If you own a newer Mercury — a Verado 150, or one of the new V6 or V8 engines — you’ve got a Fuel Supply Module sitting on it, and sooner or later it’s going to give you trouble. These aren’t the old-style units. They come apart differently, they go back together differently, and they’re full of small rubber pieces that will absolutely ruin your day if you miss one. I walked my buddy Romero through his first one, and after that he could do them with no problem. Same goes for you after this.
Why It Matters
The FSM is what feeds fuel to your engine. When it starts leaking or the pumps quit, you’re not running. And these things corrode — the copper tubes inside run fuel and water, and down here in the salt they pit and eventually hole through. The one I did had an actual hole in the fuel tube. That’s fuel spraying inside a sealed housing on your engine. Not something you want to find out about at 30 miles offshore.
Dealers charge real money for this job, and most of the work is patience and cleanliness, not specialized skill.
Products
- Johnson/Evinrude Moly Lube, part number 768306 — assembly lube for the tube o-rings. I’ve tried oil and other greases and this stuff goes together the smoothest by a long shot.
- Mercury FSM seal kit — all the o-rings and rubber gaskets. Just count on needing it.
- Replacement copper tubes — order them if yours are pitted or holed.
- Two-stroke oil — for lubing the fuel pump gaskets and grommets during assembly.
- Wire wheel — cleans the corrosion off the tubes.
- Pressure test pump that reaches at least 40 psi — 45 is better.
- Rubber caps and zip ties — block off the fuel outlet ports for the pressure test.
- Bench vise — holds the bottom housing while you stack everything into it.
Step-by-Step Guide
Getting It Apart
Pull the FSM off the engine first. I covered removal on a 150 four-stroke in our first FSM video, so check that one if you need it.
Once it’s on the bench, remove the four bolts holding the housing together. Then pry it apart — carefully. There’s almost always fuel still inside and it’s going everywhere when it finally splits. These are genuinely hard to open, so don’t be afraid to put some real force into it.
Stripping the Components
With it open, start pulling everything out. There’s a top piece that also has to come out, and it gets stuck in there — work it out without damaging it.
Fish out every o-ring and rubber gasket. There are four copper tubes running fuel and water. On top of that top piece you pulled is the fuel pressure regulator — bend the two tabs over and pull it out. It’s going to need cleaning.
Keep going until all the rubber is out of the top piece, then pull the fuel pumps and any copper tubes still stuck in the bottom housing. Count your pieces. Leaving one behind means doing this twice.
Cleaning
The tubes will be heavily corroded. Take them to a wire wheel and clean them up, paying attention to the ends where the o-rings seat. Then inspect for pitting and holes. Mine had a hole in the fuel tube, so we ordered new tubes and a seal kit.
Now the part that decides whether this job works: clean every single surface in all three plastic housings. If there’s any green buildup on a surface where a tube o-ring sits, it will leak, and you’ll be taking the whole thing back apart.
Reassembly
Do this in a vise. There are a lot of parts and it’s a delicate operation.
Start with the bottom housing clamped up. Put an o-ring on the end of an inner tube, lightly lube it with the moly lube, and press it into the housing — spinning the tube as you press so you can feel it seat. Spin it again after it’s seated to confirm the o-ring didn’t roll over.
Do the second inner tube the same way, then both outer tubes.
Grease the o-rings on the top of the tubes. Squirt two-stroke oil on the outside of the fuel pump gasket and insert it halfway into the housing. Push the pump with the wires down into that gasket and make sure it seats. Oil the gasket on the other fuel pump and push it into place. Get the top housing gasket for the fuel pump in halfway.
Now the pressure regulator — all its gaskets on, oiled, and pressed into the housing. It’s a pain. Install the plastic spacer with the o-ring on top of it, get all the pumps and tubes lined up with oil on the gaskets, then wiggle the piece and push down firmly until everything slides into place.
That’s about halfway. Install the rest of the rubber grommets halfway in, same as the pumps. Don’t forget the ring and gaskets on the pressure regulator, the two brown o-rings on the top piece, and the o-ring around the housing itself.
Hook up the fuel pump wiring, and without catching the wires, slide the top housing down. It’s tough to seat — just keep pressing until it slides. Install the four bolts.
Pressure Test Before You Install
Zip tie rubber covers onto the fuel outlet ports and hook up a pressure pump. Get it to at least 40 psi — 45 is better, because fuel pressure climbs above that in operation. If it holds 45, you’re good. I’ve never seen one leak after install that held 45 on the bench.
Pro Tips
Buy the seal kit before you start. Don’t open one of these hoping the old rubber is reusable. It isn’t, and you’ll be sitting there with a torn-down FSM waiting on parts.
Moly lube, not whatever’s on the shelf. I tried oil and other greases on these tubes and they fought me. The Johnson/Evinrude 768306 is what makes the tubes slide and seat properly.
The spin test is the whole job. Every tube gets spun while pressing and spun again after seating. A rolled o-ring is invisible once the housing is closed, and you won’t find out until the pressure test — or worse, on the water.
Final Verdict
Worth doing yourself, with a caveat. There’s no special tooling here beyond a vise and a pressure pump, and nothing requires mechanical finesse. What it requires is patience and obsessive cleanliness. If you’re the kind of person who’ll wire-wheel a tube for five extra minutes to get the o-ring surface perfect, you’ll have no problem. If you rush it, you’ll do it twice.
Budget an afternoon, order the seal kit ahead of time, and pressure test before it goes anywhere near the engine.
FAQ
How do I know if my Mercury FSM is bad?
Fuel delivery problems are the tell — hard starting, stalling, running lean at speed. When you open it up, look for corroded copper tubes and pitting. A pinhole in a fuel tube will cause pressure loss you can confirm on the bench test.
Do I have to replace the copper tubes in an FSM?
Only if they’re pitted or holed. If a wire wheel cleans them up and the o-ring surfaces are smooth with no holes, reuse them. Mine had a hole, so they got replaced.
What pressure should a rebuilt FSM hold?
At least 40 psi, ideally 45. Operating fuel pressure climbs above 40, so testing at 45 gives you margin. If it holds 45 on the bench, it won’t leak on the engine.
Check Us Out!
We hope that this has been insightful and allowed you to understand a little bit more about outboard engine maintenance and what goes into keeping your boat running right.
If you have more questions or want to learn more about your boat and its different systems, consider joining our Boating Academy where we have created HUNDREDS of video courses teaching you basically everything about your boat!
You can also find even more helpful videos on our YouTube channel where we create even more boating videos!
And if you would like to support us to continue bringing you great content, please click the link below to Amazon where we get a commission from anything you are already going to buy! Click Here To Amazon!