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mechanical fuel pump


BRDM Driver

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Would I be right in assuming that a mechanical fuel pump should be able to pull air, such that it can create a vacuum?

 

In other words if you stick your finger over the inlet and operate the lever, it should suck your finger tip?

 

My pump only seems able to pull fuel through if the outlet is not open to the atmosphere...

 

I guess the valves are leaking air.

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Does not sound good, either you have got something jamming a valve open or the rubber diaphragm has a split in it this could allow fuel into the engine sump depending how the pump is fitted to the engine.

 

You need to investigate soon and not use it the fuel will effect the performance of the oil.

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If the pump is off the vehicle you could try trying to blow through it the wrong way. The valves should stop you so if there is any air getting through with lung pressure you need to change both valves. I assume they are like most 'western' ones and can not be dismantled to get at the actual sealing faces. This will only show up if both valves are dodgy but is so simple to do that you might as well.

 

Similarly you could remove the valves and try blowing through them individually. Just remember which way up they fit!

 

Reading your post again I suspect that the outlet valve is not doing its job and when you put your finger over the outlet it is your finger that is acting as the valve. The valve may just have a bit of muck in it stopping it seating, it is amazing how small a bit of rubbish will cause a problem. Try removing the valve and blasting air through it, you might even be able to see the seat to some extent.

 

Good luck

 

David

Edited by David Herbert
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Bleeding pump! I really must stick an electric one in!

 

I have just stripped it down and the valve disks where so hard they actually snapped in my fingers!

 

The little mushroom shaped metal caps that hold the spring and valve disk are press fitted into the casting, so I wasted a bit of time making a tool to unscrew them!

 

I cut new valve disks from nitrile and cleaned up the seats. There was some signs of pitting on the seats so really I should re-cut them with an end mill or similar.

 

Anyway I opted to just give them a polish and it seems ok now. It will hold a vacuum with a finger over the inlet/outlet for ages.

 

You can hear the diaphragm jumping back when you take your finger off, so I'll stick it back in tomorrow and hopefully we'll get the beastie (BRDM) started.

 

It's been sitting for best part of six months having the brakes sorted so I really didn't need the pump to start messing me about.

 

I suspect this might also explain why I need to prime it after it's been sitting more than a few days. (I usually manually prime it to save caning the batteries).

 

Fingers crossed for tomorrow as next week it's booked into Tank Fest!

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At least you can tell what some electric pumps are doing by the rate that they tick. This aids the diagnostic process before you even break into a fuel system.

 

With old vehicles, it is not uncommon for dry mechanical pumps to not create a vacuum with your finger over the end. With one of our trucks that sits for long periods, it won't lift fuel initially. I suck a bit of fuel up from the carb end of the hose, then it will run fine all day long and continue to do so until such time that it dries out again. This particular pump has been stripped a few times and the valves are in good condition having been replaced in recent times. Lack of use seems to be the cause.

 

Vince

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