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K series coolant loss


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Following a change to radtec radiator having a challenge getting K series Caterham cooling system operating properly

Issue is after 20 mins drive and once full revs applied coolant blows out of hoses around heater and traces around expansion cap. Hose from heater to water rail particularly hot, Otherwise all looks ok.

- no symptoms or issues before radiator change

- appears NO water in oil or oil in water 

- car runs up to temperature fine

- radiator top and bottom hose get hot, doesn’t appear air lock

- fan kicks in fine

appears system is pressurised and not relieving and finding weakest point, maybe just need new expansion cap?

welcome any thoughts/experiences please



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Hi Paul,

When I've seen these symptoms before it's always come down to the head gasket in the end.

It may not be - but I think you need to be getting a "sniff test" for combustion gasses in the coolant. There are two completely different head gasket failures on a K - the elastomer beads that keep the oil and water apart (and keep the water in round the edge) can fail giving oil water mixing (which you don't appear to have) but the other, rather unrelated failure mode is a breach of the fire rings where the head seals onto the tops of the cylinder liners. In this case the symptoms are often pressurisation of the coolant when you give it full throttle and high revs (high cylinder pressures). The high pressure in the cylinder pushes gas out into the surrounding coolant. You won't see oil in the water or water in the oil with a fire ring problem.

I know it seems odd that it came on after a radiator change. Could it just be getting that bit hotter with the new radiator and tipping it over into failure where before it was just on the right side of sealing properly?

From a friend's experience, sniff tests can give false negatives and sometimes it has to have been doing this for a while before a sniff test picks up enough combustion products in the coolant to show a positive result.

If it is breaching the fire rings, you will want to get an experienced eye to have a good look to see why, just changing the gasket could well just lead to another failure in short order if there's a problem with the head or liners.


PS: Worth getting the pressure cap tested (or swap it for another one anyway, they're cheap). But if it's blowing around the heater joints that sounds like high presure rather than the cap opening at too low a pressure.

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Thanks Ian and Andrew

i am going to give it one more go, bleed system, new cap, hole in thermostat (thanks Stu, no hole at moment) and ensure valve on inlet clear. May also exclude heater for now to isolate that as an issue.

if not I am due an engine upgrade so head off it is then!



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