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MG MGB Technical - Steam from interior vents

There's always something, isn't there?

Having got the fuel pump on my '66 GT working again thanks to help from another thread I finally managed to get the B out and about today (probably 6 months since last outing). I was somewhat dismayed, however, to find steam pouring out of the windscreen demister vents whilst trundling down the A3! I immediately thought "leaking heater matrix", but I'm not so sure now:

- The heater is stuck on - a new heater valve was fitted about 4 years ago and it's never shut off properly. Piping hot air comes out of the lower vents with good flow if the fan is on, but very little air flows from the windscreen vents. Wouldn't be surprised if they were at least partly disconnected or blocked.
- The "steam" coming from the vents misted up the inside of the windscreen, but not as much as I would expect for a leaking matrix. With the driver's window down I could see out fine. There was no antifreeze residue left on the glass, nor any smell that I could notice. The "steam"/vapour was not noticably hot if I held my hand over the vents.
- Car fortunately didn't overheat, but I can't easily check the coolant level as it's a MK1 with no expansion tank and a dog-leg filler neck on the radiator. However, the top of the rad was hot, as was the top hose and the bottom of the radiator was relatively cool. Once I get it home and let it cool down I'll top up the rad and see how much it takes - however it's always settled itself to some way below the filler neck.
- Car lives in a garage and hasn't been out in the rain for years (but does get periodically washed). That's the only way water could have got in somewhere from outside.
- It's cool and very humid today - 94% with a temperature of around 13c when I left home. I guess this would make any condensation from heat much worse.
- I only noticed the steam after 5 or 6 miles, but that coincided with going from a 30mph area to 70mph (I was cruising at 60) and therefore much more forced air flow. It remained, maybe a little worse (or maybe I was just paying more attention to it) until I got to work about 25 minutes later.

So, based on that lot what does the collective wisdom of the BBS think? Is this just condensation from having been standing for a long time and then driven on a cool humid day with the heater on? Or something rather more sinister?

Thanks,
Tim
T Jenner

Remove the demist tubes and look at the mouse nest(s). They are blocking the air flow and they are wet. At high speed you have enough flow to force some heat through them, hence the steam, Then figure out how to extract the little boogers!

FRM
FR Millmore

Is "Tom's knob" blocked?

If the coolant level goes down you have a leak. Is it lower than "normal"? If not there must be "water" in the heater box/matrix. Water gets in when you wash it but exits below thru Tom's knob....
Michael Beswick

Mouse nests!? Yikes - sounds like you're speaking from experience, FRM! I'll have a look, but really hope it's not that! If I don't spot her in time our cat periodically wanders through the garage collecting stray nuts and bolts on the magnetic cat flap key she has on her collar, but she hasn't shown signs of detecting mice - I'm sure she'd notice if there were any around!

I'll also have a prod at "Tom's knob". Come to think of it it must be 10 years since I last checked...from memory it's accessible under the car next to the gearbox isn't it?

Cheers,
Tim
T Jenner

Hi Tim,
Tom's is just behind the engine bay firewall beside the gearbox, drivers side. access from below - Just don't be underneath when you manage to open the end. From experience it is not pleasant if it has been blocked for a while!.
Bernie
P.S. I hope you were not cruising at 60 on the A3 :)
B Anderson

Thanks, Bernie. 60 was on the fast section down towards Woking, not the nasty congested slow section by New Malden!

I might have a look at lunchtime and see if I can poke something down the drain hole from the fresh air vent above - however as it's a MK1 I think there's a bit of metal blocking it. :(

Cheers,
Tim
T Jenner

Jolly good. I really don't miss that road :)

I think you are correct I cant get to it on mine (70) I think they moved the deflector in '72 ish. so looks like it's under for you
Bernie
B Anderson

If it turns out Tom was blocked, it would be worth checking the state of the surrounding box after X years of living in a bath...
Michael Beswick

On MGB, you can clear Tom's urethra by inserting a piece of 1/2 tubing down the second gap from the right on the air intake grille, and applying compressed air.
But, clogged pipes here will flood the heater box - and kill your radio - but will not account for the characteristics of the problem here.

Tim - Much experience, but it can be much worse; how about having the little sweeties pack the cylinders with acorns and nest material by going up the tailpipe and through the open valves? Or filling an MGB cylinder head and a couple of cylinders plus the entire exhaust system with dog food while the carbs were off for two weeks? And they frequently tear up your seats and carpets for fuzzy stuff to put inside things - soaks up the pee and keeps the nest dry on the surface while eating hell out of your parts. Chipmunks are worse in the amount of stuff they carry around, but not so bad on getting into small holes - just cleaned about 100 lbs of pine cone hulls out of each of several cars.
And don't blame the cat (necessarily!) - the pests may have moved in and out years ago.

FRM
FR Millmore

I had a bit of a fiddle yesterday when I got home - I had a brainwave to remove the heater motor and feel inside the heater box for any coolant which might be leaking. Unfortunately it wasn't a very good brainwave as it turns out the fan which is attached to the motor shaft is too big to fit through the hole! Then when I put it back I must have moved the fan on the motor shaft as it's now rubbing on the heater casing. Sigh.

A torch down the fresh air vent showed no gunk in the bottom - bone dry, though I couldn't see the drain hole. However there are small white crystals on the outside of the bottom of the heater box in the engine bay - signs of coolant leakage after all? To be honest I'm tempted to just bypass the heater with a length of heater hose until I get time to fully overhaul the system, I never use it and it doesn't work properly anyway! Am I right in thinking it's 1/2" inside diameter hose?

Thanks,
Tim
T Jenner

By-passing will prove the point whatever, mice Tom or leak. If you can't get the impeller out, it's probably had one of the up-grades fitted at some time, which may also have included an up-rated matrix as well. Unless the early heater boxes were of a different design?? The bottom of the heater box does corrode under normal conditions. I do think you would notice the sweet smell of EG if you had a coolant leak, plus it would probably make it's way into the footwells via their outlets.
Allan Reeling

Oh, come on Tim! You can't claim t have fully experienced MGB ownership without ever 'doing' the heater. In my opinion it really separates the men from the boys! It's right up there with correctly fitting the roadster's windscreen.

Roll up your sleeves, dig into that heater and get it fixed. Please be advised that you should keep children away so they are not exposed to the foul language this job frequently causes.

Here's how it's done: http://www.mgcars.org.uk/news/news169.html
Willem vd Veer

I'm rapidly coming to the conclusion that I'll have to "do" the heater (and not just to prove my manliness to Willem!), so I'll bypass it so the car isn't unusable in the meantime. I'll check the demist vents for rodent-based blockage before committing though! Still not convinced it's the matrix leaking, as Allan says I would have thought there'd be an antifreeze smell.

One further oddity has me scratching my head (and, disturbingly, possibly supports the mouse theory). There was a good blast of warm air from the footwell vents with the fan running but no obvious steam from them - only the screen vents!

Cheers,
Tim
T Jenner

Easy to remove the demist hoses, and I expect the mouses houses are in them, or at the ends at the heater box/plenum.

FRM
FR Millmore

This thread was discussed between 22/08/2012 and 23/08/2012

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