Fuel pump behaving badly

It’s a mk7, newish su square bodied pump . Tank has got new fuel , but the pump has started hammering away as if there is no fuel :fuelpump: , but the engine continues to run . What could cause this ?

Just fired mine up after rebuild and several year slumber . You have a leak somewhere. If its not leaking fuel, probably one of the internal non-return valves (brass disks) is not sealing/seating.


One possibility…There are two one-way valves in the pump. If the inlet valve is partially held open then the pump wouldn’t reach the cut-out pressure point, but rather keep 'ticking constantly.

Edit…( And just saw Dave’s answer… Yep that’s what I meant too :+1:)

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Thank you guys , when the lift is free I’ll check it out .

I have had vapor lock with my E-Type on rare occasions. Vapor lock is caused when the lift suction exceeds the vapor pressure of the fuel. The fuel boils off and all the pump gets is vapor. Usually happens on a very hot day, and can sometimes be traced to some sort of restriction at the tank inlet, like a collapsed inlet screen.

I have a similar problem, worse after servicing/cleaning my pumps,you can blowback through them. Has anyone tried to remove the valve body from these square bodied pumps? Heat is out of the question since the valves are transparent flexible plastic. Any attempts to unscrew the brass valve body just want to self destruct the slots in the rim of the brass body.

My Mk9 has the same issue, when one tank is half full. 2 fairly new ”square” pumps. The noise is essily heard inside when drivibg.
Will follow this topic carefully….

Peder check that the pumps are flrxibly mounted with the rubber grommets not over tightened. Too firm will amplify noise transmission into the frame.

Hi Randall,

I made a key from a piece of flat steel and used large adjustable wrench to turn it. I recall there was only one cage I could not remove this way.

The newer plastic reed valves are easy to obtain, I would not worry about destroying them with heat.

If you need a pic of my key let me know.


Hi Clive, yes I did exactly the same but it wouldn’t budge & damaged the slot edge so didn’t try it on the other one.

Hi Clive, a picture of your valve key would be appreciated.


Randall Botha

Here you go, quite crude!

I found a piece of scrap metal whose thickness was the same as the keyways in the cage and whose width was the same as the cage diameter. I then filed notches into the end to locate it and prevent it from slipping.

To use, I held the pump body in a vice and used a large adjustable wrench to turn it with one hand, pushing down on the key with the other hand to prevent it jumping out.



I can live with Crude if it does the job. :+1:

Thanks Clive, I did pretty much the same except for the small cut outs at the corners, I guess that helps to locate it more securely.

So we’re saying that these things don’t come apart without a fight ! What the hell did they tighten them with in the factory , not a piece of scrap metal I guess.If left to tick away will it damage the pump ?

I think they’ll carry on ticking merriliy as long as there’s fuel in them to keep them cool. Not much can go wrong except the points burning/corroding until they stop.

The factory guy had a tool called a bung spanner, basically that flat piece with a round supporting/locating sleeve and a permanent handle to it.
Larger versions are used for the bungs on oil drums.

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Yeo that’s what I need. Another problem is to hold & clamp the body securely while exerting great force to remove that bung. The body is fragile pot metal, mine has fine stress cracks from trying to undo that valve fitting.

Hi Randall,

Given the cage contains a spring and a brass disk only, a good flush with solvent will probably save having to remove it.


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