Someone 'splain me 'vaporlock'

[mofugly13]

I think I have an idea of what vaporlock is, but I'd like someone to explain it to me.

This past Sunday, while coming home from the mountains, outside temperature about 85*, K5 temp right at the first quarter mark,right when I rolled into the first town (pop 730, EVERYTHING! closed on Sunday ), my K5 started bogging, then fine, then bog then fine, at 1-2 second intervals. So, I pulled over in the shade and popped the hood. After numerous attempts to fix the problem, including, putting on a spare fuel pump, taking the top plate off the carburetor and blowing through all the little orifices (orifi?), TWICE! Accelerator pump working fine, fuel filter fine, fuel pump pumping. After four or five "Ok lets give it a try...."'s and going great for about 1/4 mile, then the same, bog, fine, bog, fine. Really seems like it's starving. I decided to disconnect ALL the fuel lines and blow through them all. Pump to carb, clear. pump to tank (disconnected at tank), clear. Then I hooked the line back up to the tank, and blew through from th pump end. After two or three good breaths blowing into the tank, I determined it was clear into the tank. When I went to hook the line back to the carb, fuel started flowing out! My breaths had pressurized the tank enough to push fuel back through. Hooked the line up, then went and gave the gas cap a wiggle and heard come out in when I broke the seal. So, I went to my charcoal canister, disconnected the vent line that comes from the tank, and then blew a few breaths back into the tank to make sure it was clear. I left it disconnected from the canister, fired up the K5 and, four hours after I first had problems, made it the 120 miles home without incident.

This is just a tank venting problem, right? Not vaporlock?

 

[4X4HIGH]

Vapor lock is basically the fuel getting so hot it is boiling in the fuel line and causing air bubbles which will now not allow the vehicle to run correctly. From what you described it does sound like vapor lock. The cure is to move or wrap any fuel lines that are near any source of heat. It usually stems from an exhaust system that was installed too close to the fuel lines. Extreme heat itself can cause it to happen alone though but very rare.

 

[broncoman6524]

When you say "bog" what do you mean? like a jump in idle or drop in idle, or when your goin down the road it kinda dies then comes back?

 

[4X4HIGH]

Quick search found this for you.
vapor lock

The formation of vapor in a pipe carrying liquids; prevents normal fluid flow.

vapor lock
n.
A pocket of vaporized gasoline in the fuel line of an internal-combustion engine that obstructs the normal flow of fuel.

Best definition yet.

The noun vapor lock has one meaning:
Meaning #1: a stoppage in a pipeline caused by gas bubbles (especially a stoppage that develops in hot weather in an internal-combustion engine when fuel in the gas line boils and forms bubbles that block the flow of gasoline to the carburetor)

Last but not least.

Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engines. It occurs when the liquid fuel changes state from liquid to vapor while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult. The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily.

Causes and Incidence

Vapor lock was far more common in older petrol fuel systems incorporating a low-pressure mechanical fuel pump driven by the engine, located in the engine compartment and feeding a carburetor. Such pumps were typically located higher than the fuel tank, were directly heated by the engine and fed fuel directly to the float tank inside the carburetor. Fuel was drawn under negative pressure from the feed line, increasing the risk of a vapor lock developing between the tank and pump. A vapor lock being drawn into the fuel pump could disrupt the fuel pressure long enough for the float chamber in the carburetor to partially or completely drain, causing fuel starvation in the engine. Even temporary disruption of fuel supply into the float chamber is not ideal; most carburetors are designed to run at a fixed level of petrol in the float chamber and reducing the level will reduce the air:fuel mixture delivered.
Carburetor units may not effectively deal with fuel vapor being delivered to the float chamber. Most designs incorporate a pressure balance duct linking the top of the float chamber with either the intake to the carburetor or the outside air. Even if the pump can handle vapor locks effectively, fuel vapor entering the float chamber has to be vented. If this is done via the intake system, the mixture is, in-effect, enriched, creating a mixture control and pollution issue. If it is done by venting to the outside, the result is direct hydrocarbon pollution and an effective loss of fuel efficiency and possibly a petrol odor problem. For this reason, some fuel delivery systems allow fuel vapor to be returned to the fuel tank to be condensed back to the liquid phase. This is usually implemented by removing fuel vapor from the fuel line near the engine rather than from the float chamber. Such a system may also divert excess fuel pressure from the pump back to the tank.
Most modern engines are equipped with fuel injection, and have a high pressure electric fuel pump in the fuel tank. Moving the fuel pump to the interior of the tank helps prevent vapor lock, since the entire fuel delivery system is under high pressure and the fuel pump runs cooler than if it is located in the engine compartment. This is the primary reason that vapor lock is rare in modern fuel systems. For the same reason, some carbureted engines are retrofitted with an electric fuel pump near the fuel tank.
Other solutions to vapor lock are rerouting of the fuel lines away from heat generating components, installation of a fuel cooler or cool can, shielding of heat generating components near fuel lines, and insulation of fuel lines.
A vapor lock is more likely to develop when the vehicle is in traffic because the under-hood temperature tends to rise. A vapor lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heat up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitude in either case.

 

[mofugly13]

4x4 high, I guess I shoulda searched myself Thanks for the explanation and for doing my homework for me.



"Bog" as in rpm drop, but NO power at all. Maybey I donno what "bog" means.

I figured the problem to be that I was sucking fuel out of the tank, but no air was getting into the tank to replace it, or it was severely restricted. I'd get fuel until enough vacuum was created that the pump was fighting the negative pressure in the tank, and I was only getting intermittent squirts of gas to the carb.

 

[Hossbaby50]

If it happens again try loosening the gas cap up so it doesn't seal and see if it fixes the problem. If it doesn't then start looking at other issues like low fuel pressure, insufficient fuel flow, etc.

Is the fuel filter pretty clean?

Harley

 

[mofugly13]

Yes, I cleaned it befor e I took my carb apart on the side of the road. I hav one of those clear filters, with a pressure guage on it. I can tell when the pump is operating 'cause the needle will jump between 5 and 10 pounds when it's running. After it started acting up, it would idle just fine, and in neutral rev just fine, but when I tried to accelerate or cruise, it would act like it was starving for fuel for a second or two, then rev up and run fine for a second or two, then bog, rev, og, rev. I pulled it over straight after it started bogging and popped the hood with the motr running, the needle on the guage was barely deflecting, but there was fuel in the filter. After another false start, I killed the engine as soon as the bogging got bad, and coasted to the side of the road, so I could check the carb for gas. When I pulled the top off the carb, the bowls were full, its an Edelbrock 1406 fwiw. Only after disconnecting the vent line from the charcoal cannister did it run right. I was able to do 70 all the way home once I made it the 30 miles to the freeway.