496HO missing at high RPM
 

496HO, runs fine below 4k rpm but starts "missing" above that, almost like a car would if it had a bad plug. Switched out new fuel/water separaters, trying to avoid a new pump but starting to look like i may try that. I put some star-kron in the fuel to see if it was just bad gas. It didnt seem to help. Any suggestions??

Have you checked the plug wires? I had a plug wire pull off part way off of a coil on one of my 496s & it started missing at high rpm.
randy

My Starboard engine was down on power, very noticeable above 3000 rpm. I purchased diagnostic software and with the ignition coil 'misfire' test was able to individually cut-off spark to each cylinder. This identified #8 was not firing at all and #1 was sporadic. I changed both spark plugs and the problem is fixed. I don't run the engines hard and idle a fair amount so most likely plugs got loaded up....a bit surprising to me but this was my issue for what it is worth. The challenge is to troubleshoot these engines without the software. If you have a 'side by side' installation, you certainly don't want to be changing plugs and wires etc, not knowing what cylinder is causing the issue considering how difficult access can be..... hope this helps....

Do you have inline "bullet" fuel filters as well? They are about 5" long, silver; about the diameter of a stick of pepperoni. These need to be changed too.

Untreated ethanol fuel can and will destroy the fuel pumps on 496s. I know this from personal experience.

Plugs and/or wires is where I would go next, but I highly recommend checking it out with the a Rinda scanner or the software. Rinda Technologies Inc. Marine and Industrial Engine Diagnostic Tools
The scanner is money VERY well spent on these modern Mercury motors with Smartcraft. You can go in with a few friends/neighbors to buy one and share.

I do not have the in-line filters, i was surprised at this... Will be changing plugs today or tomorrow and draining water separaters... Going to take the fuel pumps off again and inspect the screens, as last time i had some crud in there.
Thanks for the help!

AJFish...pull your bananna out of the tail pipe...they tend to run better.

I would run all of the gas out of the tank first. My old 496HO would miss, and only at the higher RPMS, if the gas had sat over winter, or once when I'm sure I got crap gas. The next fill up it ran fine.

Jeb you are a homo, lol

Ignition problem needs to be diagnosed...

AJfisher and myself swapped fuel pumps side to side and the problem followed. He also pulled out the bannana. Theory is the ethanol wrecked both pumps over the winter.

Well after hooking it up to a scanner, there were no alarms. Swapped high pressure fuel pumps and the problem followed. New pump should be here this week. Thanks again everyone for your input

Edit: JRider get your ass to work and off OSO

Same thing happened to mine, from Ethanol. My fuel pump had a rattle to it when I removed it.

Is there a trick getting access to the pumps? Mine is pretty tight & need to swap for troubleshooting also

I remember just loosening a few bracket bolts and sliding it out.

I suggest looking up your engine(s) on www.mercruiserparts.com to see brackets and schematics of the front of your engine as there have been a few different pulley and bracket designs on the 496s since their introduction, What may work on one, does not even exist on another.

There are two fuel pumps on the 496. One is the Boost pump and typically primary culprit on the starboard side behind the separator filter assembly, and then there's the main pump on the port side buried deep (you should hope to not have to change that one)....Depending upon the year, you should only have to remove the fuel separator filter and mount (two bolts) and lay it out of the way to get full access. Should be a spacer in there too to keep an eye on. The newer motors with a different separator assembly I "think" may have the pump built into the filter assembly? Not 100% sure about those editions. After that pretty simple and obvious disconnecting/reconnecting the lines AND the electrical connector. While you're in there good opportunity to look around for touchups or to tighten hose clamps etc.

Another indicator would be to hook up a fuel pressure gauge to the Shrader valve on the fuel rail up top and check for consistent fuel pressure. With no alarms you're most likely on the right path.

Another note on spark plug wires or dead coils....Should one of these happen you will most likely start seeing a lot of excess soot buildup on your transom from the misfiring or non-firing cylinder.

Good Luck!