Leaking Fuel Pump

[Quickstep192]

I just went to fire up my ‘73 but stopped when I heard a dripping noise that sounded like it was coming from underneath the rear seat on the drivers side.

Sure enough, there’s a profuse fuel leak at the pump. Here’s the weird thing though - the hoses look fine; the leak appears to be coming from the pump itself.

Assuming that what I’m seeing is actually the case, is replacing the pump the fix?

My car was originally MFI but was converted to Webers by the PO. Should I replace the pump with a fuel pump for carbs?

I appreciate any insight.

Oh by the way, how does the fuel pump come out?

[Quickstep192]

After removing the pump, it appears to be leaking at the socket for the electrical connection.

Is there any way to fix this?

[Superman]

Probably not. But a FP for a car with carbs is not going to be massively expensive.

[E Sully]

Personally, I would send it out to be rebuilt, but here is a thread for an MFI pump. This is assuming it is the original MFI pump. If running carbs, I would replace it, as the MFI pump is much higher pressure than you need.
MFI Open Heart Surgery II - The Fuel Pump

[Quickstep192]

Does anyone know if there’s a carb version that’s a direct replacement? I’d like to be able to replace the pump without too much re-work of the hoses and mount.

[47silver]

1972european 911t’’s had carbs. I think you want this

#911.608.107.03

[Quickstep192]

Thanks for the part number. I notice that pump has three fittings just like the MFI pump has.

Does the fuel system for carbs still have a return like the MFI fuel system has?

[stownsen914]

Originally Posted by Quickstep192

Does the fuel system for carbs still have a return like the MFI fuel system has?

Yes, and maybe yes twice. MFI has two returns - one at the electric pump and a second one from MFI pump on the engine (and they T before heading back into the tank).

Sounds like you still have the one at the electric pump. For carb conversions, whether a return is there depends on how the conversion was done. If it has a PMO regulator, then there will be a second return line. While not required, the second return line is a good idea even with carbs. If your setup has a fuel pump intended for fuel injection (including MFI), then you need to have the second return line or your fuel pressure will be too high.

As for your leak, leaking at the plug is a common failure mode for those Bosch pumps. There's an O ring located just inside the plug and a second one for the pump body that can be replaced as part of a pump rebuild.

If you replace the pump, the return line from the pump isn't really necessary, and most (all?) non-stock replacement options won't have it. You could just eliminate that portion of the fuel circuit.

[Quickstep192]

Thanks for that.

Here’s the darndest thing -

After discovering the leak, I pulled the pump out for a closer look. I got under the car while my wife turned the key (I know, dicey move). With the pump in full view, the leak had reduced to just weeping.

So I thought, I’ll try to start the car and see what happens. It didn’t start and I discovered that all 4 float valves were stuck. After removal and a good cleaning, I put the float valves back in and the car started. Woo Hoo! So I went back to the fuel pump to ponder how to carry out the replacement. Again, I had my wife turn the key while I looked at the pump. This time, there was no leak; not even a hint.

Is it possible a seal was dried out from sitting and sealed back up once the pump got full of gas again? Could the blockage at the float valves have been a contributor? Can I trust it?

[Locker537]

Do you currently have a return line?

[Quickstep192]

Originally Posted by Locker537

Do you currently have a return line?

Yes, the car was originally MFI and has a return. There is a valve on the return that constricts the flow going back to the tank to “regulate” the pressure going to the carbs.

[stownsen914]

With a return line, the stuck float valves shouldn't have caused an over-pressure situation at the pump. The pump really shouldn't leak under any circumstance. The fact that it did, suggests it will again. Most likely when it's really inconvenient . That's not to mention the possible fire hazard of leaking gas. Personally I wouldn't trust it.

Good time to examine the fuel lines. Old hardened fuel lines are a common issue also, another fire hazard.

[Quickstep192]

Great point about the stick valves not causing an over pressure situation. It occurs to me that once the float bowls are full, the valves will be closed anyway.

As for checking the hoses, the line from the tank to the pump is new, but the return lines are not new. If a new pump that doesn’t require a return would let me eliminate the return lines instead of replace them, that seems like it would be an added bonus.

[Quickstep192]

Can someone post diagrams of the fuel line routing for both MFI and Carbureted?

On my car it looks like the return goes to both the pump and to the tank which I don’t understand.

[spuggy]

Originally Posted by E Sully

the MFI pump is much higher pressure than you need.

Well, yeh, that's true. You could replace with a cheaper fuel pump for carbs. Note that Bosch say (italics and emphasis are theirs):

Modern electric fuel pumps are “Positive Displacement” type pumps, this
means that the pump cannot produce pressure unless it is acting upon a
restriction. The only restriction in the system should be the fuel pressure reg-
ulator. The regulator controls the system pressure; the pressure ability or
flow volume capacity of the fuel pump will not alter the system pressure.

Note that current draw is related to regulated system pressure as well, eg:

Check the current draw of the fuel pump which should be less than 6.5 amps for EFI systems; for K and KE Jetronic systems 11
amps.

(From the Bosch Motorsport fuel pump document).

So if you can make a CIS or 3.2 pump fit, it'll also work, with an appropriate FPR.

Bosch motorsport pumps are widely available - and usually far less costly than the 911-specific pumps they out-perform by wide margins, thanks to the economies of scale. Just sayin'.

Although the beloved 044 pump is no longer available, having been superseded by a modular replacement; discussion about it here Bosch fuel pumps - 0580 464 200 versus 0580 254 044

Originally Posted by E Sully

MFI Open Heart Surgery II - The Fuel Pump

That's a great thread. If all that's wrong with the pump is a leaky seal, why replace it?

My tub had over 230K miles on the original pump when I replaced/upgraded it with an 044 in 2007, because I worried that it was noisy. Didn't change my CIS symptoms one jot - and it was every bit as loud as the 911S pump, LOL...

[stownsen914]

Originally Posted by Quickstep192

Can someone post diagrams of the fuel line routing for both MFI and Carbureted?

On my car it looks like the return goes to both the pump and to the tank which I don’t understand.

This is the T I mentioned in my earlier post. There are two return lines on MFI cars - one from the electric fuel pump and the one from the engine. They meet in a T before heading back to the tank.

Edit - here's a thread showing the MFI fuel schematic
MFI fuel delivery system question...

[Quickstep192]

Originally Posted by stownsen914

This is the T I mentioned in my earlier post. There are two return lines on MFI cars - one from the electric fuel pump and the one from the engine. They meet in a T before heading back to the tank.

Edit - here's a thread showing the MFI fuel schematic
MFI fuel delivery system question...

Do you know what the purpose is of having a return from both the pump and the engine? And, is that necessary with a low pressure pump on a carb’d engine?
I’m guessing the return helps reduce an over pressure situation when the float bowls are full and the valves are closed? Otherwise, the fuel might blow by the float valves, even with a low pressure pump ? Just guessing…

[stownsen914]

I've heard that while not necessary, a return line on a carbed engine does have a couple benefits:
- More accurate regulation of fuel pressure (assuming a proper regulator is used)
- Keeps the fuel cooler due to fresh supply.

If you don't have a return line ("dead head" setup), it's important to have a fuel pump rated for low pressure carb applications. Then you won't have an issue with over-pressure at least.

[47silver]

1970 911T fuel pump for Zeniths?

This is for a 1970 but would be the same as a european 1972 911t with a 2.4 liter motor like the 73