2008 Mercury 4 Stroke Fuel Pump Power Issue

New here and so far what a great forum with lots of info. Thinking I may have to subscribe to the online Seloc to keep troubleshooting my issues but it’s good to know it’s there if needed. I have a 2008 Mercury 4 Stroke (Serial # 0R241050) that is having some sort of High Pressure Fuel Pump power issue I think.

Background: I got this pontoon/motor last year and it ran fine up until now. Since I've ran it I've only used 91 octane alcohol free fuel with Star Tron Marine Fuel Treatment. I replaced the old fuel hose from the portable tank to the motor and bulb this year along with replacing the old external fuel/water filter separator.
Background of what happened: Ran the motor for about half hour and then sat in the middle of the lake
for about an hour. Went to go start it and it ran for a second and then died.
After that I tried starting it again and all it would do is turn over. I checked for spark and had spark and check the fuse and it was good. Also checked the battery and it was at 12.6 volts. Eventually was towed in.

Once back I replaced the spark plugs (as a maintenance item), took off the VST and cleaned that and it was really bad (Fuel Pump had rust all over it and was clogging the screen). Hooked the high pressure fuel pump up to a battery to test it and it ran. Put everything back together and still all the engine does is crank.

The next day I checked the fuel pressure rail and there was no pressure. I disconnected the output
on the VST where the pump should be sending fuel into the high pressure side of the system and turned the key and nothing. I then decided to try to jumper the high pressure fuel pump with a small 12v battery. As soon as I connected the little battery fuel sprayed everywhere. Which meant to me that the high pressure fuel pump is good.

I then reconnected the high pressure hose to the VST and then jumpered the high pressure fuel pump to a small battery and the pump ran and a friend turned the key and the engine ran for 15 seconds until I removed the fuel pump jumper wire.

So I’ve narrowed the issue down to I think an electrical issue but I’m not exactly sure as I don’t know enough about boat engines to determine it 100%. Is the high pressure fuel pump tied to anything else to tell it to turn on? Would the pressure regulator valve control the pump at all? I know it opens or something like that when the fuel system is around 40 PSI.

I have the battery right now connected to a charger and its charging and sitting at 12.6 volts still. I read another forum post saying a member had to replace their battery with a similar issue but isn’t 12.6 volts enough? I'm unsure of how old the battery is as I can't find a date code on it but I know it's from Walmart as its an Everstart. I also tried hooking up a jump pack to the motor and it still wouldn't start. I would think with a battery and a jump pack that should be more than enough to eliminate it being a battery issue but maybe not? The post also said to check a service bulletin about 4 stroke pcm ground but I can't seem to find anything on the topic.

<p class="MsoNormal">http://www.marineengine.com/boat-forum/showthread.php?442722-2006-25hp-4-stroke-will-crank-won-t-run</p>

Just looking to see if anyone else has any thoughts on what else the issue could be.


PS: Hope it's not too much info but I didn't want to leave anything out.

Have you checked voltage to pump?

I will give that a try tonight if I can get someone to help me turn the key while I test it.

faztbullet said:

Have you checked voltage to pump?

Click to expand...

I have a similar issue at startup. Intermittent fuel pump failure on 2019 merc 90hp four-stroke. Guardian kicks in. Eventually it starts. Looking for a wiring diagram from my dealer to do some testing as well.
May be the main power relay as well.

I was able to test the voltage to the pump and it was reading 15 volts when cranking.

Going to take apart the VST and see if the connectors inside the VST to the high pressure pump has power

Took the VST apart and tested the connectors inside and they are getting 15 volts as well. Tested the resistance across the pump and its reading 20. Not sure if that's a good thing or a bad thing yet but the pump does run. Still no pressure at the fuel rail. Going to try to probe the high pressure fuel pump connector when the VST is connected and see if I still get 15 volts to that plug.

Someone else had a similar problem I read on the forums and they had to get a better ground. I checked the cable and pinched the connector down more but still the same issue of just cranking.

Final update on my issue. Turns out it was the ECU ground to the computer. I was able to put a small jumper wire attached to the negative side of the plug for the high pressure pump and the engine runs and usually starts right back up. Sometimes it doesn't start right back up and I need to release the fuel pressure from the fuel rail. Once the pressure is released it will start up again.

Hopefully this can help someone that's having similar issues.

I'm just curious, are you running it with a jumper or you fix the ground problem? My is having the same exact problem no ground out of the ecu if there a load on the wires.

I have had this exact issue with my Mercury outboard. The ECU has a fuel pump driver that supplies only the ground circuit (NEG) to the high pressure fuel pump and the only way to fix this correctly is to replace the ECU $650.00 and up .The main question is why does so many Mercury ECU's fail.