MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fuel tank vacuum

Hi All,

The evap loss control system installed in my 1980 B seems to work properly, but a strong vacuum is building up in the fuel tank. By checking the possible cause i discovered that airflow IS possible from the fuel tank to the canister, but NOT from the canister to the tank.

Is there some kind of a check valve installed allowing only unidirectional air flow, or am i experiencing somewhere down the line a weird one way blocking?

Thank you for any input!

Reto
Reto Schlumpf

Reto. No, there is not any form of check valve involved. Hard line from the trunk to the engine compartment. Rubber hose from the tank to the vapor seperator, vapor seperator to vent hose, and vent hose to charcoal cannister.

If you can get air to flow in one direction, it might be possible to flush the contaminants out of the current hard line. I had to replace one which did not allow any airflow at all and, like your experience, kept causing a vacuum in the fuel tank. The hard line is a bear to fit. I cut the new one in the middle, making it difficult to install. Prior to cutting, it was impossible to install. Having a rack type lift available would have helped. Very interesting job when done on jack stands.

Do not know if the old style (US) filler neck and vented cap can be fitted to the later model cars. If it can, it might be worth considering as an alternative to replacing the hard line. Best, though, if the hard line can be cleaned internally and made functional.

Les
Les Bengtson

Reto,

You have not replaced the gasket or rubber underneath your fuelcap lately?

Drive the car without your fuelcap on. If all is well, this seal is giving you a hardtime.

Frank
F Navest

The carbon canister has three outlets on the top that open into an empty space (in effect the three are connected by that space), then there is a carbon filter underneath and at the bottom there is an outlet to the atmosphere.
One of the three outlets connects to the petrol tank line and another outlet is connected to the inlet manifold. The third is somthing to do with antipollution and is often plugged.

Unscrew the canister and have a look. Inside there is some actived carbon and at either end a couple of plastic grids with some glass cloth to keep the carbon in. You may need to replace the carbon or the cloth. Activated carbon is just charcol that has been heated. Lots of surface area to absorb smells/petrol vapour. I'm a chemist and at work we get it from scientific suppliers, however since it's used in water filters there must be lots of alternative suppliers.

Peter
Steve Hill

A place to obtain charcoal in the states is at a pet store that sells supplies for fish tanks.
Kimberly

Thank you for all the inputs.

I isolated the problem within the hard lines (not canister). It is strange having almost free flow in one direction and no flow in the other (like a check valve would be fitted)..

The 1980 B requires a closed fuel system (no vent in cap). To solve the problem I just fitted a ventilated fuel cap to compensate for the fuel pump suction..

Les, with what kind of liquid you suggest to flush the line?

Thanks,
Reto



Reto Schlumpf

This thread was discussed between 01/02/2007 and 02/02/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.