Technical Can fuel pumps leak air?

[smart51]

To try and solve the air leak into the fuel lines, I've replaced the rubber hoses with cavis hose and proper fuel clips. I've just been for a good run and since I've got home I've looked in the engine bay. There is a bubble of air coming up from the fuel pump towards the carb about every 3 seconds. Could the fuel pump itself be leaking air, or is it going to be coming from where the hose meets the pump?

 

[Goldnrust]

Yes it could. There is a diaphragm inside, that is moved up and down by the lever driven off the camshaft. If this diaphragm had a small hole it in, the pump could draw in air from inside the crank case through this hole.

It could also be a leak where the line meets the pump, would be on the inlet side as this is where the joint see's vacuum rather than pressure.

I would also check again with the car cold, it could be the fuel vaporising if the engine bay has got very hot after a good drive.

 

[smart51]

I ran the engine briefly yesterday, just enough to prime the fuel lines. After about 6 hours, there was a small air bubble just before the fuel pump and tiny (less than 1mm) bubbles could occasionally be seen above the fuel pump. 24 hrs on and there's about a 1cc air bubble just before the fuel pump, but the rest of the line is still full of fuel.

Could it be that the fuel pump sometime stops in the closed position and sometimes in the open position. When open, air is freer to flow?

Why does air get in anyway? Either there would have to be low pressure in the fuel lines to draw air in, or gravity would have to displace fuel downwards somehow by letting air in. No fuel is leaking out and as the cavis hose is a stretch fit, there can't be much of an air leakage path.

 

[fiat500]

The bubbles may not be air. Fuel can vaporise in the pipes, and has to go somewhere. I wouldn't be surprised if everyones' Fiat 500 did this. I've avoided it by having black pipes.
My understanding of the workings of a mechanical fuel pump is that there is an inlet and a exit valve, and both of these are held shut by a light spring. They work passively, and it is the diaphragm that might randomly stop in a half activated position. But without the vacuum it creates when in motion, both valves should always be closed.
I presume that these valves, which are simple discs, may sometimes fail or not close properly, perhaps when a small paricle gets lodged, and that may contribute to the problem of excessive drain-back of fuel when left standing.
It might be worth checking the condition of the solid fuel pipe in the engine bay as the originals are steel, and they can get quite porous.

 

[smart51]

The copper pipe is only a few months old. Fuel vapour is interesting. When the engine was hot, there were more bubbles. It could well be vapour. What I'm trying to stop is that the engine needing to reprime the fuel system after it has been sat for a few days.

 

[fiat500]

The copper pipe is only a few months old. Fuel vapour is interesting. When the engine was hot, there were more bubbles. It could well be vapour. What I'm trying to stop is that the engine needing to reprime the fuel system after it has been sat for a few days.

I think it's normal for this and some other older engines to need some degree of priming after standing for a week or more. With the very primitive breather in the filler cap there is no significant vacuum created that would keep a charge of fuel in the pipe and stop it siphoning back to the tank. And the relatively simple carburettor allows evaporation at the delivery end.

But the outlet valve of the pump relies on the weight of the column of fuel left in the supply pipe to keep it closed, and it wouldn't take much to wedge that open a little allowing that fuel to also be sucked backwards.

I find that after a week or two I have to crank thhe engine a fair bit in order to deliver fuel, which I do in a few short bursts of a couple of seconds. Without fail, the engine fires up after four or five bursts. In daily use this is never needed.

 

[Goldnrust]

How long do you have to crank the engine to reprime @smart51?

When mine has been left for several weeks, I would say it's still under 10 seconds for mine to cough into life. 10 seconds of cranking really isn't a big deal, and if its been under a week it fires right up normally.

It's worth remembering modern cars actually delay starting to allow the engine to start to move the oil round. They could fire instantly, but they run for 2-3 seconds on the starter on purpose. A longer cranking when the engine has sat for weeks might actually benefit the bearings!

 

[smart51]

Mine will need prolonged cranking after maybe 3 days. I crank it for a count of 10 cycles of the engine noise, which is probably 6 or 7 seconds, then let it rest for a moment. Sometimes it will fire up on the 3rd try, sometimes the 4th. If its been less than a few days, it fires right up.

 

[fiat500]

Mine will need prolonged cranking after maybe 3 days. I crank it for a count of 10 cycles of the engine noise, which is probably 6 or 7 seconds, then let it rest for a moment. Sometimes it will fire up on the 3rd try, sometimes the 4th. If its been less than a few days, it fires right up.

From my experien tcehat sound a bit tardy,, but compared with those who have to apply positive pressure at the tank it seems OK.