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MG MGB Technical - Fog of Confusion

Why does my screem mist up when I put my demister on? I hoped it might clear after a while, but no joy. 68 BGT.
Andy Hinson

You probably have a heater core that has "gone south". It is located inside the heater box on the shelf in the engine compartment.
rick ingram

I agree with rick.

The "mist" probably is leaking coolant from your heater core being blown onto the windshield.
S Rechter

Two things, breathing antifreeze fumes from a leaky heater core caused a friend I know to develop brain cancer.

The other, it is possible that your drain is blocked in the fresh air well and needs cleaning out.

M Whitt

Andy-
If the car is on axle stands you could slide right under there and see the drain tube hanging down beside the right front of the transmission. It should be hanging down just below the bottom of the car. On the end of it is a round bulb with a slit-like lip. In MG lore this is called "Tom's knob," supposedly after the engineer that designed it. Supposedly this "knob" was to prevent exhaust gases from working their way up the tube, into the air intake, and from there into the heater box, poisoning the air inside the cockpit. Unfortunately, the bulb can eventually get plugged up with debris, usually a compound of teeny bits of leaves and dust. This part is no longer available, so you're going to have to be careful with it.
Remove the cover and look in the corner. You'll find a small hole which connects to the drain tube on the left side of the plenum chamber when viewed from the front of the car. You'll want to avoid damaging the paint, so gently push a doubled-over electrical wire into the hole and gently twist it to clear the hole out, then add water and gently push it down through the tube while twisting the wire. Get under the car, squeeze the knob so that the slit opens, and have a partner flush water under pressure directly down the hole in the plenum chamber. This is the easiest approach and sometimes doesn't work. If it does work, you'll get a nasty bunch of crud on yourself.
If that method doesn't work, there's another that will: The drain nipple for the air intake plenum is located on the right hand side of the transmission tunnel, under the heater and just above the bellhousing. The drain tube is fitted to it's outlet tube by a hose clamp and is accessible from the side of the gearbox tunnel down in the right hand footwell area. You might have to remove the center console heater control mounting or try looking behind the hot air outlet in the footwell. It is pretty hard to see and even harder to get your hand thru to refit. Be prepared for grazed knuckles. Remove the 4 screws holding the flap to the tunnel, then you will see the tube that drains the air intake plenum. It has a clip holding the tube to the drain nipple. Undo that and reach under the car to pull out the tube. Now cleaning out the tube itself is easy. This is really the best way of cleaning the tube as sometimes there's a bend in the tube, thus it cannot be cleaned out using a wire.
I went through this joyous ritual with my second wife's car and resolved to never endure it again. I fabricated a screen using black stainless steel window screen and put it under the chromed intake grill. Nowadays you can buy one from Victoria British(Part# 12-017).
Steve S.

AF will leave a greasy deposit on the screen and smells like sweet chemicals.
The drain can be cleared quite easily by sticking a 12" length of 5/8" OD tubing (AKA 1/2" water tube) straight down the last square opening on the RH side of the air inlet grille. Wriggle it about until it goes in the hole, then hold firmly (or it will blow all manner of garbage in your face!) and apply air pressure to the tube. This will blow all the crap down and out; then flush with water. If the plenum is full of leaves and such, extract as much as possible before doing this. Note that flooded plenums can kill stereo sets in the center console by dripping water in the works.
FRM
FR Millmore

This thread was discussed on 04/02/2007

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