Evinrude VST Water Cooling Issues (150HP)


Almost A Member
  • Feb 20, 2018
    Whaler 25
    For those who run Evinrudes, the Vapor Separator Tank (VST) is water cooled and there have been issues with the tank that is now replaced with a new style. I'll describe the issue I had and what the problem was.

    When I got the engines the stbd one would idle fine and within the first minute of high speed operation it would die. I found fuel mixed with air bubbles at the high pressure test port. I'd use the primer bulb and the VST would refill. When I'd cycle the key clear fuel would come out. When cycling the key again aeriated fuel would come out of the high pressure port.

    One thing was clear, air was getting into the fuel supply somehow. I used clear tubing after the primer bulb and saw no air leaking through the fuel supply lines. Air was entering somewhere downstream of that. When I had the engine running on the garden hose I noticed a milky fuel mix discharging from the high pressure fuel schrader valve.

    I tore down the VST and discovered that the o-ring seat where the water cooling fittings connect to the coil tubing had corroded and when running, would leak a little bit of water into the VST. When you just turn the key with no water hooked up there would be enough air pulled through the system that the float vent would drop allowing air from the vapor hose/vent assembly and water coolant side, and then the engine would starve.

    That's why the engine would run on idle, then die after getting up on plane. The fuel amount in the VST would be used up and air/water would replace it. Restarts would be hard until the VST would fill back up with fuel.

    I resealed the o-ring seat and all was well for 3-400 miles. I had a check engine alarm come on and water was in the fuel filter of the stbd engine but not the port engine. Water wasn't coming in through the tank so it has to be that water cooling line. I thought to myself about why they cool the fuel there in the first place. The warm fuel coming back from the injectors plus ambient heat cause fuel to create vapor. To reduce the vapor the VST bowl is water cooled. I decided to try an experiment where I bypassed the VST and connected the cooling hoses together with a 3/8" SS tube. That way there is no water going through the VST.

    We are in pretty cool water anyway so vapor is less of an issue compared to somewhere super warm. Basically bypassing that VST hasn't resulted in any issues for 70+ miles so far. It is important that the cooling lines aren't plugged because that water flow cools the computer as well. Connecting a union tube seems to have solved this issue and so far I haven't seen any drawbacks.

    So far things have been good with the bypass hack. Hopefully this doesn't cause any negative issues and if there are similar problems then hopefully this helps.
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    I Should Upgrade My Account
    May 8, 2006
    So cal
    18.5 Wahoo cc , Evinrude Etec V6 135 HO
    That’s good info for Etec guys! Maybe share that at etecownersgroup website.all Evinrude stuff. Help more peeps. Thanks for sharing.
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