2004 Mercury 115 4 Stroke EFI problems

Aquahound

Cadet
Joined
Aug 5, 2007
Messages
10
Im at a loss and hope someone here can help. When I first go out and run, for any length of time, the engine runs fine...right up until I throttle down. After I come down to idle or neutral, it starts running rough. When I try to throttle back up, it's sluggish and takes about 30 seconds for it to fully kick in. It'll do that all day until I go back to the dock and let it sit for a while. So far, I've changed all filters, changed the plugs, changed the primer bulb, and ran Sea Foam through it using an auxiliary tank. Using the auxiliary tank served 2 purposes....it also isolated the problem to the engine. What could this be?
 

Aquahound

Cadet
Joined
Aug 5, 2007
Messages
10
This is a follow-up post to inform of the results in case anyone else searches this problem in the future. It was not the control valve. If you begin to see any sort of rust or corrosion on a spark plug, you have a major problem. I've been seeing this for a few years and never thought anything of it because the engine seemed to be running fine. That is, up until I had the problems that predicated this thread. Right after I posted this thread, I found my oil to look like a cappuccino. Not good. Turned out, I had a bad seal that slowly allowed sea water into the #4 (bottom) cylinder. Luckily, we caught this just in time to avoid having to buy a new head, but it still required some drilling and welding. Working with a mechanic friend, the project still came at a cost of $2500. Dropping the boat off at a shop could have easily cost me double that. So lesson learned for anyone with the same problem...if you have an unexplained rusty plug (mine was bottom plug), seek a professional immediately!
 
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