Vapor lock?

[quick_claude]

I have an 120hp MerCrusier on a 19' Chaparral. The engine runs great at any speed except that after I run at WOT for about 15 - 20 mins, it starts to loose RPMs and dies. If I throttle down when it starts to loose power then it will run ok., but if I keep at WOT, it dies. I checked and there is fire at the plugs and the fuel pump is pumping (although I have not checked the pressure). It will not start back up unless I wait about 30 minutes and then it will fire up and run great. The gas tank is venting good. The gas line is not collapsing. I rebuilt the carb, but that did not correct the problem. Is it vapor lock? What is vapor lock. How do you correct it? The carb is a Rochester 2GCV. Any suggestions are greatly appreciated.

 

[amtsst]

Re: Vapor lock?

I have some thoughts but somebody else is going to have to jump in here.<br /><br />A vapor lock occurs when heat from the engine is warming up your fuel line before it gets to the carbuerator. The warm fuel then begins to 'vaporize' and creates an air bubble in your fuel line.<br /><br />The only reason I know this is that on my farm, we had an old Minneapolis Moline tractor that would do it like a champ. Hot motor = warm fuel line = vapor lock. <br /><br />You can correct this by insulating the fuel line anywhere near the motor. Steel/copper fuel lines are particular succeptible to this problem. Rubber coated lines will solve the problem. On that old tractor, we bought some wrap-around insulation at home depot and insulated the fuel line. That did the trick.<br /><br />HOWEVER... I never heard of this on a boat. Is your engine compartment extremely hot? Whatever your problem, it sounds heat related since you can achieve WOT but can't stay there. If you left the cover off your engine compartment, can you re-create the problem???????<br /><br />Try it and see. If leaving the cover off solves the problem, you should work on venting the compartment instead of fooling with the fuel line. If you actually have a vapor lock, its a symptom of a larger problem!

 

[MrBill]

Re: Vapor lock?

Vapor lock, and isolating the specific issue can be a difficult diagnosis. I recently had the same type problem and a number of factors may have been at fault....95 degree day, 2 hours of hard use, bilge full of water, etc, all added up to a very hot humid engine compartment and a significant loss of power, RPM surging and a situation similar to what you described.<br /><br />I have since insulated my fuel line, cleaned the bilge and pump (so that it operates properly), and found that on very hot humid days keeping the blower on helps the problem a lot. Obviously, the things I mentioned may not be fixes resulting from good diagnostic procedure, but the problem has not come back and the cost was minimal.<br /><br />Also, temperature, humidity and atmospheric pressure play a big role in power and RPM loss. The Clymer Mercruiser Shop manual graphically shows HP and RPM loss due to regular Summer weather conditions, (10-14%) overall power loss given the right combination of circumstances. I suggest trying the quick, no or low cost fixes to cool the engine compartment before mechanical attempts to solve, if applicable be sure distributor cap is not cracked and is in good shape.<br /><br />Let us know.........

 

[denniz]

Re: Vapor lock?

could be vapor lock or my guess would be a restriction in the fuel pickup/antisiphon valve or a clogged gas tank vent.

 

[denniz]

Re: Vapor lock?

Run it on a shop tank and see what it does. Thatll tell you weather its a engine prob. or a tank problem.

 

[quick_claude]

Re: Vapor lock?

Thanks for the replies! I am going to insulate the fuel line and run it this weekend without the engine cover and see what happens. I'll let you know.

 

[Alrobot]

Re: Vapor lock?

Sounds like you have a good handle on the problem. I would like to add one thing, MAKE YOUR CHANGES ONE AT A TIME. Don't go out there and do everything described. Just like tuning a Holley, One Change At A Time...<br /><br />Al...

 

[sony2001]

Re: Vapor lock?

Adding an electric fuel pump to increase the pressure can override the lock- up. It would be best to have it with an on/ off switch of its own. Just having a second fuel pump is a good idea on its own.