Mercury 225 efi only runs if plug wire disconnected

[pmcs]

Hi,
I have a 2000 Mercury 225 EFI, S/N OT076229
Can anyone suggest what could be wrong ? Burned up the top Port piston (no oil, low oil alarm sensor cable shredded). Replaced the piston and re-honed the cylinder, but still have problems. Here is the order of events:


1. Honed cylinder and replaced piston
2. Engine will only run for a few seconds. If I disconnect spark plug wire from top port cylinder, engine will now run.
3. Verified that all 6 cylinders have compression and spark. Top Port cylinder appears to be dry, no gas.
4. Separated Intake manifold from engine. Disconnected spark to all cylinders and turned over engine. Verified that all injectors were spraying gas.
5. Inspected top port piston through spark plug hole and verified that cylinder was installed correctly.
6. Pulled Reed plate and inspected. Swapped top port cylinders reeds with another.
7. Using inspection light, compared top port and middle port cylinders and bores from bottom side of engine (via crankcase/reed valve). Looks identical.
8. Using hand, verified that each cylinders was creating a vacuum via the crankcase access.
9. Installed reed plate. Using hand, verified that each cylinders was creating a vacuum via the Reed plate access.
10. Re-installed intake and re-tested. Problem remains.

It is acting like the piston is in backwards (its not), as the spark plug is dry and backfires and dies if spark plug is hooked up .

Frankly, I am stumped and have left it with the local Merc dealer. It will take a couple of weeks for him to get to it. If anyone can think of something that I missed, please let me know.

Thanks,

Eric

[boats.net]

The best thing is start with a computer diagnostic, this should help narrow the problem. If all injectors are firing, then that cylinder should not be fuel deprived.

[pmcs]

and since I don't have the ability to read diagnostic codes, taking it to the dealer is probably my only option (don't feel so bad now...)

I will re-post when I find out what is was, but I still say it has to be the wrong piston. That is the only thing that would keep fuel from reaching the cylinder...

Eric

[pmcs]

Resolution to the problem...

Got a call from the Merc dealer yesterday and went over to check out what they found...

Seems the problem with engine not running with #2 plug wire hooked up was a bad/ wrong plug. Didn't want to use the same plug from the damaged cylinder, so used one for my wife's Merc 150. Dealer tried the plug in #6 cylinder and same result. Changed all plugs and engine runs, sort of...

Engine is now firing on all 6 cylinders, but is rough idling around 4000 RPM !
We notice that most of the vacuum lines are broke/leaking behind the oil reservoir (port side) and the electrical box (Starboard side). Dealer also mentions that the bad cylinder could be egg shaped, which in Mercury's can have the effect of great compression, but weak vacuum on the intake side (causes fuel starvation for that cylinder).

So, we decide that I am going to take the boat home do some work myself. I replace every vacuum hose with new hose and start it up. Still idling at 4000 RPM, but smoother.

I start checking out the Throttle position sensor, taking voltage measurements at different throttle openings, when I notice that the intake has a corner of the gasket sticking out right where the atmospheric sensor (airflow sensor) is mounted. I re-start the engine and start feeling around the intake. Seems the gasket is not seated properly and I can feel vacuum on both side of the front of the intake.

So, here is what is happening, with the gasket not working, air is leaking in everywhere and throwing off the air flow sensor, causing the EFI to over idle.

Re-seating the entire intake plenum gasket fixed everything.

So, I hope you folks can learn from my basic mistakes. and thank you for you help along the way.

Eric