9 replies to this topic

[StuTheGimp]

Hi,

 

I've got an MPI Cooper and a few years ago I had a right-hand fuel tank fitted by a well-known company.  The right tank has always leaked petrol out from under the cap even when the car is low on fuel.  In an attempt to fix this problem ('because it's destroying the paintwork) I bought some non-vented petrol caps and fitted them.  After a couple of days the ever-pervasive smell of petrol that has surrounded the car went away.  Soon after this the fuel gauge was reading empty so I went to fill up.  I found that the right tank was full up and the left tank was empty.  I put some petrol in the left tank to get home and after sitting without the engine running for a while the fuel level evened out between the tanks.  I took the car for its MOT and the chap doing the test found that the car started leaking fuel from the breather coming from the right tank.  He stopped the engine and opened the cap on the right tank and petrol came flooding out.  Needless to say it failed the MOT 

 

The setup is : Standard MPI left tank with factory pump and standard 3 lines coming from the top.  The tank also has a nipple fitted to the bottom corner with a hose that runs across the boot floor to a nipple on the bottom corner of the right tank.  The right tank has a breather at the top of the tank with a pipe that runs down the tank and through a hole in the boot floor.  The right tank has no pump, gauge or special stuff.

 

I believe the original idea was that the cross-feed pipe would allow the tanks to drain at the same rate, but what seems to happen is that fuel is being pumped into the right tank and is filling it up to the point where it comes out of the cap.  I can only imagine this is happening due to pressure from the fuel return pipe somehow blowing fuel through the cross-feed pipe, but I have no ideas on how to fix it.

 

I found this thread http://www.theminifo...tted-to-my-mpi/  which seems to match my problem but it goes cold before any suggestions are made.

 

I'd really appreciate peoples help with how to tweak the plumbing to get it working right.

 

Thanks,

 

Stu

[Ethel]

Is it cruel to describe that as a fun problem?

I don't think it's the fuel return, I'm putting my money on the emissions venting creating a pressure differential across the two tanks. Venting both to atmosphere would confirm and provide a quick fix.

Linking both vents together and then to the charcoal canister would be better.

[Bobbins]

My thought would be (as mentioned in the other thread) to have the fuel balance pipe as you already have it across the bottom, but the vent pipe for the RH tank should run from the top pf the RH tank to the top of the LH tank, effectively keeping the RH tank "sealed" in the system and not vented to atmosphere. Use MPi caps both sides and make sure the high level breather is a little lower at each end than in the middle so that if it does get fuel in it will drain easily to either tank. Both tanks will then maintain the same pressure and level.

 

Stu.

[StuTheGimp]

Thanks both for your suggestion of linking the breathers. I'll get some pipe and a T-piece and give it a shot.

At the risk of giving myself more work to do, do you think it would also be worth re-routing the fuel return and have it coming into the right tank?

[Bobbins]

I'd leave the existing plumbing in place to the LH tank, just link the tanks together with a fuel line at the bottom and an air line at the top. The air line will balance the pressure and allow air into the RH tank rather than open venting it, the two tanks can then function as a single tank. Pretty sure this will eliminate your pressure problems.

[Homersimpson]

Thanks both for your suggestion of linking the breathers. I'll get some pipe and a T-piece and give it a shot.

At the risk of giving myself more work to do, do you think it would also be worth re-routing the fuel return and have it coming into the right tank?

 

I think that bringing fuel into the right tank would make your problem worse.

 

I don't understand how this can be happening if the pump is taking fuel from the left hand tank and returning it to the LH tank, it must be that the system is pressurised and the RH tank is open to atmosphere and therefore the pressure forces the fuel into the other tank.

 

Is the left hand tank cap vented?  If not can you try a vented cap and see what happens?

[StuTheGimp]

I'll connect the vents together and see how it goes.

Thanks

[viz139]

I had a simular problem with my turbo although plumbed differently. Two bottom outlets were T into the fuel pump and the return was T to the top of both tanks with vented caps. Tried many things and gave up, I concured that one outlet simply flowed better than the other and the return would fill the slower tank. As a by the way the twin tanks on my Esprit Turbo are linked by a one inch diameter balance pipe at the bottom and linked breathers at the top, the return goes to one tank.

[mini13]

I would avoid linking the return feed to the right hand tank, I had a 10mm feed from both tanks to an external pump,
And 8mm feeds back to both tanks, and it would drain one tank and fill the other, ri ended up with feed and return to one tank, and a simple cross fed vent.

[Spider]

I'm not 100% familiar with the MPI tank and fuel line set up, but I can't help but think that a bigger transfer pipe, around 1/2 or 3/4" is all that's needed, along with the normal vents fitted to the tanks.