Vapor Lock in a 2001 carbureted 4.3? Is it possible?

[Big6ft6]

Hi all, can a 2001 4.3 carb engine get vapor lock? After a long run tubing yesterday, I shut down my engine, and we just floated and let the kids swim for a while, maybe 30 minutes-45min or so. When I started the motor she fired right up but died after about 10 seconds. Then I couldn't get her to fire at all. Cranking and cranking, started briefly once, but again only for a few seconds, but otherwise couldn't get it fire at all, would just crank endlessly.

Got towed in. After being towed in to the pier. I had the idea that maybe my gas tank was not venting and had developed a vacuum. So I popped the gas cap and turned the key and she fired right up. Unfortunately I didn't think to try starting at the pier before popping the gas cap so I don't truly know if that made a difference or not.

After I got home, I started thinking this had all the hallmarks of vapor lock. Running the engine hard on a hot day, then shutting down, enough time for heat soak, and then started after enough time to cool down. But I didn't think engines with electric fuel pumps would vapor lock, is this possible? If yes, where to do they vaporize the fuel?

 

[alldodge]

VL happens to EFI motors from heat soak which replaces vapor with liquid gas, so it doesn't start. When trying to restart liquid cannot get back in because the vapor pressure is as high as the liquid pressure.

Carbs may leak out fuel but the fuel pump would replace in short order.

You have something else going on, and while it may be vacuum tank issue, I would suspect it not be the issue from floating around that long. The vacuum issue normally shows up while running and then it starts loosing power

 

[Lou C]

It used to be the conventional wisdom that engines with electric pumps don't vapor lock as much because the pump is pushing fuel toward the engine from the tank but this is an automotive application, in boats they can't be set up that way so the electric pumps still pull from the tank and then push it from the pump to the carb in this case. And, since your electric pump for a carb engine should be limited to 6-7 psi just like a mechanical pump there isn't much difference there. For whatever reason I never had VL issues with my old OMC 4.3 with the mechanical pump and Qjet carb. However, I did years ago with our '70 Ford with the 302 V8 and Autolite 2100 2barrel carb. What I found was that the fuel line as it snaked over the intake manifold was getting really hot, from heat radiating off the intake. So what I did was insulate the fuel line and that seemed to help.VL is basically the liquid fuel starting to boil it gets so hot and a carb cannot run on fuel that is starting to vaporize, it has to have liquid fuel. They also sold carb spacers that were some type of plastic/wood mix that were intended to keep the carb cooler to avoid fuel boiling in the bowl. The Qjet on my engine uses a thick carb gasket, that may help keep the fuel cooler. Your 2001 Merc 4.3 is a Vortec so the intake is not heated by an exhaust crossover like the older pre-vortec engines were, so it in theory should be less prone to vapor lock.

One last point, if you are using a winter gas mix in that boat, winter gas mixes usually have lower vapor gas pressure ratings to ignite at colder temps better. Summer gas mixes have a higher vapor gas rating to avoid VL. Still running last fall's gas?

Just out of curiosity I'd take an IR temp gun and see how hot the intake under the carb, the fuel line from the pump to the carb and the carb body itself are getting. I know on my engine the intake only gets hot right where the exhaust crossover is, and that heats the base under the carb and the area where the well choke spring is located. But not so much that I'd get VL.

What's your location? High altitude makes VL worse. A hot rodders trick from years go was to ice down the intake manifold to reduce VL in very high temps, for drag racing.

 

[Big6ft6]

Thanks for the advice both of you. Thought I share follow-up.

I took her out again today and there were no problems at all, did lots of cruising, some long stops etc. However, I wasn't pulling a tube and it was cooler time of day later in the evening. My suspicion is that it was something heat related. The fuel pump is really tucked in there next to the block under some cooling water hoses. When it wasn't starting yesterday, the big cooling water hose that goes in right in front of the fuel filter and fuel pump was very hot the touch, too hot to hold my hand on,even after sitting for 30 minutes.

It is fresh fuel from this spring, so shouldn't be winter blend.

Who knows, maybe the fuel pump go too hot and wasn't running? Maybe vapor lock on the low pressure side of the pump if the fuel in the filter got too hot? Maybe clogged fuel cap vent?

I finished installing the trolling motor last night, so I have back up power if needed. I will try to get her out pulling tubes soon and see if it ever re-surfaces.

 

[alldodge]

Fuel pump should not be getting that hot