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502 Mag MPI Gen VI Hot start issue

Old 03-06-2021, 09:05 AM
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Fuel injection should be our modern answer to fix this.

Last edited by zfrilly; 03-06-2021 at 09:09 AM. Reason: Wrong info
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Old 03-06-2021, 09:18 AM
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Originally Posted by zfrilly View Post
Fuel injection should be our modern answer to fix this.
These are fuel injected engines we're referring to.
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Old 03-06-2021, 09:22 AM
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Some manufactures have fixed the issue like Ilmor which does have a return to tank. Merc just keeps changing the pump type and location instead of fixing it.
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Old 03-06-2021, 10:17 AM
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Originally Posted by Griff View Post
Vapor lock for sure. The hotter and more humid it is, the more likely it is to happen.

Not running ethanol fuel and opening hatch and running the blower for 5-10 mins after you dock helps a lot. I rarely have it happen when I do this.
Thanks...this will be my short term solution.

Originally Posted by Griff View Post
Run the fuel return lines back to the tank.
And this will be my final solution as I have an unused fuel barb off the tank for a generator that isnt installed. I take it I need to re route the lines that take the unused fuel from the injectors and instead of going back to the cool fuel pumps, run them back to the tank. Im also assuming I need to put in a check valve somewhere?

Originally Posted by AllDodge View Post
Don't see that kit helping much. The issue is there is vapor in the fuel rail developed from the heat. Your mechanical pump is working, as been said, the best method is run the return back to the tank.

A quick fix can be done with a rag, small screw driver and pair of safety glases.
Remove the cap off the schrader valve on the fuel rail, put on saftey glasses, turn key until pump stops. Place rag around the schrader valve and press in with screw driver. Repeat the step a couple times till you start getting fuel come out
And this will be my '****..forgot to keep the hatch open and run the blower after shutdown' fix.

Originally Posted by Eddienel View Post
Is this the first time this has happened? Is the boat new to you? The reason Im asking this is maybe you got some bad gas. I know the refineries had to change their ethanol formula because of the vapor lock issues and small engines having trouble with it. Mostly gumming up in the carbs. Ive also noticed that on the new boats they have a low pressure fuel pump and a high pressure fuel pump. If this is the first time and its never happened before maybe your pump is getting weak. I would definitely check your tank vent also.

Ill also have to respectfully disagree with the idle open hatch blower thing. In this day and time you just shouldn't have to do that to be able to restart your engine in this day and time. You cant tell me in this day and time engine builders cant come up with a solution for vapor lock.
Ive had the boat for about 7 months..so newish to me I guess. This is is about the 9th time its happened and ive been pulling my hair out because its usually only happened to one engine. Every time, I investigated, drained filters, pulled plugs, check wiring, only to have it fire up the next day like nothing happened. Last night it happened to both at the same time which was a blessing in disguise.

Originally Posted by zfrilly View Post
Fuel injection should be our modern answer to fix this.
Agreed 100% I think we can put this to bed gents. Big thanks for the solutions and education. Final question...About how long can I run the blower without draining the batteries assuming they are healthy?

The collective knowledge here is staggering.
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Old 03-06-2021, 09:04 PM
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Is it correct to assume the vapor lock between the tank and vapor tank? I would assume the high pressure pump can force any vapor out and I assume 45psi should compress fuel vapors. Tho OP stated that it fired then quit, is that the fuel getting used up in the vapor tank? How about insulating the low pressure fuel hose and maybe a way to return return some hot fuel back to the tank?
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Old 03-07-2021, 12:00 AM
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This is one of two things. both common to the Merc engines (big and small block) that use the VST system.


The most likely issue, as mentioned is vapor lock. During sits after running the stainless high pressure fuel lines tend to percolate fuel due to heat soak from the manifold and surrounding engine heat. The engine will typically start and run for a few seconds on the fuel on top of and near the injector. It then quits. It takes a surprisingly LONG time to recharge the lines and get the motor running again. Problem is the lines are now hot and mostly void of liquid fuel.


The other issue that can occur is deterioration of one of the rubber diaphragms in the VST assembly will allow fuel to bleed out through the yellow fuel vent line and into the intake manifold. This will bleed down VST tank pressure exacerbating the vapor lock issue AND it also floods the engine. For the small blocks this fuel enters at the throttle body. For the big blocks it enters the manifold near #7 if memory serves me correctly. Just trace the yellow line off the VST and see where it connects. Also if there is any trace of fluid in this line you have VST diaphragm issues.


One workaround is to bring the throttle up to 50-75% and attempt a start. This will force 'flood clear mode', which shuts off the injectors. Doing this will allow the flood to clear (throttle blades open / high air flow) and allow the fuel feed lines to re-pressurize easier (injectors not trying to pulse). Work the motor in this matter for a few iterations making sure to not kill the battery or starter with prolonged attempts. While doing this advance the throttle to greater than 75% or less than 50% (exiting flood clear mode). The injectors will start firing and the motor should light off. Keep the RPM up around 2000 or so in neutral for 30 seconds. You need to flow more cool fuel into the lines quickly to help bring line temperature down. Idle rpm is insufficient fuel flow to adequately cool the fuel lines in this case.


Once running get the boat up on plane and cruise as this is the best bet to get the most fuel into the lines for cooling purposes. If you go right back to idling you stand a chance of getting right back into vapor lock.


Obviously this is all a work around. I have found that fuel makes a big difference. Good fuel, preferably no or low ethanol helps (more than I had imagined). Make sure the VST internal diaphragm is not deteriorated (there are rebuild kits). Finally reroute the fuel return from the second injector rail to the main fuel tank rather than back into the VST. This will keep a constant flow of cooler fuel supplied to the injector rails rather than returning it to the VST only to be immediately reheated down the fuel rail assembly.


The fact that both motors exhibit this issue it is likely vapor lock. However, with the age of the motors that internal diaphragm could be bad on both VST assemblies.


Hope this helps.
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Old 03-07-2021, 12:27 AM
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Thanks Trash. I have Cool Fuel II systems so I dont think the 50% throttle trick would work. I have all sorts of goodies in the mail tomorrow including silicone jackets to wrap fuel lines, heat tape and thermal paste. I read an thread here about how the Cool Fuel internal mounting bracket should have thermal paste on the side that faces the engine...mine dont. Makes sense as the internal plate is mounted to the block and heat would transfer to the Cool Fuel pump and lines in the black housing. Short term plan is to wrap fuel rails with the silicone jackets, thermal paste on the side of the brackets and maybe wrap the Cool Fuel housings in thermal tape. Its a low cost experiment. If it doesnt work, I get to try the schrader valve/rag method to test it out. But I think the final fix is to run return lines back to the fuel tank.

Ill also take a look at my blower motor to see about upgrading it to a newer and or larger unit to pull more air out.
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Old 03-07-2021, 01:52 AM
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Originally Posted by carnutsx2 View Post
Is it correct to assume the vapor lock between the tank and vapor tank? I would assume the high pressure pump can force any vapor out and I assume 45psi should compress fuel vapors. Tho OP stated that it fired then quit, is that the fuel getting used up in the vapor tank? How about insulating the low pressure fuel hose and maybe a way to return return some hot fuel back to the tank?
The OP doesn't have a VST. He has cool fuel and the excess fuel (which is warm) just runs right back into the fuel system. The fuel gets heat soaked from the hot engines when you dock and essentially turns into vapor. E10 is worse because alcohol evaporates faster and at lower temps. It will run at idle for about 20-60 seconds from the fuel still left in the rails. Until the vaporized fuel gets through the rails, it won't run right. Cycling the key to kick on the fuel pump several times helps.
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Old 03-07-2021, 02:03 AM
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Originally Posted by lear60man View Post
Thanks Trash. I have Cool Fuel II systems so I dont think the 50% throttle trick would work. I have all sorts of goodies in the mail tomorrow including silicone jackets to wrap fuel lines, heat tape and thermal paste. I read an thread here about how the Cool Fuel internal mounting bracket should have thermal paste on the side that faces the engine...mine dont. Makes sense as the internal plate is mounted to the block and heat would transfer to the Cool Fuel pump and lines in the black housing. Short term plan is to wrap fuel rails with the silicone jackets, thermal paste on the side of the brackets and maybe wrap the Cool Fuel housings in thermal tape. Its a low cost experiment. If it doesnt work, I get to try the schrader valve/rag method to test it out. But I think the final fix is to run return lines back to the fuel tank.

Ill also take a look at my blower motor to see about upgrading it to a newer and or larger unit to pull more air out.
If you have the cool fuel plastic covers around your fuel filters, take them off.
I wouldn't waste your time wrapping the fuel rails or lines. That silicone jacket will hold heat in as well as keep it out.
Heck, just opening the hatch and dumping a bottle of cold water on the fuel filter and lines when you dock will probably cure it.
I have never had to open my schrader valve when I've had an issue. Just cycle the key about 5 times to the run position and on every 5th time try and start it. Repeat a few times if needed.
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Old 03-07-2021, 06:47 AM
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Is it against coast guard regulations to return the warm fuel back to the tank?
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