I'm absolutely positive that there is a black cloud over my head following me around! Boat is in fresh water and about a month ago the port engine wouldn't RPM over 3200. After much digging it was a Denso plug issue and replaced all with AC and ran fine. Let it set for a month without using and same problem--performed Rinda cylinder drop test and was number 8 cylinder. Replaced plug with new AC and still no power--old plug looked like new. Pulled plug wire and sure enough getting plenty of spark. Put a new injector in and still no power out of 8. Didn't have compression tester with me. This spring I removed turbulators and I have that sinking feeling in the bottom of my stomach that water may have dripped down and rusted the exhaust valve in #8. Engine is a 2003 with about 46 hours on it. Seems to idle fine--no backfiring. Can't really listen to exhaust as it exits under a solid swim platform. My only other thoughts are a bad wire to the injector or a pcm 555 issue. Looking for an expert opinion... Thanks

post your fuel pressure,,,on here???

You lost me on this one. If the other 7 cylinders are firing normally on the cylinder drop test and only # 8 doesn't show a drop one would assume I have adequate fuel pressure as it is a rail system.

Take a volt meter to the wires for #8 and run the engine, you should see the PCM telling the injector to open. Most are done with ground actuation (constant power using ground to open) however it could be the opposite of that as well. The biggest thing you are looking for is action.

Make sure you have a good connection for accurate readings. Also compare it too another. Results should lead you to this:

No action #8 - Bad PCM or harness.

Action #8 - bad injector(yes even new parts are sometimes bad) clogged fuel rail, low fuel volume....#8 is the last one on the rail to get fuel right? Prob why the other poster is asking you for fuel pressure.


One note, injectors are held open for a certain amount of time by the PCM. I have seen (only once, and on a car) a bad PCM that did not hold an injector open long enough. You will not be able to see this with a volt meter, you will need a lab scope with a high sample rate.

Remember KISS- Keep it simple Stupid! Use a compression guage first and make sure that cylinder is a compression range of 150-155lbs like other good cylinders. If the cylinder has a compression problem you're never gonna solve the issue with plugs, plug wires, etc.
Don't neccessarily blame turbulator removal as if this were the cause there would be other cylinders involved, not just #8. Most cylinder #8 and #7 valve sealing issues are caused by excessive moisture from running with Captain's call or Silent choice switchable systems, closed during to much idling or moderate rpm runs closed, aftermarket header systems that have water re-intrusion issues or valves hanging open in cold moist enviroments during long periods of winter layup without fogging the engine at layup.
Remember also you could be having a piston/ring sealing problem, Hope not.
Also realize that a normal 496 Mag or HO will pull above 3200 rpms even with one cylinder out of action, so I suspect this is probably a couple of things going on here.
In any event, start with compression test and go from there.

Best Regards,
Ray @ Raylar

Just returned from the boat. Did a compression test on #8 and I was shocked! 160 psi. Reinstalled spark plug and decided to go through the Rinda testing of injectors, coils, etc. Instead of attempting to disconnect both the low and hi pressure fuel pumps, I shut the gas valve off from the tank. Individually tested #8 and well as group tested injectors . In the coil testing got a backfire--I assume from 8 as there was residue fuel squirted in from the rail. Found nothing--no codes or faults. Fired engine back up again and performed the drop cylinder test a half a dozen times and guess what--#8 was now firing and dropped each time I performed the test. The only abnormality I found was a couple of times I left the throttle at 1500 and a few minutes later the rpm would mysteriously rise to 1800 without touching the throttle. A choke issue or was that cylinder cutting off and on again--never happened when I was doing the testing. To say the least, I'm beyond perplexed.....

One other item I forgot to mention is that twice with the engine running at 1500 rpm at the dock, the engine suddenly stopped running just as if you turned the key off. After the ignition was turned off it restarted normally. Something electrical?

May wish to try switching the ECU's to see if the issues follow the ECU assuming everything is fine with the other engine..

Swap two plug wires.

Then swap two coils.

Regarding the engine shutting down abruptly, search around to find a loose connection.

Side note: I know you have good fuel but if you have the latest style fuel system where both, the lift pump and the high-pressure pump mounted in the same black metal box, I'd mount a nice Raycor filter in line before the feed on the box regardless.

Those things are terrible. This will save you a lot of frustration down the road.

I'm going to the boat today and give it the once over search for a loose or chafed electrical connection. I think I'll just replace the #8 coil and will swap out the port#8 plug wire with one off the starboard engine. At that point, retest with Rhinda and see what I have. These things are frustrating--Makes me think how good we all had it with a carburetor and distributor! Thanks for all the helpful suggestions.....