Fuel Vapor Lock
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Fuel Vapor Lock

  1. #1
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    Default Fuel Vapor Lock

    I know this forum isn't for this type of boat, but some of the people on here know the Gen IV big blocks better than anyone.

    My brother has a boat with twin 454's that vapor lock after sitting for about 30 minutes to an hour. The boat has forced ventilation through two blowers and adding extra ventilation would be very difficult without hurting the looks of the boat.

    The engines are stock 1998 Mercruiser MPI inboards.

    Any suggestions on how the vapor lock can be prevented or removed easily? If possible, he would prefer to not open the engine hatch which is also the floor.

    Thanks in advance for any insight you can offer.

    Carey

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    Off Road Badass Bonzen's Avatar
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    Has he tried to just prime the fuel system before starting it (key on/engine off). That works about 50% of the time. Just cycle the key a few times to force fuel into the system and push the air/fuel vapor up against the injectors.

    If that doesn't work, there are is a myriad of issues. Could be where the fuel line and pump is mounted in relation to the gas tank & pickup tube. Could be an anti drain-back valve is letting the fuel run back into the take and not keeping the line pressurized. Could be the location of the line above the motor, and getting all the heat soak. Could also be a air leak into the fuel line.

    Check all the lines for leaks, cracks, damage. If they are old, replace them. Use J1527 USCG approved line. And if they are routed above the motor, wrap them with heat shielding or re-route them to a cooler place.
    "I'd love to help you out wags, but there is larger issue concerning breast cup sizes on another thread that we have to work thru first. This subject suddenly became a non issue." - GN7

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    Bonzen, thanks for the reply. I thought I set up the thread to alert me when a reply was provided.

    He can hit the Schrader valve on the upstream side of the fuel rail and get fuel to spray out so he knows it's not a leak down issue.

    The engines are in a V-drive setup and the fuel rail slopes uphill from where the fuel enters. The only way to prime the system to the injectors is to vent the vapors, but the fuel rail is plastic so tapping into the end of the rails to install any sort of return line or vent line with isn't practical.

    As stated above, there are no fuel leaks and the pump and pressure regulator are mounted near the bottom of the engine. The primary issue is that the engine room isn't ventilated well enough. When we last spoke, I recommended installing a vent into a storage locker that is always open when the boat is rafted up and use a bilge blower to blow fresh air across the top of the engines to keep the fuel rails cool. He'll probably get this done this weekend when he returns home.

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    Senior Member wagspe208's Avatar
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    What makes you think it is vapor lock?
    EFI, injected.... should be over 35 psi.

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    Senior Member orngxtc's Avatar
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    I have the same issue going on in our 94 Howard with a 454 Mercruiser. When it is warm out and you shut down and don't open the engine cover is when it always seems to happen. Mine has a vapor separation tank that sits directly over the intake manifold to the side of the distributor. It uses a mechanical pump to feed the VST which is where the electric pump resides. I have done quite a bit of reading on my years issue where most people recommend remote mounting a electric fuel pump on the transom away from the intake. I recently insulated my VST tank and the hard line feeding it from the mechanical pump. So far this year we have not had a issue. I realize my year is quite a bit off of your brothers but just throwing out their what I have done. Could some type of return line be set up to keep fuel flowing thru the system?
    .

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    Off Road Badass Bonzen's Avatar
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    Quote Originally Posted by orngxtc View Post
    I have the same issue going on in our 94 Howard with a 454 Mercruiser. When it is warm out and you shut down and don't open the engine cover is when it always seems to happen. Mine has a vapor separation tank that sits directly over the intake manifold to the side of the distributor. It uses a mechanical pump to feed the VST which is where the electric pump resides. I have done quite a bit of reading on my years issue where most people recommend remote mounting a electric fuel pump on the transom away from the intake. I recently insulated my VST tank and the hard line feeding it from the mechanical pump. So far this year we have not had a issue. I realize my year is quite a bit off of your brothers but just throwing out their what I have done. Could some type of return line be set up to keep fuel flowing thru the system?
    .
    Any fuel pump that is mounted up high by the intake is not getting a good gravity feed from the tank, and is exposed to hotter air between the engine and the inside of the hatch cover. I'm not saying it wont work, but it isn't ideal. If you can re-route the feed line and pump to a lower area and/or cover the fuel feed line with a heat resistant liner, that would help.

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    Off Road Badass Bonzen's Avatar
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    I use the silver colored silicone liner with the fiberglass style inside to line my fuel feed lines. Same stuff used to protect high pressure hydraulic power steering and transmission lines, as well and spark plug wires.Click image for larger version. 

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    "I'd love to help you out wags, but there is larger issue concerning breast cup sizes on another thread that we have to work thru first. This subject suddenly became a non issue." - GN7

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    Wagspe208, we don't know for sure, but the engine room gets very hot after the engines shut down. It gets even hotter if the generator is running. The engine on the generator doesn't put out much heat, but the electrical portion spits out lots of heat.

    After sitting for a few hours or with the hatch open for 30 minutes, the engines start fine. Vapor lock is the only thing that makes sense. He has good fuel pressure at the Schrader valve on the upstream end of the fuel rails, but no way to purge the vapors behind the valve.

    It would be nice if someone made a fuel rail that would allow him to purge the vapors to decrease cranking time to start.

    A fuel rail with a water line alongside it to keep it cool would work even better. Then he just needs a small water pump to circulate water.

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    Junior Member blue2184's Avatar
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    Maybe a phenolic spacer under the throttle body to stop the heat soak? Just a thought? Not sure what the engine set up is...


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    Not sure how a spacer will help in this instance. The two engines are in a typical cabin cruiser engine room (below the floor) that doesn't have enough ventilation to keep temperatures from getting too high. All of the air in the engine room is hot and will heat up anything in there. He'd have to wrap the fuel rails to insulate them, but there isn't much room for insulation between the intake and fuel rails.

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    Junior Member blue2184's Avatar
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    It gets pretty hot under the hood of a vehicle to... you would be surprised what it would do to get the throttle body of of that 160-180 degree intake... anywhere you can stop the heat soak, and they are pretty cheap to try...


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    Off Road Badass Bonzen's Avatar
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    Quote Originally Posted by blue2184 View Post
    It gets pretty hot under the hood of a vehicle to... you would be surprised what it would do to get the throttle body of of that 160-180 degree intake... anywhere you can stop the heat soak, and they are pretty cheap to try...


    Sent from my iPhone using Tapatalk
    I'm running a phenolic spacer in an under hatch engine setup. I think I paid $15 on eBay for it, fits nice with no leaks. Definitely helps with some upward heat soak from the manifold to the Throttle body.

    I still think moving the flexible sections of the fuel lines away from the top of the engine, and the fuel pump if its mounted up there and lining the fuel lines with a heat deflective material would help greatly.
    "I'd love to help you out wags, but there is larger issue concerning breast cup sizes on another thread that we have to work thru first. This subject suddenly became a non issue." - GN7

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    Boatbroke I apologize I did not mean to jack your thread earlier, I was was just referring to the issues I have had. On another note it may be a long shot but what is the possibility to just install another blower motor and blow air over the top of the intake manifold. This is what I am going to try next.
    John

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