Mercruiser 7.4 Suspected Vapor Lock (VL)

[kwells114]

I own a 1997 Cobalt 220 with a Mercruiser 7.4LX EFI/MP engine. Mechanical fuel pump and Vapor Separation Tank (VST), does not have cool fuel. Love the engine, but on a hot day (central AL) after driving a while, stop for lunch or to swim, it will not start up. Does not happen in fall, winter, or spring.

I have seen the Mercruiser fix of adding a $1k+ electric fuel pump between the fuel tank and mechanical fuel pump, and read the Mercruiser maintenance bulletin about running the fan, opening the engine compartment, throw cool water on the fuel pump / fuel line, ect.

In the forums (iboat and others), I have not found anyone state that what they did fixed this problem for good. There is a post on iboat from 3 years ago with the same engine (my serial number is 100 units from that one), but no definite "this fixed the problem".

Has anyone had this problem and installed a fix for the VL issue on this engine?

Thank you for your help with this issue.

 

[alldodge]

Welcome
This is the fix

The issue is doing it to a VST is much harder, but can be done

Another quick fix to get you running is use a baggie, a rag and a screw driver. When the problem starts take a rag and place around the schrader valve on the fuel rail. Press in on the valve to release gas and vapor out of the rail. Might need to cycle the key a couple times to get vapor out. Once started place rag in baggie

 

[cyclops222]

The right side system constantly is pumping cool fuel from the cool tank fuel.
The other fuel system on the left side is circulating HOT and MORE HOT fuel all the time. Terrible for hot weather running of the engine. DO NOT use that system.
No average mechanic will find the hot weather problem by the time he gets the boat.
Some vehicles use INSULATED fuel lines. Others use a CONSTANTLY high rate of cool gas tank fuel circulating to supply enough cool fuel to the engine.

 

[kwells114]

Thanks for the diagram. I saw that on other posts.

Do you have a diagram for the VST flow? I have not found one. Wouldn't dumping to the tank from the VST loose pressure to the the rail.

Have not seen the fuel line Shrader valve anywhere. On cars which I've done fuel injector work it's at the far end of the fuel rail, opposite side of the fuel pressure sensor. On this 7.4 fuel rails and injectors are all tucked under the intake manifold. Can't get at it unless the intake is removed, unless I'm missing something.

 

[kwells114]

There are no cool fuel components on this engine.

 

[Fun Times]

Thanks for the diagram. I saw that on other posts.

Wouldn't dumping to the tank from the VST loose pressure to the the rail.

No, pretty much the main reason Mercruiser routes the return line to the VST or fuel filter housing is convenience to keep the engine as independent from boat components. as much as possible to run on its own since a boat builder is separate from an engine manufacturer. One less component to contend with is all for this one… Return the fuel to the tank is usually the better way of a few options and possibly one of lowest cost options to try.

Have not seen the fuel line Shrader valve anywhere. On cars which I've done fuel injector work it's at the far end of the fuel rail, opposite side of the fuel pressure sensor. On this 7.4 fuel rails and injectors are all tucked under the intake manifold. Can't get at it unless the intake is removed, unless I'm missing something.

The Schrader valve should be in there, just got to get your eyes up under there to see it better but can be challenging to get at with big hands.

Schrader valve Item number 13 and 14,

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[kwells114]

I'll get back to this thread after I try a couple things on the boat this week to say what worked and/or not worked to solve this heat soak / VL issue.

 

[tpenfield]

The guy that I sold my Formula 330 to ( it had 1996 7.4 MPI engines with VST ) was getting vapor lock (Mississippi . . . so hot weather), whereas on Cape Cod (MA) I had no issues.

IIRC, he did a couple of things . . .

-> Cleaned out the VST system (I guess there is a filter in there and after 30 years things get gunked up)
-> Insulated the fuel lines (not sure how)
-> He might have replaced the anti-siphon valve on the tank.

Anyway . . . I also would run the blowers while the engines are on to get more air flow and even run them for a couple of minutes after you shut the motors down while anchored, etc.

 

[cyclops222]

When first stopped. Raise the hatch fully. Hot air rises.

 

[kwells114]

When first stopped. Raise the hatch fully. Hot air rises.

Thank you. One of the half dozen things I'm going to try to solve this. Raising the hatch is possible, but not functional with the family wanting to swim off the back swim deck because the entire back of the rear bench seat lifts up to access the engine, blocking the swim deck.

I'll update before the end of the week.