Answer
Sep 21, 2025 - 11:53 AM
When you're swapping out the water separator filter on your Mercury 90HP, you're likely gonna get some air in the fuel system — happens all the time. So here's how you bleed it out:
- Prime the bulb – After replacing the water separator filter, grab the fuel primer bulb and start squeezing it.
- Squeeze until firm – Keep squeezing until the bulb feels nice and firm. That usually means fuel has made its way back through the line and air's been pushed out.
- Check for leaks – Double check your connections on the separator — especially the O-ring or gasket on the filter itself. One tiny air leak will give you nothing but hard starts and rough running.
- Cycle the key (EFI models) – If you've got an EFI engine, turning the key to the ON position (without starting) will pressurize the fuel system. Do it a few times to help purge any remaining air.
- Start it up – Once it’s primed and sealed, go ahead and fire it up. It might stumble for a second if there’s still a little air left, but should smooth out.
Of course, that's just a general idea of it. But the full procedure can be found in the service manual of your exact model. If you'd like, feel free to reach out to us with more details about your Mercury 90, and we'll be happy to assist you further.
