Fuel reserve - Operation and usage

[alfheim]

Hi.

Being new to this car, and not having had a car with a "reserve" fuel tank before, I'm just trying to figure out how to best use this. 
Now, my understanding is that there is a single fuel tank of approximately 63 liter capacity. The fuel gauge refers to the contents of the entire tank. The normal fuel line however, exits the tank slightly above the bottom so that approximately 7 liters are left when no more fuel can be drawn from that line. The "reserve line" draws from the bottom, and enables you to use the remaining 7 liters. The reserve handle opens a valve which lets the fuel pump draw from the "reserve line". I'm not certain if the ordinary line is closed at that time, that seems reasonable to me,  but haven't managed to find anything on this. 

Is this the correct understanding? if not, could someone please correct me.

Now in terms of usage, given that the above setup is correct, there is no issue with filling up, and it doesn't matter where the reserve lever is when filling up. 
My assumption would than be that you run the car until empty on normal, and then pull the reserve lever (which I assume must stay "pulled out" for it to have any effect) at which point you know that you now have 7 liters left, and need to fill up. This means that even if the fuel gauge is inaccurate you still know when you have to fill up, as you have performed a physical maneuver, and there is a tell tale, in that the reserve lever is pulled out. 

However, that doesn't seem very practical, first you have to run the car til it stops due to lack of fuel, then you have to stop it completely, pull the reserve lever, put the car in park and turn the ignition off and on for it to restart. So I'm not sure if that was the intended usage. 

Also when I tried that yesterday, actually running the car empty on the ordinary fuel, it wouldn't start again having pulled the reserve line. (I din't try very long, because you never run out of fuel on a practical place, so I just added a few liters from a separate tank I had with me, and the car started no problem). 

This the leaves me with two questions. 1. What was the intended usage method of the reserve lever/tank? and 2. What is the recommended usage? (I see that running the car to empty probably isn't a good idea. But not using the bottom 7 liters also seem like a bad idea as all sorts of crud and water may accumulate there. )

And actually a third question, is there a way to determine if the reserve handle works without running the car to empty on the normal tank?


Sorry for the rambling and long post, but I would really like to understand this. 

 

[richardlamsdale]

Hi, your first paragraph is correct. The reserve lever operates a brass 2-way valve underneath the car (almost under the drivers seat location) that has the normal and reserve lines running to it, then a line to the engine. So when you operate the reserve lever it switches between main and reserve lines, effectively closing the one that isn't in use.

As the reserve valve is under the car, the valve or the cable can seize. It would be worth checking under the car if your valve is working or not. You should have 3 fuel lines running to the valve (two from the rear, one towards the engine), and the cable moves a lever on the side of the valve when the reserve lever is pulled.

On my car I notice a slight hesitation when fuel approaches the bottom of the main line level. As soon as I notice this, if I then pull the reserve lever the engine runs OK, and as you say I'm then on notice to refuel asap. I've not run it till the engine stops. In some ways this does seem pointless - why not just leave the reserve tap open all the time. I've always assumed it's to allow for inaccuracy in the fuel gauge.

[Brendan69]

I just keep plenty of fuel in my tank all the time and topping up each and every time I have been out in the car with some fresh fuel which will then mix with the current fuel in the tank and just keep running through the system as normal.

One thing " I DONT DO " is allow my tank in any of my cars to run low say below 1/4 full at any one time as if there is crud/sediment in the bottom of the tank there is a risk it could be sucked up and into the lines thus causing a possible fuel blockage which no one wants.

I have often looked at my reserve lever thinking what will happen if I pull it but something just tells me to leave it alone which I have done.

Regards,

[tonys]

I share Brendan's view. If it was a new car, it might be different (although I still wouldn't do it) but on a car 45-50years of age, unless the system is known to be new/clean or has been  used regularly, I would rather not rely on a reserve and just keep it above 1/4 or thereabouts. Of course, it might turn into an ideal opportunity to clean the tank and (or replace) the fuel lines. You know how easy it is for small jobs to escalate

[richardlamsdale]

"You know how easy it is for small jobs to escalate." Too true. Mind you, when it comes to petrol, checking the condition of the hose is a good idea. I think the hoses on my car were original and very brittle plastic - I assume they were flexible when installed, so I'm replacing them.

[johnwp5bcoupe]

Jun 17, 2019 16:36:11 GMT richardlamsdale said:

"You know how easy it is for small jobs to escalate." Too true. Mind you, when it comes to petrol, checking the condition of the hose is a good idea. I think the hoses on my car were original and very brittle plastic - I assume they were flexible when installed, so I'm replacing them.

The original nylon fuel pipes can be a death trap after 50 years they can be brittle so best to give them a good looking at! try bending them, mine just fell apart as did another car I worked on, the black overspill return is usually no problem but best to check.

At the same time the tank was removed, the float was removed and the tank inspected through the opening, the delivery tubes and gauze were also checked. The reserve tap was fitted with a new O ring, all these checks/replacements should be done and not "assumed" to be ok! having a tank "full of fuel" without knowing the health of the fuel delivery system could be a mobile disaster!

My reserve is often used and performs 100% I can tell when it's C/O time by the Facet Pump rate increasing! I wouldn't run on reserve all the time, the tap is there for a purpose as long as the system is working as it should! there is nothing to worry about.

[lagain]

Some years ago we had a local member who had a problem with his car conking out nearly every time he used it. He threw lots of parts at it and eventually someone suggested the O ring on the fuel tap reserve. It turned out that the O ring was disintigrating and causing the fuel to run too slowly to keep the car going. I replaced my O ring a couple of years ago and was suprised that the original was in perfect condition, but as you have used the tap, as John suggests, I would replace the O ring.
Personally I always fill up as she heads towards a quarter as the tank empties so quickly and she is rather heavy to push.

[alfheim]

Thanks for all the replies.

So my summary is that my understanding of the technicalities are right and it does switch between the two drainage points. 
On the original planned usage there seems to be a stutter that may be detected and which ideally lets you switch before the engine stalls.
On recommended usage there isn't a clear consensus, but always keeping the tank approx 1/4 full has a lot of following. Also, if it hasn't been used, don't expect it to work if you suddenly need it. 
Generally running til empty is also a bad idea.

Regardless from my now confirmed understanding it should be possible to pull the lever at any time and determine if the system works. Sounds like it's a good idea to do this while idling, close to home first time. 
Personally, I'd say I would rather know about and get rid of crud/water at the bottom of the fuel tank rather than just having it laying there, but I do see the argument for just not using it too. 

Anyways, it seems like my fuel gauge is quite accurate, so I can go with any path I choose. 
Again, thanks for all the feedback, I think I will work with this a little bit, but avoid running the engine to empty. 

Thanks

[johnwp5bcoupe]

If the system has been gone through there should be no worries about using the reserve I used it in the 70's and I still use it if necessary, if your not sure of the system 1/4 tank makes sense.

[Brendan69]

Also for peace of mind and so you know it has been done you can always still remove your fuel tank and have it flushed clean.

Regards,

[lagain]

Unless the car has been standing for many years I doubt if there is a layer of crud sitting on the bottom. As you go along the road I would imagine that the inside of the tank resembles a Kenwood mixer

[Ed from Surrey (member 236)]

Hello all,

I have a fuel leak from the fuel reserve tap on my 72 P5B.
Will a replacement "O" ring resolve this? The WSM is not very helpful.
No doubt best to replace the fuel lines as well. I assume detaching the fuel lines from the top of the tank will avoid draining the tank but how do I overcome the resulting air lock? Will my reconditioned mechanical pump be capable of drawing fuel through?

Advice appreciated

[lagain]

New 'O' rings should fix it.
I would only replace the fuel lines if they look as if they need doing, after all they don't actually do anything so no stress, they just let fuel run through them at very low pressure.
You will need to release the pipes at the top of the tank and the fuel that is in the pipe will still run out when the tap is undone.
There should not be a problem when you try to start the car afterwards.

[Ed from Surrey (member 236)]

Many thanks.

But you say O rings (plural). How many?

[lagain]

I think that you only need 1, but if I remember correctly I had 2 so put one on the outside as well, under the lever.
You may need to lubricate the seal.

[Sam Bee]

Just the one seal. Make sure it is Ethanol proof, not old stock. I find they push in OK but if needed use Red Rubber grease or even wet with petrol.

[Ed from Surrey (member 236)]

thanks chaps