2008 Merc 90HP 4 stroke runs great, then loses all power.

Ok last year the engine (Mercury 90HP 4 stroke) would out of the clear blue, lose massive power and fall to around 3K RPM or so. Then it would recover on its own and run great. I was thinking it was the infamous VST float switch, like I have seen before. But yesterday, I replaced the float switch, the engine ran perfectly for 2 hours straight, and then I lost power again approaching the dock to put it up. Damn!

How can the engine run so hard and long the way it should, and then fall on its face like that...then recover and run fine again? I replaced the fuel filter and water separator last year, but i will go ahead and do it now. Plugs were replaced last year. Would be real nice if I could simply diagnose down to a fuel or ignition issue.

Is there anyway to retrieve the trouble codes from a Merc 4 stroke outboard? I dont see how gas could do this. If it were bad gas, I would think the motor would run badly all the time. Maybe I am wrong. Any other ideas to help address this issue?

Thanks

When it loses power. Did you try to squeeze the primer bulb to see if it is a fuel issue? It's possible your lift pump is faulty.
You cannot retrieve codes unless you have a program to do so.

Boat is not equipped with a squeeze bulb at all.

Kind of wondering if the fuel line from the tank is collapsing. Can someone please tell me what size I need to purchase and I will replace the line all the way to the VST?

Thanks

I doubt your fuel line is collapsed. But if your going to buy parts. Buy a primer bulb and test to see if your lift pump is bad.

Here is an update:
Motor ran perfect 99.99% of the time over thew holiday weekend. But at random times, power would fall off to "precisely" 4K RPM each time. It seemed to consistently drop to 4K which I think might mean something of value. If there were really a fuel starvation issue, would it drop to precisely 4K? I might expect the RPM drop to be more random, or even cause the motor to turn off. Hmmm wonder if its possibly a coil dropping?

Here is a new symptom that I observed. I can start the motor for the first time, idle out of the marina, and as soon as I go WOT, I get 6 beeps from the computer, through the beeper unit. When this happens, the motor does its random power loses for about 10 seconds or so, then straightens up and runs perfectly.
From what I can tell, 6 beeps indicates one of the following three sensors are not happy:
-Throttle position
-Manifold absolute pressure
-Manifold air temperature

Some questions.
When this 90HP 4-stroke motor goes into "guardian" or safe mode. Any chance the RPMs drop to 4K RPM? I can only really see a throttle position sensor issue forcing "guardian" mode, and not the other two. If the other two were to fail, the motor could still run on baked in settings, but not so much the TPS - IMO. Im just wondering if the power fall off is really the motor temporarily simply going into guardian mode, and not something worse.

I also removed the VST/FSM and disassembled it to look for debris or other contaminates that could be clogging the high pressure fuel pump to the injectors. I saw nothing unusual at all other that some very minor varnish on the cooling coil. Since the float switch is brand new, I can see the VST/FSM being the cause of the issue unless one of the fuel pumps is occasionally dropping out. But given the small frequency at which the issue occurs, I dont think that is the case. I also replaced the two fuel filters and the water separator canister for the Hell of it too.

I really would like to have the PCM scanned, but the trailer is 100 miles away from the boat, and going to get it and taking it to the dealer, is last resort. I think I would throw a TPS at it first, before going to all that trouble too.

Thoughts?

Thanks all.

I have a 2005 Mariner EFI 60 4 stroke with a similar failure. It starts immediately every time and runs smoothly. When I go out of the Marina full speed, it after 5 minutes drops the RPM to 4K and sometimes much lower but never stops. Yesterday it dropped to 4K for 15 sec and then went to 5K and again to 4K. This 2 - 3 times. I have measured the fuel high pressure in the fail situations and it is 42 psi(filters are all new, air inlet tank filter is cleaned). It think, it must be an electrical failure. All electrical ground connections are now cleaned. Tested the MAT sensor ok. Next I will test the TPS sensor.

Guardian mode will set rpm to around 2000 rpm and an alarm will sound. Are you sure a cylinder isn't firing. Or water in the fuel isn't the problem?

The fuel tank has been emptied and cleaned and now with new fuel. The alarm unit is ok but there is no sound. A local dealer also recommended to look for a defect coil.