replacing low pressure in-tank pump with high pressure unit

[cunninglinguist]

With early cars (325e, M3, etc) that have the low pressure in-tank pump, this pump is supplying less than 8psi to the main (in-line) pump.

On later single-pump cars such as the 318iS, this in-tank pump is supplying much higher pressures. I have repaired such higher pressure pumps by replacing the dead motor with a Walbro 255lph unit or similar.

What would be the result if the low-pressure pump was replaced with a 150-250lph pump? What would be the effect on the main (in-line) pump?

[jeffnhiscars]

I would think you would want to delete the main pump and run a single. 2 high pressure pumps are redundant and may not play well together.

Just my 2c

[dnguyen1963]

If you want a two high pressure system then both pumps must be perfectly in sync or you are going to have fuel spraying everywhere. Why would you want to do this?

[StereoInstaller1]

With early cars (325e, M3, etc) that have the low pressure in-tank pump, this pump is supplying less than 8psi to the main (in-line) pump.

On later single-pump cars such as the 318iS, this in-tank pump is supplying much higher pressures. I have repaired such higher pressure pumps by replacing the dead motor with a Walbro 255lph unit or similar.

What would be the result if the low-pressure pump was replaced with a 150-250lph pump? What would be the effect on the main (in-line) pump?

You do understand that the fuel pressure regulator sends excess fuel back to the tank, correct? Having more fuel come from a transfer pump won't help a thing, except cycle more fuel through your system. What have you done to your motor that you need that much fuel?

[nando]

If you want a two high pressure system then both pumps must be perfectly in sync or you are going to have fuel spraying everywhere. Why would you want to do this?

what? No.

[Gary Horneck]

TRE 255 in tank pump, can the inline.

[cunninglinguist]

I'm not looking to supply extra fuel. I'm simply looking for an alternative to buying an expensive in-tank pump from BMW. The VDO pumps seem to be unavailable from the usual parts houses at the moment.

I was just curious whether anybody had tried to run two high pressure pumps.

[dnguyen1963]

what? No.

Example...external pump is dying and cannot pump as fast as a brand new high pressure in tank pump. Now you are putting a lot of pressure onto the external pump and things can go bad unless the external pump has a by-pass. Someone had tried this setup before and the result was not good.

[nando]

yeah, I don't think so. Unless the external pump is completely dead, all it will do is take all the load off the pump. I'd bet if somebody put in a high pressure internal and left an old external they probably had 25 year old fuel lines too.

there are some great pump curves out there for using walbros or the Bosch race pump with a lower capacity secondary pump. What actually happens is you can run a heck of a lot more volume of fuel at a given pressure than you could with a single pump.

[jeffnhiscars]

I'm seeing a dead or weak inline pump with no built in bypass reducing fuel flow dramatically. It maks no sense to leave it there if you have an adequate high pressure in tank pump