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MG TD TF 1500 - Fuel filter bypass?

This is a purely theoretical question. Reading about fuel filters, the argument is that a clogged fuel filter can destroy the fuel pump; so my question is has anybody heard of a fuel filter with a bypass? I know that oil filter or oil cooler systems sometimes have bypasses for exactly this reason, so that a clog won't end up starving the engine of oil...
Obviously the bypass would only operate above a certain pressure.
Just wondered if these exist?
Geoffrey M Baker

Put the filter after the pump and there will be no issue. You don't need one before it. The tank and pump both have screens in them to block larger debris, and the smaller stuff will pass right through.
Steve Simmons

It was just a theoretical question. As I just rebuilt my tank and know it's squeaky clean, and have a brand new filter in the tank, I should have no problems for many years :)
Geoffrey M Baker

Well, never heard of such a thing but if you can imagine something then someone else probably has too! So maybe is has been done, but I don't see why it would be worth the effort. The bypass in the oil system exists because an oil blockage will pretty much destroy your engine. A fuel filter blockage does nothing except make the car sputter a bit. No harm there, and bypassing it would just mean that the average person would never replace the filter.
Steve Simmons

Geoffrey M Baker

Seems that it is so easy to change out a clogged filter that a bypass wold not be necessary. My thought.

... CR
C.R. Tyrell

Geoffery's question has merit when comparing it to the oil filter bypass valve in an engine. Following the reasoning for a bypass valve to save an engine, why not one to save a fuel pump. The answer has to be an oil bypass will save an engine that costs up into the thousands of dollars, whereas a bypass valve for a clogged fuel filter would probably cost more than a new pump ':). The real fix is either no filter or one that is course enough that it won't clog easily. Cheers - Dave
DW DuBois

I don't think a clogged filter would kill a pump though, unless you're talking about the unusual issue of an SU pump and a pre-filter. In ordinary circumstances the pump would simply stop pumping, figuring the floats were full.
Steve Simmons

Currently, I'm running with a large glass bowl fuel filter (originally for 50's Chevys). I installed it to replace an inline filter which clogged up regularly, as the tank was filled with junk. Since I've had the tank rebuilt completely (cut open in every baffle, sandblasted, soldered shut and recoated) and installed a new screen in the tank, it's all been perfectly clean. I may remove the filter as it seems to no longer be needed, and I do worry about damage to the pump.
Geoffrey M Baker

Steve - "I don't think a clogged filter would kill a pump..."
A clogged filter on the inlet side of the pump will cause the pump to stall in a current on condition. It doesn't take very long for a pump stalled in this current on condition to burn out the swamping resistor that is hidden inside the coil housing. Once the swamping resistor is burned out, the excessive arcing will cause the points to become excessively pitted and in need of renewal. I see this often in the pumps that I restore and I have a stock of carbon resistors (the original swamping resistor is a length of resistor wire wrapped around the coil) on hand as replacements. at one point, I used an O'scope to check the reverse spike produced when the points open during normal operation to see just what kind of an inductive spike was being generated when the points opened and was surprised to find that with no arc suppression of any kind, the spike was over 500 volts at a very short duration. Just having the swamping resistor in place, the spike dropped to around 100 volts and the capacitor or the diode/resistor assembly would further drop the remaining spike to less than 50 volts. With the swamping resistor, the arc suppression devices would only drop the voltage to somewhere between 100 and 200 volts. This the reason that some pumps seem to always burn up the points in a short period of time. Cheers - Dave
DW DuBois

If the filter is placed on the inlet side of an SU pump, and it becomes clogged, then yes it can damage the pump. But a clogged filter after the pump will make no difference whatsoever to the pump, except that it doesn't have to work so hard anymore. ;)
Steve Simmons

"But a clogged filter after the pump will make no difference whatsoever to the pump, except that it doesn't have to work so hard anymore. ;)"
That is absolutely correct. I think I lost track of which side of the pump we were talking about. Cheers - Dave
DW DuBois

This thread was discussed between 06/10/2015 and 08/10/2015

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