Fuel pressures

[Ivelosttheplot]

Hi all 

does anyone know what the return fuel pressure is from engine back to the fuel tank

also if the nut and tail fitting onto the return of the tank can be acquired separately and not with the hose

regards

keith

 

1976 2.7s

k injection 

[PeterK]

IIRC the return to the tank is M14 (fine), with the connector having a convex orifice

I bought a 90’ tube with appropriate end shape and a 14mm nut separately from fleabay when I put new flexibles on the ends of my ‘through tunnel’ lines

[Ivelosttheplot]

Yes it’s that convex fitting that’s awkward 

obviously required to make a good seal.

Flow line is no problem 

Return line to tank:          there’s not much room around the pump and tank, plan is to utilise an existing fitting by brazing the connector that I need to it.

Im using -6 stainless covered fuel proof Goodridge hose for flow and return

if anyone knows the fuel return pressure that would be good

regards Keith 

 

 

[apesman]

Do you mean this?

https://www.design911.co.uk/fu/prod144558/Banjo-Fitting-M14-to-M10-90--Tube-Porsche-911-69-75/

The banjo bolt and washers are available too

[PeterK]

Unfortunately that’s the fuel pump feed side (tank outlet), not the tank return connection

[apesman]

I used this on mine

https://www.design911.com/Fuel-Tank-Installation-Kit--Porsche-911--20-27--10-63-07-73/prod132733/

[PeterK]

Thinking back, & IIRC, on my tank return the metal pipe was OK, so I cut off the crimp, replaced the rubber pipe and just needed a new M14 x 1.5 nut on the old metal pipe.

[Strictly]

22 hours ago, Ivelosttheplot said:

Yes it’s that convex fitting that’s awkward 

obviously required to make a good seal.

Flow line is no problem 

Return line to tank:          there’s not much room around the pump and tank, plan is to utilise an existing fitting by brazing the connector that I need to it.

Im using -6 stainless covered fuel proof Goodridge hose for flow and return

if anyone knows the fuel return pressure that would be good

regards Keith 

 

 

The fuel return pressure on the return end is very low (estimate of 0.25 to 0.5 bar.g max)

When we talk pressures is a flowing system, it is really a "back pressure" (i.e the pressure is the counter force created by forcing a liquid through a system). If we just think of a pipe, this counter force is the drag on the pipe wall (which creates the back pressure from flow through the pipe). In addition to that you have fittings, heights, equipment, etc)

Lets start at the fuel tank end and work back. At the fuel tank end (which is vented to atmosphere), as the return pipe enters the tank, it is only the head (height) of fuel in the tank (say 200mm), which is acting as against the flow into the tank. This is the back pressure just inside the pipe before it actually enters the tank.

Estimate height of 200mm of petrol which is approximately = 0.014 bar (based on a SG of 0.72)

Now now add the return line to get an estimate for inside the engine bay just after the FD or WUR. 

• 3m pipe (guess)
• 10mm ID (guess)
• pump 2 L/min (from TSB)
• Viscosity of 1 (near enough for this)
• This works out as 0.01 bar.

Add these together (0.01 + 0.014) = 0.024 bar. Very low, now add a safety factor for the fittings which we didn't bother to calculate, and even so you are at 0.25 to 0.5 bar.g max, so overall the return side is a very low pressure.  Certainly enough to use a rubber pipe and fuel clip. I would not rely on an interference fit, but a clip with barb will be fine. I have done it myself on my own SC, and other 911's!

----

Just for interest, the pump side is quite different. 

On the supply side of the pipe, again low pressure (the height of fuel - fittings and pipe). On the delivery side of the pump however it is more like 6 to 7.5 bar.g at the very end of the pump (just an estimate). The fuel distributor (Aka system pressure) is regulated to say 5 bar.g at the fuel distributor end, so then you add the 3m of pipe + accumulator + fuel pump check valve  + fittings, and I estimate 6 to 7.5 bar at the pump discharge end (just before check valve). It is an estimate to illustrate how different it is to both the pump supply side, and the return side. Even though the FD may be at 5 bar.g, and the WUR at 3.g bar (warm), once the excess fuel is diverted into the return leg, the only "back pressure" at this point is the from the return pipe drag, fittings, and height of fuel still in the tank. If the tank was pressurised then it would be different. 

---

I work for a pump manufacturer, and calculate pressures in pumped systems on a daily basis. I also work on 911's, but that is just for fun =P 

Luke.

 

[Ivelosttheplot]

Wow what an answer but we’ll explained that I understand 

my reason for asking is this:

for my 76 2.7s backdate project I’m using Goodridge-8 stainless hoses suitable for modern fuels something I used for my GT40 for many years . I’m using this on the Porsche For flow and return lines. Changing this of course creates the problem of matching fittings from Goodridge AN to Porsche. 
At the tank return inlet is a 12 mm male and I have to either make a fitting to suit or use a fitting that fits tank and jubilee clip to the return stainless line.

if hose and clip will suffice this may be an answer if I can’t make one. if the return pressures are this low the jubilee clip would be more than suitable as you said

thanks

Keith 

 

 

[Ivelosttheplot]

Sorry I used -6 not -8 

typo error

[Ivelosttheplot]

Thankyou for the replies received 

although after researching the viability of using Goodridge fuel hose it transpires it’s not suitable for modern fuels such as E10.

I find it confusing that they advertise as for all oils and fuels yet mention not suitable for E10 which is a fuel 🤔 

anyway they offered a full refund and I’m going back to genuine Porsche fuel lines thus making life easy

i would suggest fuel lines be researched as the outcome could be disastrous 

im also finding as a Porsche club member the genuine parts are often cheaper than other suppliers.

regards Keith 

 

  

[eebee]

On 23/10/2022 at 23:29, Strictly said:

The fuel return pressure on the return end is very low (estimate of 0.25 to 0.5 bar.g max)

When we talk pressures is a flowing system, it is really a "back pressure" (i.e the pressure is the counter force created by forcing a liquid through a system). If we just think of a pipe, this counter force is the drag on the pipe wall (which creates the back pressure from flow through the pipe). In addition to that you have fittings, heights, equipment, etc)

Lets start at the fuel tank end and work back. At the fuel tank end (which is vented to atmosphere), as the return pipe enters the tank, it is only the head (height) of fuel in the tank (say 200mm), which is acting as against the flow into the tank. This is the back pressure just inside the pipe before it actually enters the tank.

Estimate height of 200mm of petrol which is approximately = 0.014 bar (based on a SG of 0.72)

Now now add the return line to get an estimate for inside the engine bay just after the FD or WUR. 

• 3m pipe (guess)
• 10mm ID (guess)
• pump 2 L/min (from TSB)
• Viscosity of 1 (near enough for this)
• This works out as 0.01 bar.

Add these together (0.01 + 0.014) = 0.024 bar. Very low, now add a safety factor for the fittings which we didn't bother to calculate, and even so you are at 0.25 to 0.5 bar.g max, so overall the return side is a very low pressure.  Certainly enough to use a rubber pipe and fuel clip. I would not rely on an interference fit, but a clip with barb will be fine. I have done it myself on my own SC, and other 911's!

----

Just for interest, the pump side is quite different. 

On the supply side of the pipe, again low pressure (the height of fuel - fittings and pipe). On the delivery side of the pump however it is more like 6 to 7.5 bar.g at the very end of the pump (just an estimate). The fuel distributor (Aka system pressure) is regulated to say 5 bar.g at the fuel distributor end, so then you add the 3m of pipe + accumulator + fuel pump check valve  + fittings, and I estimate 6 to 7.5 bar at the pump discharge end (just before check valve). It is an estimate to illustrate how different it is to both the pump supply side, and the return side. Even though the FD may be at 5 bar.g, and the WUR at 3.g bar (warm), once the excess fuel is diverted into the return leg, the only "back pressure" at this point is the from the return pipe drag, fittings, and height of fuel still in the tank. If the tank was pressurised then it would be different. 

---

I work for a pump manufacturer, and calculate pressures in pumped systems on a daily basis. I also work on 911's, but that is just for fun =P 

Luke.

 

Hi Luke, just seen your expert post and wondered if you had any thoughts on an issue with my ‘74 Targa.  It was a complete (nuts and bolts, including new fuel tank, accumulator, WUR and pump) rebuild between 2015 and 2018 and ever since then has had a random fuelling issue.

When I fill the tank it sometimes creates an airlock in the fuel feed line causing the pump non-return valve to clatter and the engine to cut out, a few minutes later. I had to leave it to sit for a while to wait for the air to clear before attempting to start again.  I later found that pumping (literally with a bicycle pump) air down the fuel return line from the engine bay, the blow back would clear the problem and the engine would start and run fine until the next refill.  I’m thinking it might be a tank breather issue? The expansion tank is also new.  Any thoughts on this, and what else I could check?

Marcus

 

[Strictly]

On 20/11/2025 at 12:12, eebee said:

Hi Luke, just seen your expert post and wondered if you had any thoughts on an issue with my ‘74 Targa.  It was a complete (nuts and bolts, including new fuel tank, accumulator, WUR and pump) rebuild between 2015 and 2018 and ever since then has had a random fuelling issue.

When I fill the tank it sometimes creates an airlock in the fuel feed line causing the pump non-return valve to clatter and the engine to cut out, a few minutes later. I had to leave it to sit for a while to wait for the air to clear before attempting to start again.  I later found that pumping (literally with a bicycle pump) air down the fuel return line from the engine bay, the blow back would clear the problem and the engine would start and run fine until the next refill.  I’m thinking it might be a tank breather issue? The expansion tank is also new.  Any thoughts on this, and what else I could check?

Marcus

 

Hi Marcus....quite an odd problem. What happens if you fill the fuel tank. Then leave the fuel cap off and go and start the car. Don't drive any where just start the car with the fuel tank cap off. The idea is there can be no excess air or fuel vapour pressure this i believe should escape from the tank out the fuel pump nozzle entry point. I think anyway! Maybe not

If this fixes the problem it must be something to do with your fuel breather system..perhaps the pipes are pinched.  You will need to find a way to pressure or smoke test it safely. Im not familiar with what that looks like on a 74.

If leaving the fuel cap off makes no difference then I don't know....let's deal with that after!

Edited December 5, 2025 by Strictly

[Strictly]

P.s

Odd problem for sure. Im clutching at straws at best on this one.

Hot start problems can be related to fuel evaporation in the fuel lines this is why they are pressurised by the FD/ accumulator.

My question to you would be does this problem really only occur when you fill the fuel tank?

What if you only partially filled the fuel tank or you tried a pretend fuel stop when you take the fuel tank cap on and off but don't put any fuel in. Odd idea I know but the purpose is to determine that it is ONLY the action of fully filling the fuel tank that creates the problem you have. This then provides one variable to solve for

[eebee]

Thanks for your thoughts, Luke.

I will try leaving the fuel cap off at the next fill. I’m at 3/4 tank now, so will fill soon.  If that works okay, I will try again at 1/2 tank and then 1/4…. This could take some time as I don’t use it in foul weather. 😂

I will also check the breather hoses and expansion tank, which I replaced these with new when I rebuilt the car.

Edited December 8, 2025 by eebee
Typos

[Strictly]

4 minutes ago, eebee said:

Thanks for your thoughts, Luke.

I will try leaving the fuel cap off at the next fill. I’m at 3/4 tank now, so will fill soon.  If that works okay, I will try again at 1/2 tank and then 1/4…. This could take some time as I don’t use it in foul weather. 😂

I will also check the breather hoses and expansion tank, which I replaced these with new when I rebuilt the car.

Keep us posted we will fix it eventually even if next summer. 

Weird problem for sure. Maybe a blockage in the breather system...but im really not sure. I've never had that particular problem before 

[eebee]

On 05/12/2025 at 15:38, Strictly said:

P.s

Odd problem for sure. Im clutching at straws at best on this one.

Hot start problems can be related to fuel evaporation in the fuel lines this is why they are pressurised by the FD/ accumulator.

My question to you would be does this problem really only occur when you fill the fuel tank?

What if you only partially filled the fuel tank or you tried a pretend fuel stop when you take the fuel tank cap on and off but don't put any fuel in. Odd idea I know but the purpose is to determine that it is ONLY the action of fully filling the fuel tank that creates the problem you have. This then provides one variable to solve for

 Being a 74 model, the fuel pump comes on with the ignition switch, there is no relay link to the alternator like later models. If I turn on the ignition and hear a nice even humming from the pump, I know the car will start fine, hot or cold. If I hear an erratic, staccato sound, I know there is air in the fuel line and will have a problem. Out comes the bicycle pump, a couple of pumps into the return line. Listen for air bubbling in the tank. Reconnect the lines. Turn on, listen for the fuel pump to run smooth and off we go all fine.  Of course I may be deluding myself and masking the real root cause of the issue.  I’ve had this problem on and off since the rebuild in 2018, Its intermittent.

[Strictly]

12 minutes ago, eebee said:

 Being a 74 model, the fuel pump comes on with the ignition switch, there is no relay link to the alternator like later models. If I turn on the ignition and hear a nice even humming from the pump, I know the car will start fine, hot or cold. If I hear an erratic, staccato sound, I know there is air in the fuel line and will have a problem. Out comes the bicycle pump, a couple of pumps into the return line. Listen for air bubbling in the tank. Reconnect the lines. Turn on, listen for the fuel pump to run smooth and off we go all fine.  Of course I may be deluding myself and masking the real root cause of the issue.  I’ve had this problem on and off since the rebuild in 2018, Its intermittent.

Is it only on full fuel tank fill up? If it can happen anytime...this could be explained buy fuel evaporation in your feed line between the tank and the pump. This would create a gas lock. I moved the pump on a 75 to the front like 77 and later. I can explain that if needed. Cold start is quicker but better with later alloy crossmember. No gas lock...but maybe the check valve helps that 

Also if it happens at random....some fuel pumps have internal check valve. It varies by part number...could you have double the check valves (external and internal)...

[eebee]

It is only related to filling the tank. In the early days, I was at a petrol station having just filled the tank.  Wouldn’t start, called the AA, who disconnected the fuel line at the filter and Accumulator, to no avail. Then after about an hour, it finally started. Can’t recall if he disconnected the return line.  If ran fine, until the next petrol station visit.  When I got home, I checked all the fuel lines, and at the FD and each injector.  Replaced the fuel pump (AA man’s recommendation), the Accumulator and the fuel filter - all genuine parts direct from Porsche Silverstone.

Next fail was after a fill at a motorway services.  About 1/4 mile down the road, backfiring, lean running and finally died in the hard shoulder. This time I used the bicycle pump hack, which worked (having read about somewhere, possibly Pelican forum) and off I went running fine for days.  Since then, I dread refuelling, as I don’t know what will happen - but have a hack that always works.

The dual check valve thing you mention is interesting, as the airlock sound does maybe sound like a chattering ball valve.  I will check the part number.  I do have a check-valve in the connector, which I checked for cleanliness at the suggestion on another forum.

Thanks, for your ideas.  I’m determined to crack this issue - the car runs perfectly otherwise!

[eebee]

A further update…

I rummaged through my receipts and found that a new banjo/check valve (p/n: P911.608.901.00) was installed on 06/08/2018 at the same time as a new Porsche fuel pump (p/n: P911.608.110.06). Does this have pump an internal CV? There appears to be so many pump variations. If so, then I do indeed have 2 CVs in series and could this be the problem?

I also found this receipt , when I later asked my restoration builder (who did an awesome job btw) to re-examine the fuel system.  They didn’t notice or mention the possibility of having two CVs.

 

Edited December 9, 2025 by eebee

[Type911]

I dont believe they have an internal valve, it should be outside of the pump on the banjo or close

[eebee]

On 09/12/2025 at 16:23, Type911 said:

I dont believe they have an internal valve, it should be outside of the pump on the banjo or close

I was hoping to have identified a possible cause of my tank-filling problem.

Edited December 15, 2025 by eebee

[Strictly]

On 15/12/2025 at 21:29, eebee said:

I was hoping to have identified a possible cause of my tank-filling problem.

I will have a look in the manual see if I can find further reference to the internal check valve. Im thinking its mostly the 73 to 76 CIS pumps located at the rear torsion bar. I will dig some more.

In the meantime here is a bosch list. See atfached picture

Note the M12 x 1.5 (F) on some of the pumps with the (F) referring to the internal check valve.

Ill try and post up a bit more in a little bit.

 

 

Edited December 19, 2025 by Strictly

[Strictly]

Ok had a more detailed look.

Your fuel pump part number does show it with an external check valve which does match the part you have. I confirmed by looking at the 74 part diagrams. Interesting the 75 which I know better uses the same pump BUT does not show an external check valve. I could not correlate the pump part number to a bosch number.

See pictures. 

I included a picture in the troubleshooting section of the manual where the fuel pump check valve is mentioned...but it indicates this is a hot and cold issue and probably is just 1 large red herring.

My question to you is what does your fuel evaporation system look like? Compare it to the part diagrams. I've posted a couple. I've actually never owned a 911 that had the charcoal canister but incidentally on my maserati granturismo which does have one a symptom of needing a new filter is the car not starting after filling the tank. It's something to do with the filter becoming blocked which some how results in the fuel vapour flooding the intake system. Im not sure how it actually works on the 911. I would look for a blockage in the fuel tank vent system or the pipes not being connected correctly.

There is another explanation but its still clutching at straws. If you look at the picture showing the yellow line which is the fuel return line. Essentially the extra head of fuel in the tank somehow stops the spring in either the fuel regulator or the warm up regulator or even the throttle valve working correctly. 

I think ignore the check valve focus on your tank vent system. Maybe convert it to the far less complicated version which does not use the charcoal canister.

If you can get anywhere with this start looking at the wur or fuel pressure regulator.

I've done quite a bit of work on 75 to 79 CIS..but I've never actually worked on a 74

 

[Strictly]

Final thoughts for today. See if your 74 tank vent system can allow fuel vapour into the engine air intake. As it could be making the air too rich to start. If that won't explain it then something to do with the extra head of fuel in the tank somehow stopping the fuel pressure regulator or wur from working properly

Post pictures and don't give up until we beat this thing