Fuel return line ?

[jury]

So I fitted a new fuel pump to the ModRod.

2.2 T/E
Weber 40 IDA

Previously had fitted a FACET pump in the engine bay, which was loud !! I updated to fit a PIERBURG style and mounted near top the petrol tank, similar to the original Porsche location. The PIERBURG has been great, quiet and a constant flow, no banging noises.

Drove it yesterday, amazing pull and then cut out (on the Motorway again!). The pump was completely dead, managed to get a tow home, started to have alook underneath. All the wires are still connected, turned on and off a few times and it started working ! It drove great to the motorway, but when started to step on it, it started to stumble before cutting out.

First thing to look at is the fuel pump relay, or lack of ! Now realise there is not a separate relay fitted ...Maybe this is the problem.

OR

Is there supposed to be a return line somewhere to the tank ?

The pump has 1 inlet and 1 outlet, I have seen that the original pumps have 3 connectors . Some research says that no return line is needed, some say to connect a T-union to allow a return to the tank.





Anyone had any experience with this ? Any pictures of similar setups would be great


Cheers
Chris

[Strictly]

The cut out could be due to

A) General wiring (power/earth)

B) Overpressure/Blockage

Im thinking you have a 72 on carbs? As you said, the original pumps located near the torsion bar, had a return built in to return the excess flow (basically it was a pump pressure relief so the pump could produce a constant flow at a set pressure). Im fairly sure a 72E and S was the same, just higher pressure, but ive not owned one, and a bit rusty so I cant remember.

If I was running carbs, YES i would use the return feature. I would do it one of two ways, use a PMO regulator which has this feature or use an original pump which had it built in. I don't know that it really matters, like you I have read conflicting opinions, but I have decided this is the better way to do it, and incidentally the way I will go in a few months.

I have also run a 67 911 with a 73T engine it it (2.4 carbs) which did not have a return (but I did measure and regulate the pressure) so as to not over power the needle valves in the carbs....I had no problems with it over 3000 miles, so you can get away without a return certainly....

Regardless of which way you decide to do it, you need to measure and regulate the pressure.

===

To me it sounds electrical. I cant imagine you have an over pressure issue....Make sure you have a good connection on both ends (power), for earth I would just earth near the pump. You can just about see my brown earth right on the cross member, on a 75' I moved the pump to the front on in the picture below (CIS fuel set up, so fuel return from Fuel Distributor itself). SC suspension cross bar would have been neater (to tuck pump out of the way), but I used what I had. PS, after I took the pic, I put the inlet line over the torsion bar like yours! Otherwise the cover hits it.

In terms of problem solving...you could perhaps have a dodgy ignition switch. This is what I would do to troubleshoot. I would put a regular on/off switch inline to the pump and connect it to a constant 12v (i.e battery terminal), with or without a relay. Then simply turn the switch on or off, but regardless of the ignition/key. Drive again, and if this time it works fine, then you can conclude it is ignition wiring related, rather than a direct problem with the pump.

If it is the same regardless, and you are certain you have good electrical connections, then I would focus on over pressure as being the problem....so start by measuring your pressure (inline while the pump is running, and engine is running)...and then go from there. The pressure at a pump output point will always be the highest, so if you want to be through because you think overpressure is what is cutting out the pump....then measure inline at the pump output, and at the point where you actually want to regulate the pressure (i.e at the carb needle valves). You can then compare your actual pressure to what the pump should be producing the flow against, and see if you have an overpressure situation.

If its neither...then try another pump

[rhd racer]

My experiences of both fuel systems in the same car - 911SC engine in a 914/6 competition car;

- Fuel injection (certainly CIS) pumps produce very high pressure required for the FI to work
- The PMO regulator solution (usually fitted to the engine bay firewall) throttles down the pressure so the carbs get a much lower dose of fuel. They are really designed to convert one system to another, (and therefore saving you the expense of a pump), but I must confess I was not keen on having very high pressure fuel in the engine bay when I didn't need it to be
- Does require a return

Onto carbs, early 911s ran a low pressure fuel pump like a Webcon
- These provide both lower pressure and a flow rate matched to the carbs
- Does not require a return (seems from the below some did, some didn't)
- Are obviously safe with a jubilee clipped rubber hose, rather than HP hose and unions etc

So when I converted from CIS to carbs (40 IDA3C) on a 3.0SC motor, I changed the pump (Webcon - as recommended to me) to match the carbs, blanked off the return line and left it redundant. I did however use an electric pump too, as this was to fill a swirl pot to prevent fuel starvation when competing. So my set up was;

Tank > Facet > Swirl pot > Webcon > Carbs

For road use you should be just fine with a low pressure pump and carbs. I think Filter King make a filter and pressure regulator unit in one, which to Strictly's point gives you the best of both worlds. I ran a relay for my pumps to ensure a reliable feed, but would have thought that this would have manifested itself with your old set up unless something has changed (eg. poor connection, corroded fuseholder or connections)

I hope that helps.

Cheers
Wayne

[rhd racer]

Just had another thought. Could the fuel tank breather be blocked, or got kinked whilst you were under there, so when there is a big draw from the pump that the tank is pressurising cutting delivery?

[jury]

Thanks guys, much appreciated !

Started to go through everything, all connectors are in place. The new pump is good for 2-6 PSI delivery, I have a Malpassi filter setup up which allows the pressure to be set at 3PSI.

I have a 70a Relay on order and have a plan to re-route the fuse panel to connect it efficiently, get rid of some unnecessary pigtails connectors. It is currently running through a separate 20a toggle switch, which I thought would be ok, but maybe it does need the Relay after all

I re-routed the tank breather hose when it cut out the first time around.....The original setup had made a P-TRAP !! The new breather is now much simpler and direct....essentially just a piece of fuel hose (to prevent any kinks) that exits past the steering rack.

It currently starts fine and sits quite happy at idle...800-1000 rpm and 3psi fuel pressure.....It is fine if I stay on side streets, 2nd gear, its only happened when I step on it...over 4500-5000 rpm and shift up to 3rd and 4th.


Ideally need a rolling road !


Will report back

Cheers
Chris

[Strictly]

It currently starts fine and sits quite happy at idle...800-1000 rpm and 3psi fuel pressure.....It is fine if I stay on side streets, 2nd gear, its only happened when I step on it...over 4500-5000 rpm and shift up to 3rd and 4th.

If we focus on that....Can you get it to cut out with the car stationary and revving to 6000 like if you were checking your ignition timing?

Besides current draw perhaps being too high at maximum flow....the only other thing I can think of to cause the pump to cut out at 5000 rpm is some sort of fuel starvation, which because the pump uses the fuel to cool itself, results in too high an internal temperature...what about if you drive with the fuel cap off? ...not ideal but worth a quick test....you need atmospheric pressure to push the fuel in (high to low pressure). We don't think of the air we breathe as high pressure, but it is, pumps don't suck (even though they we call it a suction line), they just create a low inlet pressure. It looks like they suck, but in reality it is high to low as the driving force.

Perhaps try a flow test of the pump, with and without the fuel cap fitted so the tank is 100% breathing, even if it doesn't show anything, worth a quick blast to see if it makes a difference. If not, check for a blockage maybe....what about the center of the fuel tank....that strainer?

If not and you keep having the same problem....then try turning your pump 180 degrees, so you have a shorter suction line. I DOUBT it is the problem, but if you are really scratching your head after checking everything then worth ago. I work with pumps for a living (industrial process), and if anything I know the flow/pressure calculations inside and out. Although the system is "flooded" you can have a situation where the pipe length/diameter results in too high a pressure loss, and basically the pump cavitates.....normally that would just be noise, like your old one (and the long suction line). It sounded like is was running with some cavitation...this is why I doubt it is the issue, but worth ago.

Perhaps other explanations could be a blockage or other restriction on either the inlet or discharge.