See full version: Mercury 4 stroke efi fuel pump symptoms
If the fuel system is getting air in it, it may run at slow speeds, but stumble at high speeds when more fuel is needed and it can't be pulled in by the fuel pump with an air leak. Cheap suff to replace before taking it in. here
Did you change out the fuel line and primer bulb like the other guy did? It was not mentioned in your post. here
Edit: Also check fuel lines all the way back to the tank. Any pinched line, or cracked line, or poor connection can all give the same problem. [links]
Maryland, does not require the pumps be labeled that ethanol was added.
I don't know much about these engines but the first summer we had it ran like a champ and very quiet. I'm not touching a thing. I've learned my lesson the hard way in the past. We'll see what the next mech says. a couple hundred dollars later.
I have a 2005 Mercury 90HP four Stroke EFI that I purchased on a 17' boston Whaler summer of 2005. Only have about 25 hrs on it. Had the 10 hr maintenance done on it by a certified mech. and winterized end of 2005. Beginning summer of 2006, I've started having quite a few problems with the engine.
Of course, once gas is PS/WC, your only (safe) option is to discard it and start over with fresh fuel -
Forget the "miracle products" that use deceptive marketing to try to convince you they can "fix" bad gas, and make it "good as new again", (they can't - not possible from a scientific, chemical standpoint).
I have a 2005 Mercury 90HP four Stroke EFI . rebuilding my carborators here
MANY people are having problems, mainly because the delivery truck drivers that add the ethanol in at the pump, incorrectly, and added more than 10% - Over 10% will cause both performance issues and later serious engine damage. here
90% of all outboard EFI "fuel" problems, are really ignition problems.
Your ECU is connected to your ignition, and a problem there will cause EFI problems.
If you have a spark gap tester, check at cranking speed for a strong 7/16" open air spark on all cylinders.
If one bank of cylinders shows weak, you might have a bad switchbox. If all cylinders are weak, you might have a bad stator.
An individual weak cylinder might be a bad coil.
Be sure that you're running resistor spark plugs. RFI will cause the ECU to go bonkers.
Check your voltage rectifier for proper function. An erratic tachometer reading is a sign of a bad rectifier.
Unrectified 12 volts will cause the ECU to act up.