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How to troubleshoot possible vapor lock issue.

B

Bajaoutlaw

New member
Hello to all. I recently experienced VL symptoms with a 5.0 MPI for the first time in several years of operating this boat. After a few inquiries, I was given these likely culprits:
1. Running winter blend fuel.
2. Defective fuel cooler check valve.
3. Defective temp sensors.

I am not much into just throwing parts at a mechanical issue and am looking for some further troubleshooting advice. Thanks.
 
I am not exactly familiar with that one, but I did have 43 psi at the fuel rail.
The engine did restart after a couple of hours period.
 
Take a fuel sample- the gas may have water in it and if the first sample comes out cloudy, you have phase separation and that needs to be removed from the tank.
 
jimn said:
Take a fuel sample- the gas may have water in it and if the first sample comes out cloudy, you have phase separation and that needs to be removed from the tank.
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That was my original thought. I checked the separator filter and it was fine. If it was VL, it took quite a while to cool off. First time in 10 years of boating that I ever needed a tow.
 
makomark said:
are you sure it is experiencing vapor lock??
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No. I just know that I experienced VL type symptoms. Boat wouldn't start after running on a hot afternoon. Spit and sputtered at idle but subsequently died and simply woyld motcstart. Boat finallt restarted after about 3 hours of cooling off. This boat has never had a VL issue before, but I ran it on a full tank of winter gas from storage without adding any fresh fuel. Never did that before either.
 
all fuel injection pumps are water cooled with raw water to cool the fuel . no /low water cooling the fuel means the fuel gets too warm.
Some have used this type to cool the fuel before it enters the fuel pumps
1752595171656.png
 
Bt Doctur said:
all fuel injection pumps are water cooled with raw water to cool the fuel . no /low water cooling the fuel means the fuel gets too warm.
Some have used this type to cool the fuel before it enters the fuel pumpsView attachment 35652
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Thanks for the reply. Since this boat has never had a VL condition - and used in hotter conditions - I don't think that I need to do any modifications to the fuel delivery system. I will likely start with fresh fuel and go from there.
 
I would remove the winter blend from the tank and start with a fresh, full tank...especially if it wasn't stabilized....
 
makomark said:
I would remove the winter blend from the tank and start with a fresh, full tank...especially if it wasn't stabilized....
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It was definitely stabilized. I just didn't put in any fresh since the tank was so full. I did find a small rubber hose connection that was disconnected. Am in the process of tracing where each end goes.
 
I had the same thing happen on my 5.0 MPI. I replaced the check valve assembly in back of the fuel pump. Problem went away. I think it stops hot engine water from backing up into the fuel pump.
 
Mark S. said:
I had the same thing happen on my 5.0 MPI. I replaced the check valve assembly in back of the fuel pump. Problem went away. I think it stops hot engine water from backing up into the fuel pump.
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I was told that valve was a possibly by a fellow Baja owner. What confuses me is how cooling down of the engine fixes (if only temporarily) a faulty check valve.
 
It doesn't. The valve stays broke. The vaporized gas cools on it's own then the boat restarts. The problem doesn't reoccur until you shut down again. Mine would only do it on the hottest of days. Come fall the problem went away. Once I figured out it was a vapor lock issue I would unscrew the fuel filter, let it breath for a few, re-tighten and boat would restart. I guess this would cause the vaporized part to move out of the way so to speak. Hope this is your problem. Cheap easy fix...at least for me I had access to that part of the engine at least after I removed a panel.
 
Mark S. said:
It doesn't. The valve stays broke. The vaporized gas cools on it's own then the boat restarts. The problem doesn't reoccur until you shut down again. Mine would only do it on the hottest of days. Come fall the problem went away. Once I figured out it was a vapor lock issue I would unscrew the fuel filter, let it breath for a few, re-tighten and boat would restart. I guess this would cause the vaporized part to move out of the way so to speak. Hope this is your problem. Cheap easy fix...at least for me I had access to that part of the engine at least after I removed a panel.
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Duh! Give me the "bonehead" award for that comment. I was thinking in a completely and wrong troubleshooting context. Mine is easily accessible as well, but I do want to trace out a discovered disconnected hose line first. Not sure yet what it does.
 
If you find where the other end of the disconnected hose goes, let us know..

My hunch is the winter fuel is the issue...in the future you can help prevent by running at high idle, in neutral(say 1500), for several minutes, and then shutting down. Opening the engine hatch will also help..
 
makomark said:
If you find where the other end of the disconnected hose goes, let us know..

My hunch is the winter fuel is the issue...in the future you can help prevent by running at high idle, in neutral(say 1500), for several minutes, and then shutting down. Opening the engine hatch will also help..
Click to expand...
This was the only time that I have not put in some fresh fuel after winter storage. As such, I had a full tank of winter blend fuel on a 90+ degree day.

My regulator valve (which has the return side blocked off) has a short piece of tubing with a "t" in it. One side of the "t" has a short piece of tubing that goes into the manifold. The other side of the "t" has a long piece of tubing that runs down the side of the engine block and is attached to a plastic barbed fitting that hangs loose under the exhaust manifold. The other end of that fitting has another piece of tubing that looks like it attaches to the fuel cooler assembly. This connection at the barbed fitting might have become detached when I did some engine cleaning in the spring, or it may have been that way for awhile. I have no idea of the function of that long run of tubing that came undone at the barbed connector, or how it may affect engine performance.
 
Looking at the service manual and the parts breakdown, the vacuum hose does appear to go to the fuel cooler, more specifically to the pressure regulator on the fuel cooler.

If the vacuum hose became disconnected, you could have two issues - a minor vacuum leak and the pressure regulator would not have the correct reference so the pressure delivered could be off.
 
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