496 MAG HO Problems Help!
 

Of course it always happens on a vacation.

Ran like a top to start with. Then started to get a little hesitation when accelerating. Would clear up an run fine. Acting just like an old carb motor with the plugs loading up. Progressively got worse. This is over about 5 or 6 running hours. Now it is stuttering pretty bad accelerating...at about 4000RPM's it just starts cutting out and won't do any more(strated doing that 1st time I noticed at 4500 RPM's). When you come off plane and are idling for a minute or 2 it will backfire 2 or 3 times at about 10 second intervals(like 1 clyinder is firing late). The it will idle along with alot of surge. Once on plane it will clear up and run fine from 2500-4000RPM's.

I have checked the plug wires,changed the fuel filter, and looked for any connections that might be loose...but there are many. It is not sounding any alarms. The transom is as white as snow. Oil pressure,temp,and voltage are normal.

Obviously I have no scanning equipment available. Is there anything small or stupid that could be causing this? Would also love opinions as to what it might be. I believe the boat has probably not been run much for close to a year. I've put 8 hours on it. 2005 model. Now here's the really dumb one that I know the answer to bit need to appease my wife. How much risk am I taking to run the boat like this? It idles like a champ when first started.

idle air valve,,iac valve,,,,trying tapping on it ,,,sometimes it helps,,,easy to replace

This is a EASY fix.....Same thing happend to me, starboard motor kept popping, cutting in and out, wouldnt run, and so on....

Take the main engine harness that looks like a big trailer wire connector and unplug it, once you have it apart your going to work with the male side of the plugs. Using a thin flathead screwdriver gently spread everyone of the male connectors. Use some wire/connector grease (lithium grease) and reconnect the plug and your problems should be gone....

Like I said, same exact thing happend to me and I ran into someone on the water who fixed mine when we were tied up....

Good Luck

Thanks, I'll give your suggestions a try!

Also check both (HP and Low P ) fuel pumps.

This sounds like bad fuel. I had similar happen to both engines last year. I got 10% Ethanol fuel from somewhere that was not labeled as such. After I returned from a trip, the boat sat for a couple months, and the Ethanol absorbed a LOT of moisture. It ruined my F/W separators, inline fuel filters, AND my fuel pumps.

If you have replaced your F/W separator and your inline fuel filter, buy 5 gallons of known good gas, and run the boat off of the can for a few minutes.

EVERYTHING changed with the introduction of Ethanol to our fuel supply.

If your IAC was bad, or you had other electrical issues, the alarm would be going off.

Where do you live? (Is it humid?) How was the boat stored (outside, inside, climate controlled)? How much fuel was left in the tank? (Either leave the tank FULL or EMPTY. Anything in-between is bad news, as the more surface area the top of the fuel supply has to absorb moisture, the more moisture it absorbs.)

I just had the same problem IAC was no good and under the breather there is a little white filter going to the IAC very small. If that gets glogged, dirty she will cause the same problem.

Well, the diagnosis is in. The boat shop said it's the fuel pressure regulator. Cleared the codes 3 times and kept throwing a fuel pressure regulator code. Does this sound like it will solve my issues????

Fuel pressure is very important to the proper operation of a fuel injected engine. If your regulator is kicking out a code, there's a very good chance a new one will solve your problem.

I have to admit that while reading your original post bad gas occurred to me too like Brian said. Could be a combination of regulator AND spoiled fuel.

I thought bad gas too but I'm sure I ran all the old out before. Filled it up from a 1/4 tank and ran a couple or so hours before we went on vacation(took 80 somin gallons). Ran without a flaw the 1st time out! Then I filled it back up...around 40 gallons to top off 110 gallon tank. Put the same gas in my Suburban. Fuel filters were clean with absolutely no sign of water. What bugs me is not the loss of RPM's but the occasional backfire at lower RPM's......seemed more like an ignition or valve problem to me. Guess I'll find out when they get the part and I give her a go! He did say there was a cam and crank sensor code with an overrev code. Cleared the computer and did not happen again. He said the overrev would trigger the cam and crank sensors. I guess when I hit 5000 RPM's it set em off? Was in the time frame since I've had the boat!