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'73E Fuel Pump Question

 
Old 04-04-2013, 09:53 AM
  #1  
justacoupe
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I'm involved in the slow and methodical process required to "wake up" sleeping beauty: our one owner 66K mile '73 911E that has been "sleeping" in climate controlled storage for the past 24 years.

I have a question re. the Bosch fuel pump (between tank and MFI pump): Bosch P/N 0580 970 001 (Porsche part 901.608.105.00).

I removed the pump for bench testing yesterday, and good news so far. First, the gas that remained in the pump (and drained out when inverted) was "gas color" clear with no flakes or debris. I hooked her up for bench testing, using a clear container filled with a 50-50 mix of diesel fuel and SeaFoam. I used clear tubing on all three fittings, drawing from and delivering back to the container.

Positive to positive and negative to negative (off a trickle charger) found her spinning happily from the get go. The mixture circulated as expected and remained the same color as when added: no stains, rust, debris were flushed out. I ran it about 3 or 4 minutes and she stayed cool to the touch and no leaks.

Today I will measure fuel delivery (I've read that it should be 900 to 1000 cc in 30 seconds). But, before I do that, I have a question regarding the "return" line (that returns fuel to the gas tank).

When bench testing, the intake line (from tank) draws steadily and the output line (the one that delivers to the MFI pump) flows as expected. But, there is also a more or less small but steady flow on the return side (maybe just 20% or less of the amount being pumped to the MFI pump).

Is that to be expected (normal)? For some reason I thought that line would only return fuel when there was a certain pressure exceeded. In this case, with totally "free" flow and no restrictions, I expected no flow via the return line.

I'd appreciate any input as well as suggestions for additional testing before I do the cosmetic restoration.

Thanks in advance...

Bill
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Old 04-04-2013, 11:46 PM
  #2  
pu911rsr
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Bill,

I'm pretty sure those pumps use a recirc circuit that is used to keep pump cool.

Phil
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Old 04-08-2013, 11:44 AM
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justacoupe
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Thanks, Phil.

The bench test of fuel pump output was as follows:

Spec. = 125 liters per hour (equates to about 35 ounces per 30 seconds).

Actual Output = 138 liters per hour (fed 39 ounces in 30 seconds) or about 10% over specification.

Relief valve "test": When the output line was pinched off, the valve opened as expected. However, even when the output line is not restricted, there is a small dribble into the return line (from relief valve). The flow rate is just 3.5 liters per hour (or about 2 ounces per 30 seconds).

I'll assume the pump is "good to go" unless someone lets me know otherwise. Any comments welcome.

Thanks,

Bill
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