1997 502 MPI Issues - Vapor Lock?
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08-07-2008, 09:55 PM
#32
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hey folks............i am having VERY similar issues on my 1994 454 magnum MPI / EFI. it has the old style VST on the rear of the motor. please check out my post from another site. i copied and pasted it for anyone's review. thanks. it's a very long post. sorry.
ok gang...........after 6 more hours of trouble shooting today, it comes down to this......the guy from mercury was there for 5 hours of it.
fuel pressure reads good on the mechanical and electrical pumps (running on the boats gas tank OR a separate gas tank with inline primer)(makes no difference which fuel source is used. the problem still occurs).
after about 15-20 minutes on the hose, pressure on the high fuel pressure pump starts to drop off (its good for 15-20 minutes) and it eventually quits running shortly thereafter. we physically primed the low pump when high pressure would drop and the high pressure pump would recover (get fuel).
air or vapor was discovered after removing the plug on top of the VST. after allowing the air to escape, the pressure on the high pump picks up again. we ran the boat without this plug and the pressure would fluctuate 5-7 pounds.
air or vapor seems to be somehow building up inside the VST causing a drop in fuel pressure (high pump runs outta gas and can't get anymore until the air is released from inside the VST). the pump was taken apart, inspected and looked good. a vacuum line on the VST was found to be where it shouldn't have been. this was corrected and the problem still occurs. the only difference is that "heat" seems to have been removed from the equation.
after bleeding off the air from the VST, OR physically priming fuel through the low pressure pump, the motor will immediately run until air builds up again (seems to take about 10-20 minutes) and the high pressure pump is starved for fuel.
the problem "seems" to have been found, but noone can seem to figure out whats allowing air or vapor into the VST and why it cannot escape without force feeding it fuel or removing the plug on top of the VST allowing air to vent.
we are down to 3 remaining "fuel" possibilities....
1) fuel line between the mechanical and high pressure pump (didn't have the fittings to by-pass this line). maybe monday.
2) fuel / water separator assembly (seems a VERY remote possibility)
3) the high pressure pump itself (doesn't make sense since it picks up after priming low pump).
these are the only 3 fuel related issues left to check.
one thing that was odd is that when a digital gauge was put on the fuel line from the low pressure pump, the motor never failed. the pressure readout was a constant 10 to 10.4 on the low pressure pump.
i hope i made sense
suggestions anyone? thanks.
ok gang...........after 6 more hours of trouble shooting today, it comes down to this......the guy from mercury was there for 5 hours of it.
fuel pressure reads good on the mechanical and electrical pumps (running on the boats gas tank OR a separate gas tank with inline primer)(makes no difference which fuel source is used. the problem still occurs).
after about 15-20 minutes on the hose, pressure on the high fuel pressure pump starts to drop off (its good for 15-20 minutes) and it eventually quits running shortly thereafter. we physically primed the low pump when high pressure would drop and the high pressure pump would recover (get fuel).
air or vapor was discovered after removing the plug on top of the VST. after allowing the air to escape, the pressure on the high pump picks up again. we ran the boat without this plug and the pressure would fluctuate 5-7 pounds.
air or vapor seems to be somehow building up inside the VST causing a drop in fuel pressure (high pump runs outta gas and can't get anymore until the air is released from inside the VST). the pump was taken apart, inspected and looked good. a vacuum line on the VST was found to be where it shouldn't have been. this was corrected and the problem still occurs. the only difference is that "heat" seems to have been removed from the equation.
after bleeding off the air from the VST, OR physically priming fuel through the low pressure pump, the motor will immediately run until air builds up again (seems to take about 10-20 minutes) and the high pressure pump is starved for fuel.
the problem "seems" to have been found, but noone can seem to figure out whats allowing air or vapor into the VST and why it cannot escape without force feeding it fuel or removing the plug on top of the VST allowing air to vent.
we are down to 3 remaining "fuel" possibilities....
1) fuel line between the mechanical and high pressure pump (didn't have the fittings to by-pass this line). maybe monday.
2) fuel / water separator assembly (seems a VERY remote possibility)
3) the high pressure pump itself (doesn't make sense since it picks up after priming low pump).
these are the only 3 fuel related issues left to check.
one thing that was odd is that when a digital gauge was put on the fuel line from the low pressure pump, the motor never failed. the pressure readout was a constant 10 to 10.4 on the low pressure pump.
i hope i made sense
suggestions anyone? thanks.
Last edited by bigblueheaven; 08-07-2008 at 09:58 PM.
08-08-2008, 08:12 AM
#33
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Just a note. There have been many threads concerning VST tank problems. One very common problem has been the VST screen inside the VST tank. This screen is extremely fine and will clog and restrict engine running very easily. A solution I have posted here before still works great for me. I installed an additional Racor fuel filter "after" the standard filter for each engine. The Racor is much finer and catches the finest of contaminants. Placing it "after" the Merc filter keeps me from having to change it so often as the standard filter catches the bigger debris. This doesn't address all the VST problems but certainly addresses a very large one. I also keep a spare VST screen and O-ring set onboard at all times. By the way, after installing the Racors I'm trouble free for over 300 hrs.
08-08-2008, 02:00 PM
#34
Jerry B
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I thought the purpose of the diaphram and spring was to dump pressure or vacume from the vst after the engine is switched off. It appears to me that vacume is applied to the diaphram with engine running to close the vent. With the engine off why isnt the pressure vented? I do know that I have had both diaphrams rupture and caused alot of fuel to be sucked into the vacume line and then into the engine causing a couple of cylinders to run very rich. I still get what I think is vapor lock at times. I added mystery oil this year but am still running on last years fuel. I put stabil in but I know that gas is getting old. Needs a fill up soon and then will add the mystery oil again to see.
08-08-2008, 09:27 PM
#35
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well fellas, i found that i have the old style VST. the spring in the vapor regulator assembly was on the bottom of the diaphragm (wrong, it should be on top). i put it on top of the diaphragm and the vapor lock issues went away!.
it took 2 months and 90 hours of labor (i only paid 6 hrs.). the repair shop and a mercury field rep. never found the problem.
i was led to this thread from another web site and whammo.....problem solved.
thank you very much 28eliminator.
it took 2 months and 90 hours of labor (i only paid 6 hrs.). the repair shop and a mercury field rep. never found the problem.
i was led to this thread from another web site and whammo.....problem solved.
thank you very much 28eliminator.
08-09-2008, 07:36 AM
#36
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well fellas, i found that i have the old style VST. the spring in the vapor regulator assembly was on the bottom of the diaphragm (wrong, it should be on top). i put it on top of the diaphragm and the vapor lock issues went away!.
it took 2 months and 90 hours of labor (i only paid 6 hrs.). the repair shop and a mercury field rep. never found the problem.
i was led to this thread from another web site and whammo.....problem solved.
thank you very much 28eliminator.
it took 2 months and 90 hours of labor (i only paid 6 hrs.). the repair shop and a mercury field rep. never found the problem.
i was led to this thread from another web site and whammo.....problem solved.
thank you very much 28eliminator.
09-02-2008, 08:55 PM
#37
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Bigblueheaven
I have been fighting this problem also.I have a 96 baja with 454 mags. I just moved the spring in my VST tank today. It was on the bottom and I move to the top. If this is wrong what is going to happen. I will bring it home and start in driveway before going to the river. If this is wrong will fuel pass through the vent?
I have been fighting this problem also.I have a 96 baja with 454 mags. I just moved the spring in my VST tank today. It was on the bottom and I move to the top. If this is wrong what is going to happen. I will bring it home and start in driveway before going to the river. If this is wrong will fuel pass through the vent?
01-21-2009, 12:08 PM
#38
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Which is the return line? I believe it is the lower one per the schematics. I would like to be sure from an experienced eye however. Thanks
Upper or lower is the return line?
Upper or lower is the return line?
01-21-2009, 06:32 PM
#39
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Vapor lock issues....
Hello I have a 1995 Formula 271. Basically the same issues, took another rout, $hit canned VST tank, sea water transfer pump, the whole 9 yards and started over. Run an Aeromotive fuel pump, (filters fore and aft the pump) -8an to the fuel rail, -6an on the return to the boats fuel tank, through a fuel pressure regulator set at 48psi. then to the tank. No more "cool fuel" or any other crap. The fuel from the tank is always fresh, you eliminate the fuel on the pressure side getting hot from the engine and just "circling round until you get on the throttle. It's clean and simple, your fuel rails should already be drilled for 1/4 mpt.
01-22-2009, 06:19 AM
#40
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Hello I have a 1995 Formula 271. Basically the same issues, took another rout, $hit canned VST tank, sea water transfer pump, the whole 9 yards and started over. Run an Aeromotive fuel pump, (filters fore and aft the pump) -8an to the fuel rail, -6an on the return to the boats fuel tank, through a fuel pressure regulator set at 48psi. then to the tank. No more "cool fuel" or any other crap. The fuel from the tank is always fresh, you eliminate the fuel on the pressure side getting hot from the engine and just "circling round until you get on the throttle. It's clean and simple, your fuel rails should already be drilled for 1/4 mpt.
Thanks
Last edited by Pismo10; 01-29-2009 at 07:51 AM.
