My buddy has 2003 mercruiser 8.1's. one engine runs great. The other one has a definite miss. When we hook a timing light around the front left(looking forward from flywheel) and and second one on the left, the timing light will flash erratically, sometimes even quitting for a second or 2. We changed the coil pack and it flashes better but still missing. Can a bad plug cause this? If not, what can be causing this?

Did you check the harness connections? I would run a scan tool or Diacom to test each coil pack one at a time. The spark plug wires also burn up on those engines.

Yup, wires and plugs are common too of course.. Make sure to use OEM AC Delco plugs. You can get these anywhere that sells ACDelco plugs. If you need them BUP has OE wires for a few million $$$ less than OE.

If still issuse, the steel injector line can rust and clog injectors, the cool fuel module can flake and fill injectors full of unk...etc,etc.

So, time to start diagnosing vs throwing parts at it and/or just thinking LOL.

A scanner may be a good tool to buy or borrow,

The Crankshaft Position Sensor can do weird things (meaning it is bad), without throwing a code.
But you need a Rinda or similar scanner to properly troubleshoot.

With the the Rinda scanner, you can shut down individual cylinders, to see if you can find which one is missing. Shut them down one at a time. When it's down 2 cylinders, it will not be happy. But when you shut down the 1 cylinder that is already missing, you will know it because it's not choking hard.

Also, try running it in the dark, and you may can see if you have bad spark plug wire arcing.

U can OHM test each spark plug wire. Check the boots thats were alot issues can be cause they tear.

These are gen 1 coils and they do have issues.for failing. Scan tool will code fault which coil is bad by number as well. The easiest thing to do is connect inline sealed spark checkers for ign. spark output while running the engine. Buy all 8 and watch the spark or lack of.. .

This is GEN 2 cool fuel and no paint peeling issues but under the fuel pressure regulator is a small seal and a screen. When the screen starts to get even slightly restricted can cause problems from rough running to running rich.

The internal fuel rails become a issue especially if you have a fuel water mix inwhich accelerates the internal aluminum oxide corrosion in return starts to restrict the fuel injectors. All 496 fuel injectors suffer contamination issues over time in marine apps. . That,seems to be the norm.

No one ever changes the rubber fuel line from the gas tank to first inlet side of the engine. That is a 4 to 7 year item change regardless period. Also the boost pump canister fuel filter 100 plus micron rated is the biggest piece of junk on the market., So is Merc spin on fuel water seperator filters. Anyways the list can go on here..

if the crankshaft sensor goes completely bad you will have no tach and no start. That sensor also provides the tach signal. When it is having issues the engine can seem like its lagging plus show less rpms.

Also here is the boost pump filter some of these right out of the Merc box. . And yes these are my pics not net pulled The tie wrap thru the filter cracks me up and the these filters are not cheap but yes cheaply made. Brand new out of the box. Seen so many installed this way as well..

Thanks guys. I swear we've done most everything. We have a rhinda and no codes found. We have switched just about every sensor with the good engine and PCM as well. We tried to run on remote gas can and no change. We removed the fuel rail and did find some crap but before we put it back on the manifold we cycled the pumps. No leaks, then touched them all one at a time to a little battery. They all had a good spray pattern. Its frustrating! Mechanic is coming on Monday so he should know how to use the rhinda scan tool better than us. Full disclosure: when we start it with all sensors hooked up it runs for 3-4 seconds then shuts off. It will never stay running. We unplugged the map sensor and it starts up. If we plug it back in it will die. After it warms up it stays running with the map sensor plugged in. Not sure if that is a clue or not.

Where is the cool fuel 2 module located? I have a 2003 Baja 275 with a 496 HO that would run 67 mph and now only runs 62. I think I may have a partially clogged filter.

Are you close to North Texas Dallas / Ft Worth ?

Hit me your engine serial number - I think I know what is wrong here. and hopefully the tech coming can fix it. - if not I would like to fix it. I like the ones no else can fix. Put me in line for that.

It is best to get the service manual and follow the proper procedures because there is alot more to it than taking stuff apart. . I have all service manuals who ever needs them.