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1995 5.7 Mercruiser EFI

D

drummondgang

New member
Hello. Our situation is that engine will start no problem, you can go 3000 rpms for about 5-10 minutes then all of a sudden it quits like you turned off the key. It will restart and you can do this again and after a third time you are dead in the water. We were able to idle or very low rpm our way back to the ramp. Changed mechanical fuel pump, still happened. Added electric fuel pump.....still happened. I think I have now tracked to down to having to do with the Vapor Separator Tank. I am not familiar with this system and not sure why there is an additional electric fuel pump inside this unit. Any help or explanation would be greatly appreciated. We have more than a month to try to get it in to our local shop. As many of these parts are either no longer available, obsolete, and very expensive, I would rather find the actual problem rather than just start replacing expensive parts. Thanks again.
 
An EFI uses an electric fuel pump not a mechanical one
The VST has a high pressure pump and filters inside it and is used for MPI motors
post the engine serial number
 
Thanks for the reply. Engine # OF360552. It is an early system that appears to have fuel injection into the what I would call a throttle body. Yes, there is a high pressure pump inside the VST. Thanks again.
 
drummondgang said:
Thanks for the reply. Engine # OF360552. It is an early system that appears to have fuel injection into the what I would call a throttle body. Yes, there is a high pressure pump inside the VST. Thanks again.
Click to expand...
I think on these early ones there was a low pressure mechanical pump feeding a VST tank which essentially was a float bowl with a submerged fuel pump. not sure if they were trying to mimic the in tank fuel pumps commonly used in automotive applications. @pwskicat on here had some issues with his on a similar vintage TBI . He may be able to offer some assistance

Couple of questions - what makes you think it is fuel related and not electrical? have you measure fuel pressure or look for fuel out of the injectors when this happens? Have you verified you have spark when this happens with a spark gap tester ?
 
Was it pretty hot outside during the events?

If it is truly a VST issue of getting to hot / vapor locking then the best way to try to resolve that issue is to reroute the fuel return line that runs from the VST to the fuel filter housing assembly and run the line safely back to the fuel tank itself however possible you could engineer.

Vapor Separator Tank & Fuel Lines for Mercruiser (5.7l (Alpha) (Efi-Tbi)5.7l (Bravo) (Efi-Tbi) Engine) Engine

www.marineengine.com www.marineengine.com

Fuel Pump & Fuel Filter for Mercruiser (5.7l (Alpha) (Efi-Tbi)5.7l (Bravo) (Efi-Tbi) Engine) Engine

www.marineengine.com www.marineengine.com



Mercruiser (5.7l (Alpha) (Efi-Tbi)5.7l (Bravo) (Efi-Tbi) Engine) Engine Components

www.marineengine.com www.marineengine.com
 
I think Hardin makes a reducing T made to go into fuel tank fill like so you can easily splice a vapor or fuel return into any tank
 
Yes, I live in AZ and it's generally between 102 and 110 even out on the lakes. I guess my main theory is if it is overheating, is there a sensor that is shutting the engine down? Could the float inside the VST be bad not allowing enough fuel in?
 
drummondgang said:
Yes, I live in AZ and it's generally between 102 and 110 even out on the lakes. I guess my main theory is if it is overheating, is there a sensor that is shutting the engine down? Could the float inside the VST be bad not allowing enough fuel in?
Click to expand...
yes could be or vapor is returning to filter housing. in automotive applications the fuel line return goes back to tank. I suspect plug and play for merc with boat builders meant then cant require a different tank so single line hook up...

I think you need to determine am i missing fuel or spark. defintiely temp could be an issue there maybe leave blower on full time to see if it helps.

look down throttle body when it happens to see if fuel is being injected when it wont start. if you can access the schrader on fuel line to tbi vent it with small screw driver - if gas comes out instead of vapor you are not vapor locked. of course be careful as this is a flame hazzard

also test spark with spark gap tester when this happens should be able to jump 3/8" at least nice blue spark that you will hear crack.

have u tried opening throttle wide to restart if the injectors are leaking fuel into intake after shutdown could be flooded opening throttle could help to clear it
 
Yes, there is a strong spark to the plugs. I really don't think it is a true vapor locking situation as it happens after only being run for about 10-15 minutes. This is why I think that something is shutting down the fuel system. I will be visiting family over the weekend and may not be able to reply in a timely fashion until Sunday or later. Thank you all for your input.
 
you wanna make sure the spark is STILL present when the engine quits....

Getting some fuel pressure measurements would help as well....

With the early ECUs, the ECU turns on the Fuel pump based on detecting engine rotation via pulses from the distributor...so its not always an easy task to ID the root cause...
 
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