Pontoon Boat & Deck Boat Forum

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. Mayb I am wrong. Any other ideas to help address this issue?

Thanks

The Mercury vessel view would be able to tell you what's going on with the motor as far as codes. It sounds like a fuel issue but sounds as if you have already addressed that.

Maybe gas line "liner" is collapsing.?

Have you tried a portable tank with it's own fuel line? This could eliminate part of the fuel system.

bansil wrote: ↑

Mon May 20, 2019 3:32 pm

Maybe gas line "liner" is collapsing.?

with my limited knowledge, I would sure think this could be an issue, especially if he's been running fuel with ethanol in it.. From what I've read ethanol really does a number on rubber hoses and fuel lines.

Especially the "atwood" labeled 2nd's that walmart carried last 5 years or so, I got one about 2 years ago

Thanks for the input guys.

Can someone please tell me what size fuel hose I need and I will purchase about 8 feet of it and replace it all the way to the VST?

Reading this thread gives me flashbacks and feelings of deja vu......Mine was a 90 hp Merc on a 21 ft tracker. Damned thing would run great for awhile and then just flat die. Wouldn't start no matter what for about 45 minutes sometimes. As long as you didn't run the shit out of it , it ran great. If you cracked it for any length of time, it would go face down again.
Replaced fuel lines, bulb, filter, fuel tank cap over the course of a month. All but pulled the tank to see if the sending unit in the tank was the culprit. When the S.O.B. stranded us for the 3rd time I finally got it started , limped to the marina and promptly traded it off for a new boat with a Yamaha on it ...lol

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.

Ask over at thehulltruth dot com in the engine area, basically cut and past the last post you did