correct position of fuel filters

[lilianroyle]

The fuel filters on my (bmc 1.8 diesel) go as follows Fuel Tank to water seperator filter to lift pump to second filter to injectors. Is this correct?, or,as has just been suggested, should both filters be before the lift pump. Does it make a difference?
Many thanks

 

[Guest]

Your engine is in the "Normal" sequence of:

1. Tank.

2. Water Separator/Filter (should be about 30 microns).

3. Lift Pump.

4. Fine Filter (should be about 2 microns).

5. Injector Pump.

All filtration is a compromise. If you make the Filter too fine then you are forever changing out the damn thing. Too coarse and you damage the Pump it is designed to protect.

The Water/Separator Filter is usually 30 microns to give adequate protection to the Lift Pump and at the same time minimise change-out times.

The Fine Filter is 2 microns because the Injector Pump works to much finer tolerances (and costs a bomb if you damage it).

The reason they are fitted in the order indicated is that the suction to the Lift Pump only has atmospheric pressure driving it.

Fitting two filters in the suction can damage the pump due to what is known as "cavitation".

When the pressure into the pump falls (due to the flow restriction caused by the fine filter) the pump pulls a small vacuum "bubble" in the liquid on the suction stroke.

The vacuum "bubble" then collapses when the pump piston reverses and "implodes".

These tiny implosions can cause more damage to the piston, suction valve and cylinder than the dirt you are trying to remove.

Suggest you leave it as it is.

Best regards )

IanD

 

[vyv_cox]

Yes, I agree, this is the correct order. My last two boats have been equipped like this and I have yet to encounter any fuel problems, in 10 years.

 

[mtb]

Don't forget your lift pump has a gauze filter as well , you'd be surprised at how much gets past the agglomerater to the lift pump. Quite often I have found fuel starvation problems due to the pump's filter being blocked.
Also one last point the fuel pump pressure is in the region of 1.75 to 2.5 psi so don't expect the earth as the none return valves wear this gets lower.
There are now modern type pumps ( no not Su ) that generally pump over 3. psi which certainly for petrol engines is to high, the needle valves cant seat properly !! just one more useless bit of info BUT IF you consider the amount of times I recount this to customers who ring me with the same problem .Its handy to know.
Mick

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