Vapor lock -- boat, not airplane

Ken Ibold

Ken Ibold

Final Approach
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Ken Ibold
In the last two days I have gotten what I suspect was vapor lock in my boat twice. Both times it was the same scenario -- headed to an offshore spoil island for picnic/swim etc, and on trying to come back the engine would not start. Cooled it by opening the engine hatch (sterndrive) for a while and repeatedly dumping the raw cooling water manually. Then it would start and operate normally. This is a 50-hour Mercruiser sterndrive, 5.0 liter MPI engine. In two years this problem has never surfaced before. Ambient temps were in the low 90s, water temp was high 70s/low 80s. The engine has run flawlessly in all conditions since new except for these two instances.

Does my vapor lock diagnosis sound reasonable? Any other possibilities? At first I considered water in the fuel/fuel filter, but because it runs perfectly under both high and low power I discounted that.

If it is vapor lock, what might solve the problem? I am running 93 octane gas in it.
 
Ken Ibold said:
If it is vapor lock, what might solve the problem? I am running 93 octane gas in it.
Click to expand...

Sounds like it's choking on the high price of gas... :D

I'd suspect vapor lock, too. Are all the various baffles & cooling "stuff" still in place? That shouldn't happen with a motor of that newness....
 
Ken Ibold said:
In the last two days I have gotten what I suspect was vapor lock in my boat twice. Both times it was the same scenario -- headed to an offshore spoil island for picnic/swim etc, and on trying to come back the engine would not start. Cooled it by opening the engine hatch (sterndrive) for a while and repeatedly dumping the raw cooling water manually. Then it would start and operate normally. This is a 50-hour Mercruiser sterndrive, 5.0 liter MPI engine. In two years this problem has never surfaced before. Ambient temps were in the low 90s, water temp was high 70s/low 80s. The engine has run flawlessly in all conditions since new except for these two instances.

Does my vapor lock diagnosis sound reasonable? Any other possibilities? At first I considered water in the fuel/fuel filter, but because it runs perfectly under both high and low power I discounted that.

If it is vapor lock, what might solve the problem? I am running 93 octane gas in it.
Click to expand...

JOOC, why 93 octane? Most standard production Mercuiser engines will run on 89 R+M or even 87.

I'm not all that familiar with Merc's recent engines, but the MPI marine systems I've seen can vapor lock if the engine compartment isn't well ventilated. Putting a wet towel or rag on the injector lines usually "cures" the problem. Also a lot depends on where the fuel pump(s) are located. If there's one in the tank, a low fuel level can make things worse. Some systems have a pump/reservoir on or near the engine and if that gets hot enough to boil fuel, the engine won't start. Can you make the pump(s) run by "bumping" the starter? If so doing that several times before starting may solve the problem. Did you try running the blower for a while after stopping to cool things down?
 
lancefisher said:
JOOC, why 93 octane? Most standard production Mercuiser engines will run on 89 R+M or even 87.

I'm not all that familiar with Merc's recent engines, but the MPI marine systems I've seen can vapor lock if the engine compartment isn't well ventilated. Putting a wet towel or rag on the injector lines usually "cures" the problem. Also a lot depends on where the fuel pump(s) are located. If there's one in the tank, a low fuel level can make things worse. Some systems have a pump/reservoir on or near the engine and if that gets hot enough to boil fuel, the engine won't start. Can you make the pump(s) run by "bumping" the starter? If so doing that several times before starting may solve the problem. Did you try running the blower for a while after stopping to cool things down?
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The book recommends premium, and 93 is what the gas station had last time I fueled it. I do note the marinas typically carry 89, so I'll try dropping down to that. There was about 3/4ths of a tank -- ~45 gal on board, so low fuel level was not an issue. I tried to start it, oh, probably 10 times before it dawned on me what was happening. Must have been the rum!

I'll try the blower on shutdown, that seems reasonable. The engine compartment is very well insulated -- keeps noise down a lot -- and perhaps it's TOO well insulated. These were the hottest conditions we've had it out in in which we shut the engine down for a while in the middle.
 
Does ethanol have a higher vapor pressure than gasoline? Could ethanol have been introduced in marine fuel? Sure sounds like vapor lock. Running the blower for awhile after engine shut down (when it is the hottest) should help.
 
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Graueradler said:
Does ethanol have a lower vapor pressure than gasoline? Could ethanol have been introduced in marine fuel? Sure sounds like vapor lock. Running the blower for awhile after engine shut down (when it is the hottest) should help.
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I too suspected ethanol as a cause of the vapor lock, but not knowing the vapor pressure of ethanol, I kept quiet.

With regard to running the blower: I can't say it won't help, but I can say it won't help much. Gasoline vapor is heavier than air, so marine bilge blowers are rigged to extract air from the low point in the bilge. It will take a long time on the blower to extract the heat at the top of the engine.

-Skip
 
Skip Miller said:
I too suspected ethanol as a cause of the vapor lock, but not knowing the vapor pressure of ethanol, I kept quiet.

With regard to running the blower: I can't say it won't help, but I can say it won't help much. Gasoline vapor is heavier than air, so marine bilge blowers are rigged to extract air from the low point in the bilge. It will take a long time on the blower to extract the heat at the top of the engine.

-Skip
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Typically the air inlet(s) are near the top of the compartment with the exhaust (blower inlet) down low. I'd expect that 10-20 minutes should do. Alternatively opening the engine compartment cover until the engine cools off would probably do as well, but in many cases that get's in the way. If you are leaving the boat when you stop, opening the hatch makes the most sense.
 
Most of the BB guys around here dumped the high octane and went to 87 oct and octane boost, that took care of the vapor lock problems. Anything over 89 octane around here has 10% or more of ethanol.
 
Ken,

That GM MPI engine has a recirculating injection system. Perhaps the injected Continental hot start procedure will help: turn the fuel pump on for at least 90 seconds before you start cranking. This circulates cool fuel through the system, flushing out the vapor and cooling things down. In the boat you'd simply turn on the ignition but not crank, it should have the same effect.

FWIW I run marina gas (87-89 octane) in my Crusader MPI. The only modifier is that I put in gas stabilizer to keep it from gumming up. No octane boosters. Been working fine for 7 years now.

Regards,
Joe
 
Is there a way to run the pump without cranking the engine? Normally there's something to kill the pump when the engine isn't running. In my boat (351 MPI) just tapping the start position with the key runs the pumps for 3 seconds.

Joe B said:
Ken,

That GM MPI engine has a recirculating injection system. Perhaps the injected Continental hot start procedure will help: turn the fuel pump on for at least 90 seconds before you start cranking. This circulates cool fuel through the system, flushing out the vapor and cooling things down. In the boat you'd simply turn on the ignition but not crank, it should have the same effect.
Click to expand...
 
lancefisher said:
Is there a way to run the pump without cranking the engine? Normally there's something to kill the pump when the engine isn't running. In my boat (351 MPI) just tapping the start position with the key runs the pumps for 3 seconds.
Click to expand...
Ditto mine.
 
Skip Miller said:
I too suspected ethanol as a cause of the vapor lock, but not knowing the vapor pressure of ethanol, I kept quiet.
Click to expand...

I suspect the vapor pressure of ethanol is a bit lower than normal gas.

I've had a very severe case of vapor lock in my jeep in the summers since they started forcing ethanol sludge on us. If it's hot out and I have a low fuel flow rate (stop and go traffic vs highway) it'll vapor lock every time without fail. No ethanol gas = running ok. Ethanol = quit repeatedly. (the record so far is 12 times in 5 hours)
The next step in the fix it procedure is to take my currently installed bypass fuel lines (fuel supply and tank return that's further away from the engine and heat sources) and add an electric fuel pump at the tank to force the fuel to keep cycling back to the tank. Several other people with the same type jeep have found that to be the real world solution.

If the fuel lines are in the vicinity of the heat sources, that could be the problem.
 
lancefisher said:
Is there a way to run the pump without cranking the engine? Normally there's something to kill the pump when the engine isn't running. In my boat (351 MPI) just tapping the start position with the key runs the pumps for 3 seconds.
Click to expand...

Rewire the pump? :dunno: In mine the fuel pump is on whenever the ignition is on. Dunno more, it just came from the factory that way.

Regards,
Joe
 
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