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K-Series Coolant Eruption


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Hi all,

Just wanted to ask a few questions then get a sense-check on my plan please.

Last week my 7 got a bit hot on a run. It was a hot day (28ish?) the car was sat whilst I fitted a passenger, then a 20 minute blat, 5 minutes sitting in a car park (still running) then 20 minutes slow drive home. Temp got up into the red when at traffic lights, but dropped to 80-90 when cruising at around 50-60mph.

My suspicion was that the fan wasn't kicking in, so I took it out yesterday for a 30 minute blat - temp stayed around 80-85... all good. When I got home, I deliberately left it running whilst I took the nosecone off, to inspect the fan. As I got the cone off, with the temp showed around 90 (maybe near 100?), the expansion tank was in full volcano mode. Turn the engine off and ran cold water of the radiator to help cool it.

Once cool, I topped up the expansion tank with just water and started it again. Temp seemed happy to sit at around 80-85, but there was a tiny bit of steam coming from somewhere in the radiator (no idea if this is normal or not?). Holding a mobile down the back of the fan, I could tell that it wasn't spinning.

My first step is to make sure that there's not an air gap at the top of the radiator. My understanding is that this would stop a true temp reading being taken which, in turn, might confuse the fan. The expansion tank shows fluid up to the max line, but I understand this can be deceiving.

Question 1: What's the easiest way to top up the radiator and remove any air gap? Is it going via the hex screw at the top (#1 in photos)?

Question 2: Could someone name parts 2-5 and what they do please?




I've ordered a new expansion cap as a precaution.

After inspecting a coolant container in the garage, it clearly only contains water :(. I used this to top up the coolant a few weeks ago. I wonder if that diluted it too much and lowered its boiling point? I've checked the fan relay and fuse - which are both working.

Question 3: What's the best way to test the fan? I've seen snippets of people talking about shorting wires, but I've not found an explicit set of instructions.

Thanks all *thumb_up*

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With ignition on, unplug connector off thermo switch "2" and short the wires in the connector together. Fan should run. Your "4" is the single wire sender to the temperature gauge. Your "3" is a small bore hose to let air out of the head. "1" is a bleed screw to help get air out of radiator. You should have used yellow or orange for your labelling !! Yoh really want a 16mm bleed ted in the highest heater hose.
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Pulled out both wires going into thermo switch (#2 in pics), and held them together with some locking pliers. Turned on ignition and fan came on straight away.

I then started the car and it took about 4 minutes for the temperature gauge to read 60. Not sure if that suggests whether or not the gauge is working correctly though. The top red hose (which, I believe, comes from the engine?) got warm in places, but the radiator stayed cold.

As soon as it was turned off I also took the liberty of checking the oil - which was spot on.

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The top red hose (which, I believe, comes from the engine?) got warm in places, but the radiator stayed cold.

I'd say that was normal if the gauge reading of 60C is anything like accurate.  IIRC, the stat is 82C, so until that opens, the top hose (from engine to rad) will get warm but not hot.  The hoses to/from the heater (the smaller red ones at the back of the engine in your photo) should get hot, however, as that's the bypass circuit until the stat opens.


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Just re-reading #1:

... the expansion tank was in full volcano mode.

Was the "volcano" inside the expansion tank?  That is, the coolant wasn't pouring out?

From your photos, it looks like you have a Supersport-R.  Those rollerbarrels replace the standard inlet manifold, and thus the manifold connection to the small-bore pipe to the expansion tank will no longer contain the standard "jiggle valve".  As a result, there will be a constant flow from the engine to the expansion tank.


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Hi John, I didn't check those - but I will tomorrow. I'd only checked the other large pipe connecting to the rad.

So, I must confess that I don't really know how this works. When you say "until the stat opens" do you mean that the coolant flow doesn't start (going through the rad?) until it reaches a certain temp? Presumably, there's then a second, higher, temperature where the fan kicks in (82?).

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82 degrees.

re post #18 thats how the cooling systems work on most water cooled internal combustion engines. If there was free flowing fluid in the system engines would never reach optimum operating temperature for best fuel economy. The stat blocks 90% of the flow (the 10% is the heater, or bypass, which is a secondary circuit before the stat) until the water around the block is hot then opens up to the rad.

Hope that makes sense.




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The screw in the top of the radiator is lower than the expansion tank- so unless you are parked on a very steep hill, or have anti-gravity, any water poured in there will have no effect on the level in the expansion tank.  What should happen is when you remove it, assuming the expansion tank is full of coolant,  air, flowed by coolant should escape from the radiator, the level in the expansion tank dropping as coolant escapes.  At this point, replace the screw.

While plain water does have a lower boiling point than water and anti-freeze, its not making it boil over - the duff fan switch is not operating the fan, so its over heating.  Your local motor factors will have something suitable if you wander in with that one - I believe the standard one is VW Golf Mk2 ish.  If you go too low on the "on" temp, it will run all the time - a cheap replacement claiming 88 on did that to me.

And the steam on the radiator - just the water you tipped over it to cool it evaporating off as it warms up when you ran it.  If you wanted to cool it, you should have left the engine running to pump coolant around the radiator as you ran water over it - assuming of course, there was still enough coolant in the system to do so.

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