The expansion tank thinks its a radiator.

[banjomandolin]

Hi. I have recently carefully read the description of the Bis cooling system in the workshop manual and was horrified to learn that, when the thermostat opens, "the pump sucks coolant from the bottom of the radiator AND THE EXPANSION TANK". and "THE RELATIVELY COOLER COOLANT FROM THE BOTTOM OF THE RADIATOR, RADIATOR TANK AND EXPANSION TANK IS DRAWN UP BY THE IMPELLER PUMP".  The pipework layout does indeed lend itself to creating this circulation. The circuit through the tank becomes a sub circuit of the main circuit around the radiator. To confirm this strange state of affairs I removed the left hand grille and waited for the thermostat to open. Both the expansion tank and it's flow and return pipework became red hot. I can only guess that the original designer found himself short of cooling surface and resorted to this expedient to get himself off the hook. The tank cooling will be assisted by the flow of the engine combustion air as it passes through the grille and squeezes past the tank on it's way through the hollow body member to the air filter inlet. I intend to carry out a further test involving fitting a clamp across the flow to the expansion tank in order to halt this circulation in the hope that it will divert more circulation through the heater (so I don't have to buy a blanket Mr. fiatfsm !). I would be interested to know if anybody else has gone down this road.
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[Chris Lakin]

hi John a very clever analysis, i envy you!  

I dont fully understand what you mean, so any pics/further details to explain the pipe layout would be great.  Ive always been interested to find the fatal flaw of the BIS, afer finding my  thermostat clogged up with a porridge like substance after 16 years (presurmably corrosion deposits) i put the over heating/head gasket troubles down to thermostat blockages from the corrosion preventing the thermostat to open. Its recommended the thermostat is cleaned out once ever couple of years. It obvious that the radiator is too small, and inadeuqate and in the wrong place! (like the hillman imps which were prone to headgaskets blowing being a rear engined car!)

However i like the sound of your new theory!  

wouldnt it be great to find a simple mod (for the sake of a couple of pence, and not a thermostat relocation kit costing £120, in many cases not making any difference) which would make the BIS as reliable as the air-cooleds!

Thanks John! keep us posted

regards

Chris

[126dave]

mmmnn.i dont think the rad is too small chris,dont forget all the thousands of 850/600s etc that run fine with everythoing in good nick,i just think that the whole bis cooling system is just too close to its maximum even when driving normaly and in good nick.....other cars are more ''squady'' proof and can take abuse easier.

as banljo describes,its over complicated as well.
 
the heater has never been a prob on mine,except when the dreaded air blocks turn up.....no heater=impending doom...... :shock:
 
the rad is bigger than my 900cc fireblade one,and much bigger than the two on my 1000 VTR SP1.......so should cope ok even stuffed in the back,just keep the fan over-ride switch on boys!!!

 :idea: another aproach would be to bin the water pump idea and fit an inline electric pump to keep the circulation high even when idling in traffic :idea:

[126dave]

they would prob benifit from an oil cooler as well.......maybe cobble something together from the 2cv one's?........... :cool:

[ciaoflyer]

Hi  Since I have fitted the thermostat kit and rewired the fan so you can manualy switch it si it is on all the time I have had no bother at all.  Temp gauge stays just above a quater.  If you turn the fan off it rises above halfway very quickly but once it is switched on again it drops like a stone and stays just above a qauter.  I must admit the thermo kit is a little pricy but a bit of wire and a switch is about £3, I am sure you could just fit this and not the kit.  I just bought it for piece of mind, as I said no more overheatiing probs.

[Chris Lakin]

hi mate

only prob with that is its a 2stage fan and if you burn the fan out (because its always running) you might not know, luckily it makes some noise so will be ok.

I cant see my BIS ever over heating, as i look at the temp gauge more than the road ahead!!  

hows the engine going youg to from Jogn Bitmead? I got the gearbox from that engine i still havnt fitted it yet. Did you fit the copper headgasket?

regards

Chris

[ciaoflyer]

Hi

Engines is great!  Not fitted copper gasket yet as it all seems to run fine.  Only prob is a dripping sump plug :sad: but will fix this soon!  
Why would the fan burn out?  Anyway I do have another.  What I did was to completely remove the thermoswitch and replace it with a manual one.  The thermoswitch still sits in the rad but isn't wired to anything.

[banjomandolin]

Once again thanks again for the outpouring of knowledge in response to my post. I never fail to be astounded at what emerges from the woodwork on a forum. I don't think the circulation of coolant through the expansion tank would increase the sludge or corrosion problem because it is a sealed system, there is no air and hence no oxygen for the coolant to absorb. I had thought about the idea of using an electrical fuel pump to tickle up the circulation to the heater and intend to do some calculations to ascertain the volume of water required but fear it may require something impractical like a bank of 17 fitted in parallel or alternatively a single one which had previously been fitted to a 38 ton earth-moving machine. I noted the suggestion of having the fan running permanently, also the idea of running the fan from a manual switch as and when the temperature gauge dictated but prefer to have automatic control from a thermostat. One never knows what could distract one's attention on a hot sunny day in summer !.

[Chris Lakin]

sorry to hear about your sump bolt, yep another reason why i took my engine out, i was going to fit a spare sump ive got, but worried about the state of the water pump seals so going to just get my sump thread helicoiled. You can take the sump off with the rest of the engine left in. Remove the rad it gives more room. Use araldite for now to hold the bolt in (just incase) i even used plubers tape to wad the bolt in, this stoped the oil seaping out, then just araldite around it. If the bolt comes out while you going along thats it, your engine will be shot.

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John i will look forward to what you discover, i dont like the sound of "calculations" ive never been any good at maths!  

regards

Chris

[126dave]

hi john,i meant a dedicated lecky water pump......i think demon tweeks sell them for compition use.i know drag cars use them..............

[banjomandolin]

Hi Dave, sorry I didn't read what you said carefully enough. Demon Tweaks' electrical pumps, do you know any costs ?.

[126dave]

no john,sorry i dont.....be easy to find there site tho? google them?