'73 911E Fuel Pump Question...

[justacoupe]

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 been taking a lot of photos as I run through the various steps, and have benefited a lot from postings already made on this forum. I do plan (when I'm finished) to start a thread and show all the photos, etc. But, don't have time to do that now, as it would interfere with getting it done.

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 diesel fuel and SeaFoam (in a 1:1 ratio). I used clear tubing on all three fittings, all drawing from, and delivering to, the diesel fuel/SeaFoam 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. In other words, 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.

My question: tomorrow 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 steady small flow (maybe just 20% or less of the amount being pumped to the MFI pump) that steadily flows through the return line.

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

[E Sully]

I'm not sure about the MFI fuel pump, mine is similar design CIS, but the return is from the fuel distributor pressure valve. I would think that your design with the return line being part of the pump that the pump is designed to exceed the requirement of volume and pressure for the fuel system at full throttle so there would always be some fuel in the return line.

[Rusty 356]

Bill pretty sure what you are seeing is correct. There is a pressure relief valve in the pump and that may be the small return line to the tank. I would be more concerned with the MFI injection pump following your delivery pump testing. Seals, etc. Not really a waste of money to have it freshened prior to installing as well as pressure testing the injectors and checking atomization and leakage.
How could you keep your hands off this baby for 24 years?

[Matt Smith]

I'm no expert, but I wouldn't expect that line to be flowing unless the pump was overpressured. Grady had a good post about this. I think from memory that the overflow comes in a 2 bar pressure? Operation of the pump normally is 1 bar.

[Porboynz]

On my pump a small amount of fuel did escape through the over pressure return valve outlet when the pump was operating. Just a dribble though, so why not get your pump running again and block the output line a few times to see if the pressure relief valve reseats itself. I did this myself after the pump had sat unused for 10 years, I think I pumped kero through it as I had heard it was a lubricant and certainly less volatile than petrol. I used a pair of vice grips to crush the output fuel line a few times while the pump was operating, it fair groans when you do this and the pressure relief valve opens, but then you know it's not frozen shut.

[justacoupe]

Thanks to everyone who replied!

My testing of the output was delayed... will happen today. When I do, I will also measure the amount that the return side is releasing (really just a "dribble").

I did find a couple of threads that had a lot of useful info:

Porsche MFI fuel pump conversion

Electric MFI pump by Holley adapt for Porsche

From those threads, it is clear that the "primary" pressure relief in the MFI system is the one located at the fuel filter. If pressure exceeds spec, it releases fuel back via a connection to the same line the pump is attached to.



Looks like a "split decision" from your four guys regarding the small amount being released on the return side.

Rusty & ED: As you have stated, as long as the pump volume tests good today, it seems a small amount of flow from the relief valve shouldn't be a concern. It sounds like the release valve at the pump is there for pump protection while the one at the fuel filter is the "final" check of pressure before the MFI pump.

Matt & Poorboy: Even though a small amount of return may be OK, based on your comments I will test the relief valve as Porboy suggests. Perhaps the valve isn't seating properly.

Matt: is the thread you are referring to one of those listed above? I am not sure I have found the one you are referring to. The forum member using "Grady" as a handle has not made any fuel system related postings.

Rusty: Is it still Pacific Fuel Injection as the recommended MFI pump rebuilding service? Anyone else I should consider? And yes, it was hard to have her stored for so long, but my work kept me away from her (plus I spent a long time restoring a Sunbeam Tiger).

I'll post the results later today.

Thanks again!

Regards,

Bill

[justacoupe]

Matt: Just found the thread you were referring to (and now realize the "Grady" you were referring to is Grady Clay). Thanks for your help!

Bill

[justacoupe]

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

[Rusty 356]

Looks like you should just keep moving forward to the injectors. Your local Bosch diesel shop can pressure check your injectors and with a little more time with test fluid can improve atomization.
Keep us informed as to your progress.

[Porboynz]

Originally Posted by justacoupe

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

Sounds good to me. Maybe there is a small bleed quantity of fuel that flows into the return line even when the relief valve is closed, mine was about the same.

[Matt Smith]

I think Porboynz is on the right path.
You're attempting re-invograte an ancient, expensive and wonderful fuel injection system (which is extemely cool), and there are a thousand posts- and the equal quantity of helpful people- on Pelican willing to help. I suggest you continue posting and asking for help. The answers will flow.

[justacoupe]

A "before" shot when removed, and an "after" shot following testing and cleanup. Now on to the fuel filter...





Thank you, again, to all who responded to this thread.

Regards,

Bill

[Porboynz]

Very nice finish, thanks to you for the feedback.