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Mercury FourStroke 90 EFI - Strange Intermittent Problem

S

sprawldog

New member
Hi everyone,

I have a Mercury 90 Fourstroke EFI on my 2008 170 Montauk. About a month ago I have had an intermittent issue which occurs every few days within the first 5-10 minutes of running. Engine starts up and sounds fine and as I leave the harbor the motor completely loses almost all power at full throttle and barely moves. The engine does not rev high and sounds very rough almost if it is only running on one cylinder. If I leave the throttle in the WOT position, within 5-10 seconds, the engine seems to recover and full power returns, runs perfectly for the rest of the day and the issue never happens again for a few days. Shutting the engine off and restarting it seems to correct it as well. I run the boat probably about 100-150 miles a week and after this initial episode within the first few minutes the boat runs like new. During the episode I never get any warning alarms.

I have changed the fuel filters, tried an engine cleaner, changed fuel tanks just to rule a few things out and none of them have made a difference. I bought the boat used last year and do not know when the spark plugs were changed last. I spoke with a couple of Mercury mechanics who state it would be difficult to diagnose until whatever the problem is becomes constant. The strange thing is is that it starts up quickly, does not smoke, or has not lost any top end speed.

Does anyone have any other ideas I can check into? Thank you
 
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You may have gotten some contaminated fuel. Do you have a fuel/water separator filter? It should have a 10 micron element. Do you use a fuel preservative that prevents water separating out in the tank?
 
guyjg said:
You may have gotten some contaminated fuel. Do you have a fuel/water separator filter? It should have a 10 micron element. Do you use a fuel preservative that prevents water separating out in the tank?
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Thanks for the reply. I do not have a fuel water separator however I do have a 20 gallon fuel tank and use approximately 40 gallons of fuel a week. I usually run the tank to about half full before refilling it. So I would say the fuel is usually fresh. I did try to use an engine/fuel cleaner in the last 20 gallons I used with no effect. The problem has gone on for about a month or so.
 
MAP sensor starting to go would be my first guess - but that really is just a guess.

You can try unplugging it and see if the problem goes away (anytime a sensor fails completely or is disconnected, the ecm "pretends" it's there and fakes readings it expects to see. Running it disconnected would cause the ecm to record a "fault" for that sensor, but after connection it will eventually clear or can be cleared by a DDT at a dealer).
 
There may be some dirt in the system. This is what you should use. http://www.marineengine.com/products/accessory.php?in=3672052 Get the 10 micron filter because you have an EFI engine. Keep an extra filter on board. There is also the type w/a clear view base and water drain. You'll need 2 nipples. Mount it level w/the top of the fuel tank and route the fuel line so there isn't a dip in the line. Lube the "O" ring on the filter and prime it w/fuel before installation. I change mine annually. With the quantity of fuel you use consider changing twice a year.
 
galamb said:
MAP sensor starting to go would be my first guess - but that really is just a guess.

You can try unplugging it and see if the problem goes away (anytime a sensor fails completely or is disconnected, the ecm "pretends" it's there and fakes readings it expects to see. Running it disconnected would cause the ecm to record a "fault" for that sensor, but after connection it will eventually clear or can be cleared by a DDT at a dealer).
Click to expand...

Thanks for your reply. I have never heard of a MAP sensor but I googled it and finally found the first post online from someone who describes a very similar problem. I am not sure what the sensor even looks like or how to disconnect it but it is a start. If I were to disconnect it would there be any risk to causing damage to the engine by running with it disconnected? Thanks again, this has been driving me nuts
 
There may be some dirt in the system. This is what you should use. http://www.marineengine.com/products...php?in=3672052 Get the 10 micron filter because you have an EFI engine. Keep an extra filter on board. There is also the type w/a clear view base and water drain. You'll need 2 nipples. Mount it level w/the top of the fuel tank and route the fuel line so there isn't a dip in the line. Lube the "O" ring on the filter and prime it w/fuel before installation. I change mine annually. With the quantity of fuel you use consider changing twice a year.




Thanks for responding and the link, I will look into possibly installing a filter. Have you ever had any issues with yours? I am a bit hesitant to install something that didn't come with then engine.
 
guyjg said:
Filter? No. I wouldn't run my boat engine w/o one. Millions of them are in use around the world.
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I had just listened in the past to a few friends who complained that sometimes when they were added aftermarket there was potential for fuel restriction issues in some of the newer engines. Thanks for responding
 
I just checked all of my plugs and I am starting to think it is a sensor of some sort. Anyone have any opinion on the condition of these plugs? Bad enough that it could be source of issue? Thanks in advance IMG_7672.jpg
 
when they were added aftermarket there was potential for fuel restriction issues in some of the newer engines.
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Ask your local Merc. dealer what his opinion is about them. Your plug looks worn...electrode is rounded off in the picture. I would replace them just because of the quantity of fuel you consume and because they are cheap...you want it to burn the fuel w/o wasting a drop.
 
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