MFI electric pump safety pressure relief valve

[Grady Clay]

MFI electric pump safety pressure relief valve

A friend wrote me:

I was testing the fuel pump on my '69E by closing the pump output line with a gauge attached. The pressure went VERY high, something like 100-200 PSI for a few seconds, then a bang, then the pressure dropped to the proper pressure. Obviously the pressure relief valve stuck closed; in proper operation it should never get exercised. The problem with this is, if something went wrong in the return or if a downstream fuel pressure regulator malfunctioned closed , the fuel lines , filter and other fuel components are not speced for the pressure.
I believe the pressure regulator should be periodically exercised, just like one on the top of electric water heaters. I think this would be important to all 911 electric pumps except possibly ones with CIS.
What do you think?

This is the MFI electric pump 901.608.105.00 (Bosch 058097 0001).
It is rated 125 liters/hour, 2800 rpm, 3.5 A @ 12 V.
The “Operating pressure” is 1 atm (14.2 psi).









While the MFI system pressure is regulated to 11.8 ± 3.0 psi by the fitting in the outlet at the fuel filter console, the electric pump ‘safety pressure relief valve’ never opens during normal operation. Porsche specifies it operating at “approx. 2 atm (28 psi)”.

I have never measured the ‘stall pressure’ of the pump. That is the 100-200 psi you measured with the safety valve stuck closed. I guess I have never had a pump where the safety pressure relief valve stuck closed.

I guess the downside of having the pressure spike like yours is it could start a leak at the O-ring or the cap over the pressure relief spring.

If the pressure in the MFI supply system were to spike to 100-200 psi, every hose and the fuel filter would be subject to failure.

I think the CIS pumps have a stall pressure considerably lower than the MFI pump. Since the CIS system pressures are considerably higher than the MFI supply pressure, the integrity of the plumbing is much better.

The carburetor pump (similar to MFI) has the pressure regulated by the pressure relief valve in the pump. Porsche specs that at “0.2 – 0.3 atm (2.8 – 4.3 psi)”. The pressure relief valve is operating continuously.

I agree.
Whenever the opportunity presents, the safety “pressure relief valve” should be tested for function.
I would never do this in the car as the MFI supply system could be damaged if the valve fails to open.

Who has more to add?

Best,
Grady

Mechanical fuel injection resource index

How To: MFI Electric fuel pump overhaul

[356RS]

Well Grady I think your friend has made an excellent point on the fact that the pressure relief valve can get stuck closed because of never being operated. I agree that the safety valve should be exercised/opened at times to make sure it functions properly. Although the chances of having an over pressure event happen are very rare, the fact that this could cause a dangeres fire hazard definitely merits the testing of the valve.
Removing the pump for testing would be the safest and using something like ******** Solvent with a small amount of Marvel Mystery Oil mixed in would make it a little safer than gasoline plus adding some lubrication to the safety valve components. With the proper gage at the outlet port you could then monitor the pop-off pressure point.
Any pictures or drawings of the pressure valve components around?