5..0 MPI won't start back after sitting

[spdracr39]

Well I thought it was a vapor lock issue and have been trying to repair it as such but now I am not so sure. Last weekend after running at cruise for a little while we stopped to swim. I didn't run the blower or let the engine idle and we swam longer than I thought we would. It was probably about an hour. The engine would not start back. We had to get towed in. The next morning on the muffs it cranked on the first try and purred like everything was perfect. I bought a fuel pressure gauge and checked the pressure it was 40 lbs.. When I hit the bleeder the pressure vented immediately and the fuel only flowed from the clear vent tube for a few seconds. All looked normal to me. We went back to the lake today. It was not hot outside at all. We cruised for a while and stopped. I did let the engine idle for 5 minutes and ran the blower for 10 minutes. I cranked it and it started fine so I turned the blower off and we swam about 15 minutes. Ready to go and no start. I hooked up the gauge and it would not make any fuel pressure. What was odd was that even with no pressure on the gauge if i hit the bleeder valve a constant flow of fuel would run through the tube and would not stop. I was draining it back to the tank from the fuel filler and I don't think it could have been siphoning because it was higher than the tank. Eventually after about 45 minutes it started back and ran fine. We just didn't shut it off again. What could be causing this ??? What is the pressure on the rail supposed to be? Why would it keep flowing from the relief when it won't build pressure but when it is working the flow stops when you bleed the pressure? Is there a pressure relief that could be sticking open when it is hot keeping the rail from building pressure? I am really grasping at straws now. Now that I think about it I never actually checked the pressure with the engine running I guess I should check that also.

 

[Scott Danforth]

Re: 5..0 MPI won't start back after sitting

40 psi is low. check your filters, fuel plumbing, and tank fittings for restrictions. could be your pressure regulator is bad.

 

[spdracr39]

Re: 5..0 MPI won't start back after sitting

I am going to check the anti-siphon and fuel pick-up screen. The water sep filter is new. What should the pressure be? Will it be different when running at idle or wot than just pressurizing with the key ?

 

[spdracr39]

Re: 5..0 MPI won't start back after sitting

checked the valve and it looked fine. Checked the pickup it was clean. The pressure was actually 43 psi. When I started it and ran it up to about 1500rpm the pressure remained constant at about 39psi. I replaced the anti-siphon temporarily with an open fitting to see if maybe it was not working correctly and I just couldn't tell by looking. The pressures remained the same. My gut tells me the problem lies in this area. Maybe it's denial because I don't want to have to remove the cool fuel box to change the regulator.

 

[Scott Danforth]

Re: 5..0 MPI won't start back after sitting

EFI fuel systems are either 1 bar (15psi) TBI systems or 3 bar (45psi) MPI systems. there are higher 4 and 5 bar systems, however these are not used in marine motors. Direct injection is much higher at 120 bar (1800psi)

since boats use MPI, your fuel pressure should be 45psi. any less and you have a problem somewhere. since your only getting 39psi, I would suspect either a filter or the regulator.

 

[spdracr39]

Re: 5..0 MPI won't start back after sitting

Will a regulator issue show up on a vacuum gauge? That was what I was going to try next since I can't physically see any restrictions. The boat runs like a champ as long as you don't shut it down and leave it off for any length of time while its at operating temp..

 

[spdracr39]

Re: 5..0 MPI won't start back after sitting

Well I ran again without the anti-siphon valve and it was no difference. I did raise the engine cover the second time and let the heat out and it did not do it. It is definitely a heat related problem. I guess a fan mounted somewhere to circulate some air will be the solution. I don't believe there is a fuel issue because it runs perfectly at WOT 50mph at 4500 with 1200 lbs fuel, people, and gear and pulls great out of the hole.

 

[Scott Danforth]

Re: 5..0 MPI won't start back after sitting

if your vaporlocking due to heat, fix the issue, not introduce a fan. how is the fuel cooler?

 

[spdracr39]

Re: 5..0 MPI won't start back after sitting

Its down where you can't access it with the motor at temperature. I have not used a vacuum gauge yet but there are no restrictions in the lines that are visible. When it is acting up you can hear a difference in the pump and I can tell when it gets fuel and is going to start. There is zero fuel pressure on the rail at that time. I have a gen III cooler and it has a check valve in the line already to prevent hot water siphoning back to the cooler. The only other solution I have found is to add a low pressure pump between the tank and the water sep filter for a mere 400.00 plus labor. Even Mercruiser service letter says this is a patch not a solution so I am not inclined to do it when I can just open the hood or run an additional fan for very little or no cost. I also read somewhere that the kit they recommended is NLA but I have seen them around. Just FYI I run at 160 cruising or at WOT so it is not a cooling issue. The design of my hull is such that there are four 3 inch holes by the swim deck and two of them are blocked by the bilge blower hoses. There is really no way for air to circulate back there so I can see how things would get heat soaked when sitting still. There seem to be a lot of folks with this problem and opening the hood seems to be the only good solution because running the blower and idling the motor doesn't work.