Strange intermittent issue with Mercury Fourstroke 90 EFI?

Question

Hi everyone,

I have a Mercury 90 Fourstroke EFI on my boat. Starting 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 this sound like a sensor problem?

Does anyone have any other ideas I can check into? Thank you

Answer 1

There is a service bulletin concerning the FSM (fuel supply module) on some 90 EFIs. The symptoms are almost exactly as you have described. Edit your question with your serial number, I'll see if this pertains to your outboard.

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Yep, that's the SB. It does pertain to your model. I would show this to your tech (I'm surprised he/she did not check for SBs), and have your fuel system parameters checked by a CDS scan (only a dealer will have access to CDS). If your float switch is faulty and giving a "high" reading, the PCM will read this as "system ready", and no fault will be stored in the PCM, nor will the guardian alarm activate.

Answer 2

Could be ignition or fuel. You should go ahead and try the plugs first. I am kind of surprised the mechanics won't take a stab at it since it does act up right after startup. They could get the test equipment set up and should be able to find the problem. It would be fairly easy to find out if it's a spark problem. If it's not spark they could concentrate on the EFI. Problem is most techs are not up for a challenge. They just want to fix the easy ones. You are probably going to need to dealer help to repair it. There are just too many components to deal with when you have EFI.



One other thing you can try is unplug and replug every connector you can find. Shutting off and restarting leads me to think maybe you have bad connection. Look inside the connectors for corrosion and that they are in the right position. I have seen pins pushed back many times. Keep your eyes open and don't miss something easy.

Answer 3

It sounds like the engine is going into shut down or limp in mode.I think that Mercury calls it guardian system.It does sound like a sensor or connection problem.Water in the fuel is usually there until you drain or remove it and bad spark plugs on four strokes are usually just that,bad.They don't just come and go.The best thing to do at this point is to have it scanned at a dealer to see if it set a code.I do remember a problem similar to yours that was caused by a heat sensor.Not that the sensor was bad,or that the engine was overheating,the computer thought the the temp was rising too fast.The problem was called gradient heat.

Answer 4

I have to agree that a problem like this could be extremely hard to find and fix until it becomes a constant problem. It sounds like it could be a little water in the fuel tank. The water will be like a bubble in the fuel tank but will still be picked up and sent through the fuel system. You might try installing a fuel/ water filter in the fuel line. This really don't sound like any type of sensor problem.

Answer 5

Mercury 90 Four Stroke