5.0 MPI vapor lock question

[spdracr39]

I have all the symptoms of vapor lock on my 2008 5.0 MPI runs like a champ till you stop for a few minutes and then no start. Let it sit for a hour or two and it runs like a champ. If I open the engine compartment when I stop it never misses a beat. I have tried idling and running blowers that doesn't work. I have checked for fuel system blockage and had the pump pressure and leak down checked at the dealer and it is perfect. Changed the water filter. If you check the pressure on the rail when it is acting up it is 0 and if you try to bleed fuel off it does not have air bubbles although due to lack of pressure it barely flows out. The pump is running but you can tell it is not pumping fuel because just before it is ready to start you can hear the pumps sound change because it is beginning to pick up liquid fuel. My theory is that it is boiling the fuel in the lines before the cool fuel box as it passes over the engine and the pump cant pump the vapor. I am going to try insulating the lines or building some heat shields but I don't think that will work. Has anyone had any success with this fix :

"you may also try adding the merc low pressure fuel pump kit to help prevent vapor lock and maybe the merc cool fuel cannister. sounds like its a fuel boiling in the lines (vapor lock) problem. remember fuel under a vacum can boil at 50*F. good luck and keep posting"

I am willing to try anything at this point because leaving the hood up attracts many fellow boaters trying to help because it looks like I am stranded. I have also considered mounting a radiator cooling fan back there somewhere to circulate air from the gunwales but I don't know exactly how to mount it yet. There is very little ventilation in my engine compartment and I am surprised more people don't have this issue.

 

[Don S]

Re: 5.0 MPI vapor lock question

You need some air circulation and ventilation, not more pumps. What you need is approved bilge blowers in the engine compartment, not automotive fans. These don't need to be sparking and igniting any possible fumes in the compartment.
Maybe just leaving the bilge blower on and the engine hatch open an inch or two will be enough to cool it down.
Do you let the engine idle for a minute or two to cool down? That might help some.

 

[spdracr39]

Re: 5.0 MPI vapor lock question

Yes I have done both of those but it is not enough. Any suggestions on something approved that will move more volume of air and bring it in across the top of the motor where the heat is at ? I suppose I could mount another double blower and route the intake hoses to the gunwale and aim the outflow towards the area around where the fuel lines cross. The extra air pressure should go out the other gunwale.

 

[Don S]

Re: 5.0 MPI vapor lock question

What's the serial number of your engine, I will see if there are any service bulletins on your specific engine and cool fuel system.

Here is an old (99) service bulletin from Merc on Vapor lock.

View attachment EN_07.pdf

 

[spdracr39]

Re: 5.0 MPI vapor lock question

I have seen that and is what I am using to base my diagnosis. It doesn't really apply to my engine but makes a lot of sense. my engine s/n is 1a348745. Thanks !!

 

[Don S]

Re: 5.0 MPI vapor lock question

I just checked the service bulletins from 2008 to 2013, and nothing amiss there.
Somehow you are going to have to get some cooling air coming in down low and the heat getting out the top.

 

[spdracr39]

Re: 5.0 MPI vapor lock question

I may try adding the extra double blowers and see how that works. Thanks

 

[BIGALF]

Re: 5.0 MPI vapor lock question

My 5.7 Mercruiser EFI had that problem for a few years. I was driving me crasy. I thought I had a vapor lock but I found that the fuel filter screen inside the fuel tank got clogged after the fuel level was half full alowing the fuel and junk in the tank to clog the screen. I took out the screen and that worked fine. There is enough filtering in the fuel water filted to catch any junk before it gets to the nozzles. Also, I added a fuel buster pump in the fuel line and relocated the inlet air hose and exhaust air hose inside the engine compartment to get a better flow of air in the engine compartment. The last thing I did was change all the spark plugs. I suggest you try all of the above except leave the added buster pump to last since this is the most expensive part of the job.
Boat runs fine now in very hot weather down here in Florida. Can't wait to use it this weekend for the first time this year.

 

[spdracr39]

Re: 5.0 MPI vapor lock question

I have removed and cleaned the pickup filter and made sure the factory bilge blowers were working and moving air. Since their real purpose is to vent gas fumes that accumulate down low I don't want to move the hoses. I am going to add some additional airflow. If that doesn't take care of it I am going to add an inline pump as a last resort. Where did you plumb your pump? Was it between the tank and filter or the filter and cool fuel box? Also how did you energize it ?

 

[spdracr39]

Re: 5.0 MPI vapor lock question

Also just found this ? Have you seen it before DonS ?
http://www.boatfix.com/merc/Bullet/02/02_03.pdf

 

[BIGALF]

Re: 5.0 MPI vapor lock question

I used mercruiser Service Bulletin No. 99.7 dated November 2001. My blower intake and exhaust were both faced in the same direction next to each other so when the blower was on it blow the hot air right into the air inlet. I moved the inlet vent to face forward so when the boat moved forward air was forced into the engine compartment and out the exhaust vent that faced in the opposite directionwithout the blowed being turned on.

 

[spdracr39]

Re: 5.0 MPI vapor lock question

Oh i see. Well the genius that designs the newer boats put both the inlet and outlets on the swim deck where the vacuum keeps the air from going anywhere when under way. I can't move the holes Perhaps boat designers need some training in aerodynamics.

 

[BIGALF]

Re: 5.0 MPI vapor lock question

Sorry I can't find the pictures I took that show what I did. I found a cup holder on the stearn that I used to run the inlet air line to and put a crome flow fitting forward where the cup holder was. It Looks like it belonged there. The new the boat is the smalled the engine compartment is so they better do something about the heat buildup there.

 

[BIGALF]

Re: 5.0 MPI vapor lock question

Sorry I can't find the pictures I took that show what I did. I found a cup holder on the stearn that I used to run the inlet air line to and put a crome flow fitting forward where the cup holder was. It Looks like it belonged there. The newer the boat is the smalled the engine compartment is so they better do something about the heat buildup there.

 

[spdracr39]

Re: 5.0 MPI vapor lock question

Sounds similar to my plan but I'm going to use the existing openings in the gunwales for intake and exhaust air flow we will see how it goes. The added fuel pump is still a backup plan. I hope a couple of 4" 250cfm blowers will move enough air I just hate all that noise though.

 

[tomasz]

Re: 5.0 MPI vapor lock question

Hi,

I had same problem and what I did was to shorten the fuel house (from tank to pump) and I've replaced the antisyphon valve and no problem since.

 

[spdracr39]

Re: 5.0 MPI vapor lock question

I replaced the anti siphon with an open fitting for testing and it did not make any difference so I put a new one back in. My lines are all metal except for the tank to filter so I don't know how I would shorten the run but perhaps relocating the lines might be an option. I'll have to think about that. Thanks for the reply.