Very, very strange: started tye car for the first time in 2 months. Both tanks full. As the car was idling outside, I saw the petrol pouring out of the angled fixed pipe under the rear end, under the right tank.
I had the left tank operating.
When I switched to the right tank, the pouring stopped.
I opened the lid and cap, on the right tank, and saw the level of petrol all the way up to the opening, where the cap sits. I never fill it this much at the station.
So, it is as if the left tank´ s pump shoots across to the right tank, as well as forward to the carbs.
Is there perhaps a one way valve or something to prevent this happening……???
I have 4 of these cars, one of which I bought in 1988. And an S-tupe and Mk9. Never experienced this😰
The SU fuel pumps do have one way valves internally that are supposed to stop this exact scenario. The overflowing tank’s pump must have a stuck or damaged valve.
What/where is that part located?
It’s part of the pump itself. Internal. See pic - I circled the valves.
You might be able to tap on the pump to get it unstuck or run it a while on the tank with the pump that has the stuck pump valve. The flow of fuel through the valve may clean it.
BTW I should mention that even if you have modern electronic SU style pumps they still have the one way valve system.
The problem you’re experiencing is pretty common in dual tank carburetted cars if they sit - especially with ethanol laced fuel.
Butwhy does the left pump push petrol into the right tank, which overflows through the tank´ s cap?
When I swith to the right tank the overflow (on the right tank) stops,but there is never an overflow on the left side.
I am confused……
Because the pump feed lines that go to the carbs connect in a simple ‘Y’ (3 way connector) - only one line goes forward to the carbs - so the only thing stopping the pumps from filling the opposite tank is the one way valves inside the opposite fuel pump. If those valves are malfunctioning as yours seems to be, the left pump pushes fuel into it’s outlet fuel line, then through the 3-way connector and into the stuck right-side-pump’s one way valves without anything to stop the flow. It just pumps fuel through the pump filling and overflowing rhe right tank. It also pumps to the carbs as it should so the car will still run.
There are also other failure scenarios with those pumps that might cause it but if one pump is filling the opposite tank the problem lies with the pump on the side that is overflowing.
A simpler fix than replacing or rebuilding the fuel pump might be adding one way valves to both fuel lines. I think you have a 4.2 Peder? getting those pumps in and out over the tops of the rear fenderwells is no fun. I have to squat inside the trunk to do it - getting too old for that.
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Another issue can be the fuel caps, they must be ventilating types.
I seem to have cured it by tapping on the right pump with the end of a rubber hammer. A bit like tapping on it with a large spanner when it doesn´t work, after a longer period of inactivity:sweat_smile:
The new Purolator pumps have a better design, better flow control and low psi although they are not stock. I just added them outside of the tanks rather than messing with the internal pumps. There are also one way valves you can install to prevent this from happening. Problem with the temporary fix is you will not know if this is happening while you are driving and losing copious amounts of fuel on the roadway.
And now the problem is back…
Tapping on the pump doesn’t seem to cure it, as it did in April.
I have a spare, new SU pump, but it’s a real bummer to change it😱
Peder, see my discussion about this problem with my MK IX under the heading
Why does my right fuel tank overflow when I use the left tank?
I had the pumps out and cleaned and polished the valve discs but one of them still didn’t seal perfectly. Since, like you, I didn’t want to keep removing them, I bought a new low-pressure Facet-style pump, the one that looks like a beer can, and an electric change-over valve. The total cost was less than $70US and it is now impossible for one tank to fill the other. Now I have two additional SU fuel pumps in the box with all the other unreliable ones that I have removed from all my cars.
Let me know if you want more details.
Thanks Mike. Yes I did read the thread, but on the Mk9 the pumps are very easy to access from underneath. Not so on the Mk10/420G.
In Germany they like the Hardy pump, which I have on one G. They look like the SU pump.
fre 16 juni 2023 kl. 04:03 skrev Mike Eck via Jag-lovers Forums <noreply@jag-lovers.com>:
Hullo Peder,
Is it Hardy or Harding. I have a 6v German made Harding I used on a car I had in the '60s. You are right, apart from the name, you would swear they came from the SU factory. Maybe they did.
It is Hardi fuel pump, have them om most my cars.
Okay, thanks.
Do not be afraid good sir,
the job is not as bad as I seemed to remember the first time, I removed and installed the RH side one recently on my 420G.
It only took me about 30mins ( the boot card that covers that area is no longer present)
nevertheless, with that removed, one just have to have the right tools and you do have to sort of lay down in the boot
1/2" socket with slight extension to remove the 2 hold down nuts from their studs
13/16" socket for the pipe banjos, I lifted mine clear of the studs, but if they are tight, you may wish to try and break them free while still on the studs
Have a small bowl or rag to absorb the spilled fuel
Fit the new one you have, I would fit a TVS diode or electronic module at that time if not already fitted
You can dismantle and examine your old unit at your leisure
I drain my tanks on a semi regular basis.
This is mainly due to the fact that moisture in the air condenses in the tank and settles at the bottom, however, each time I do it, also find small particles
These tiny particles could cause those valves to leak
Peder,
Maybe a couple one way valves would be an easy solution for you.
Amazon.com: Mesee 2-Pack 3/8" 10mm Fuel Non Return One Way Check Valve for Fuel Line Oil Petrol Diesel Water Aluminium Silve : Automotive
One Way.pdf (278.2 KB)
Micah
These are the ones I bought but didn’t end up using.
There are a lot of them to choose from but I was concerned about the opening pressure, which was 2 Kpa (0.3 PSI) for most of them but these were only 0.5 Kpa. We don’t have a lot of fuel pressure to lose.