Chicagoland MG Club: Tech Tips

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Here's Mud In Your Drain Tube

Have you ever heard the old joke about getting mud in your dust valve? Well, it can happen on any MGB! If your floor mysteriously gets wet after a rain or after a trip through the car wash and you can't seem to locate the leak, it is probably due to the fact that the drain tube/dust valve (Moss #363-171, page 67 of the MGB catalog) is plugged.

The reason that this drain tube gets plugged is that the air intake grille at the base of the windshield has holes in it large enough to let in small critters, bugs of all sizes, leaves, dust, or anything else that can get airborne. Mix these things with a little water and it quickly seals off the very small opening at the end of the tube. When this happens, the well at the base of the air intake will fill with water and overflow through the fresh air opening behind the center console onto the transmission tunnel and down to the floors. All without you ever seeing it!

To clean out the obstructions you must remove the drain tube and clean it out. The clamp for this tube is loosened through an access hole just aft to the heater opening in the right-hand side footwell. You must also clean out the metal drain at the bottom of the air intake. There is no easy way to do this, but the least painful method is to remove the defroster tubes from the base of the heater and run a wire up through the metal tube until it is cleaned out. You will have to remove the defroster tubes to replace the clamp on the drain tube, so don't feel to bad about this; it beats removing the heater!

Note: If your car has never been restored or if you have had this leaking problem for some time, you should remove the heater and inspect the area beneath it for rust.

After you have cleaned everything and replaced all of the tubes, you can then stop any more infiltration of unwanted matter by attaching a piece of door screen to the underside of the air intake grille. Paint it black before you put it in and it will look like it belongs there. If you use the original style blind speed nuts (Moss #326-665) reinstallation even with the screen is a snap.

You may also want to remove the bulb at the end of the drain tube so that what goes in can get out. The opening from the factory is only a 1/16" slit! This may help save a few more MGBs from rotting away.

More on drain plugs

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