Mercury 50hp EFI Intermittent Rev Drop

[MC01]

Hi all,
Looking for some advice. My 2007 Mercury 50hp EFI has an intermittent issue where it suddenly drops to 0 RPM, then comes back alive with a single beep (same as the system check tone) within about a second. Sometimes it doesn’t come back and I have to shift to neutral and cycle the key, but it always starts fine. The issue usually happens 10–20 times over a few minutes, then might not occur again for 5 minutes to an hour — it’s very inconsistent.

I've traced the cause to the main relay 'blipping' off and back on. This briefly kills power to the spark plugs, fuel pump, injectors, and other systems. From video analysis, the dropout always lasts exactly 66 milliseconds.

I’ve had the fault codes read — the only code that appears is a Fuel Pump error whilst the engine is in a dead state, which makes sense as the pump loses power during these events. However, I haven’t had live diagnostics running during the actual fault yet.

It feels like an electrical issue, possibly something causing the ECU to momentarily shut off the relay, but I haven’t found the root cause yet.

What I’ve tried so far:

• Fresh fuel (drained and replaced)
• New fuel filters (both)
• New spark plugs
• Checked/wiggled all connectors and wiring for corrosion or looseness
• Replaced the main relay

Any suggestions or ideas would be greatly appreciated!

Video 1:*https://youtube.com/shorts/Vb5vJAXDxd0?feature=share
Video 2:*https://youtube.com/shorts/8-sk4o0IdqA?feature=share

[Fenlander]

>>>However, I haven’t had live diagnostics running during the actual fault yet.

Hello and welcome to the forum. A very frustrating fault. I am not experienced in larger efi outboards but very familiar with car diagnostics. Reading the live data as it faults would be very informative.

[spartacus]

You say you’ve changed the fuel filters, does that include the one in the VST (vapour separator tank)? Useful video on Dangar Marine here. Looks like there’s a couple of filters on it. https://youtu.be/eVdJ4BvRVYQ?si=99MfwzWklCDalz2k

Pity their diagnostic software isn’t more affordable. Mercury M.E.D.S to run on windows 10 with cables etc is £800. Similar to Fenlander, I’m experienced with car diagnostic software on VAG cars and BMW and live data would pinpoint it, or certainly narrow it down.

[MC01]

Thanks both.

I haven't touched the VST filter as I was warned its a bit of a tricky part to get back together but maybe something to consider.

Frustrating with the diagnostics for sure as I can't just trigger it with any action meaning it has to just sit running until it happens which gets costly in time at the service centre. Worth considering again though.

[spartacus]

It's going to be a process of illimination otherwise. The VST looks interesting, in the respect there's two filters in it. If the gause mesh is clogged, then that could be culprit. If you're spanner handy, then it should be straight forward. Work clean, and photograph if unsure for reverse fitting. You might need a gasket to rebuild.

Are you able to check HT leads? Go around the engine and check all the electrics. Outboard manufacturers don't used tinned wiring, so anything that looks oxidised, take a closer look, wire brush, seat and tighten. You can use a multimeter to check continuity too.

[spartacus]

At the end of that Dangar Marine video, Stu references the low pressure pump. There’s nylon washers on it which were worn. Again, worth investigating.

[beamishken]

Without running diagnostics your realy shooting in the dark. Stored faults only give a little of the story, the fuel pump fault could be many things from a blocked filter to a faulty ecm, just pulling things apart without a running diagnosis you risk inducing extra faults. My advice would be get the required diagnostic kit first & see what it throws up.
Rinda technologies seem to do a reader for around £440 plus shipping & whatever import taxes are applicable nowadays
Rinda stuff used to be good but no idea on the newer stuff.
I'd be surprised if you couldnt find a laptop lead & pirate software a lot cheaper with a little internet searching, someone usually manages to clone these things after a few years if not sooner