Mysterious fuel problem

[djhammy]

Hi guys i have followed advice in previous threads and changed all fuel hose etc though i found no sign of a petrol leak.

Here's the problem/scenario.

I can start the van fine normally at home, drive miles and miles with no problem infact I drove to Warrington and back last month from Southampton without a problem until i stopped.

Any way often but not always if i stop and say go shopping or take a break when i get back in and drive off it will go 20 yards and the van cuts out. Guess its using fuel in the hose from the pump to the carb.

When it does happen I've found there is no fuel in the line to the pump from the tank so is it draining back to the tank?

I have a priming pump so i attach it and pump the fuel back and it runs fine. It runs fine when driving and never cuts out unless i stop and then drive off again.

It was mentioned about the tank causing a vacuum. I have opened the filler cap and there's no hiss?

Hope you can help.

Cheers

Dave

[kevtherev]

Hmm either there's an airleak in the line to let the fuel run back (now sorted) or the diaphragm is faulty in the pump

If you can't afford a pump then fit a one way valve to the fuel line near the pump

[djhammy]

HI thanks for the advice this is a new brickwerks pump built like a tank and the previous one was less that a year old and only replaced a few months back it as a method of elimination. Think the non return valve is a definite goer i think.

[djhammy]

Happened again tonight 3 times.

Fine driving to town, stop at in laws, get back in van 20 minutes later get 10 yards and cuts out. Pump up fuel again and its all ok. frive off, stop go shopping. get back in and same thing happened.

Later when picking wife up from college same thing again

If i leave over night though will be fine.

Could this be the petrol vaporising as the engine does seem very hot despite the guege saying other wise.

Did have a squeak a week or so back from what i thought was the water pump and have replaced the old and shabby belt to see if thats the problem. I wonder if its a cooling problem not being pumped around?

[djhammy]

Sounds exactly what is happening.

http://en.wikipedia.org/wiki/Vapor_lock" onclick="window.open(this.href);return false;

Vapor lock was far more common in older gasoline 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 bowl 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 fuel in the float bowl and reducing the level will reduce the fuel to air mixture delivered to the engine.

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 bowl 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 bowl 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 fuel 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, or using an active carbon filled canister where fuel vapor is absorbed. This is usually implemented by removing fuel vapor from the fuel line near the engine rather than from the float bowl. 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 an electric submersible 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 positive 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.

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.

[bumble bee]

ive had this happened to me tonight got going home from homebase and cut out a matter of yards down the road on a junction with a trailer on didnt realise it was a common thing gona change fuel lines and filters tomorrow just incase

[kevtherev]

It's not common
what makes you think that?

I have never read of a proven vapour lock problem with this engine in 5 years of posting here.