - on the way back - started to sputter - would not pass 3K RPM - NO GUARDIAN MODE CODE.

I replaced the inline merc filter and the spn on water separator - no change -

I don't know what to do at this point. I don't understand why the codes are coming up as inactive - or what that means.

The next step I'm going to try to swap my fuel rails - but I wanted some other thoguhts before I did this. Anyone have this issue?

Any help is greatly appreciated.

Eric

Eric,
I know what you mean about not wanting someone else to touch the boat, I am the same way with my cars but ..... with a modern fuel injected engine, you can waste a BUNCH of money throwing parts at it trying to find a problem. Never mind taking the chance of ruining the new engine by running it the way it is. It doesn't take very long to burn a piston etc if its lean and you keep running it while trying to figure out the problem.
If you are willing to buy the correct scan tools, gauges and manuals to troubleshoot it properly, then go for it. Otherwise I would say it would be money well spent to pay a dealer to at least look at it for you, even if you end up fixing it yourself.

Just my opinion,
Bill Koustenis
Advanced Autommotive Machine
Waldorf Md

Same problem 3 weeks ago and a set of plugs cured it. Good luck...

I had close to the same problem it was the transfer pump. I would pull one off your old motor. It may pump enough fuel to get the motor to run 3000 rpm. I didn't have a spare pump so I went to the local parts store and bought a cheap $25 electric inline fuel pump. That showed me the Mercury one was bad.

I would install mechanical fuel psi gauge there is a spot on rails,install scan tool test run boat make sure fuel psi is up to spec if it is ok and no codes i would call merc tech line,they have been very helpful troubleshooting efi engines saving time and money getting to the problem,there is a dealer tech line not sure about the consumer tech line

Find every place there is a ground wire from the harness and clean all the connections. Can't hurt.

I had this same problem last year and it was impeller starting to go bad and when motor started spinning in upper rpm's would loose water pressure and set codes of with cam sensor and knock sensor. A few dollars and two hours fixed it.
Also could be bad thermostat as 496's have there own closed cooling system.seen this also once.

X 2, The cam and crank sensor share the same ground.

You may have a bad or loose wire connection in the wire harness someplace after the new engine installation.

You should consider doing an inception for a backed out wire in a wire plug connector on the wire harness by following the wires from the sensors.

Carefully wiggling and pushing in wires at the wire connectors may help you find the problem to.

The mercruiser service manual that will help you through this process is part number 90-8637571 otherwise known as service manual # 33 PCM diagnostics, $35.22 from BAM marine.
http://www.mercruiserparts.com/partsByNumber.asp

Good luck with it.