Bizarre cooling system problem

[Mike Bees]

It's a K-series (what else would have a bizarre cooling system problem...). As expected the level of coolant in the header tank rises with engine temperature. What's unexpected is that when the engine cools it doesn't draw the coolant back from the header tank into the engine. Instead it's getting air in somewhere. I'm frequently having to bleed air out of the system. No coolant is getting out, but air is getting in. How does that work then?

 

Mike

[Peter Carmichael]

Micro leaks from the compression ring. Get the coolant tested for combustion by-products.

 

Alternatively (and just a hunch) try a different pressure cap on the expansion tank.

 

Give me a quick call anyway.

[R2D2]

Mike,

 

Is it a "header tank" or an expansion tank? Where is the pressure cap?

 

The old crossflow system has a vacuum valve in the thermostat and the expansion bottle isn't pressurised.

 

If you have a pressure cap on the header tank then the system is sealed and must always return to an identical level and pressure if there only thermal energy in the system.

 

I had a problem with a Twin cam about 2 years ago. The head was very slightly porous and at high BMEP levels (full load) the porosity allowed combustion gas into the water jacket.

 

At idle in the garage combustion pressure levels were insufficient to cause a leak.

 

The effect was to continuously over-pressurise the system and lose fairly small amounts of water.

 

I only found the real probelm when I checked the gases in the cooling system with a "block tester" which gives a colour change when there are hydrocarbons in the water.

 

I replaced the head and all has been well since.

 

I have a block tester if you need to loan one.

 

Good luck.

 

[Mike Bees]

I think I meant "expansion tank". It has a pressure cap. The coolant I'm using is supposed to change colour if it encounters combustion gases, it's failed to change colour thus far.

 

When I first ran it (after it's recent rebuild) it weeped coolant from the water rail/head interface. I tightened it and it stopped. Clutching at straws, but I wonder if the gasket there is somehow sealing under +ve pressure but not under -ve pressure.

 

Mike

[David Hopkins]

Mine did this , tracked it down to the fact that the jubilee clips supplied by Caterham were fine for the hose diameter when the hose is virgin but after you have compressed using a jubilee for a few weeks the outside diameter of the hose was borderline in terms of matching the clip min diameter .

[John Vine]

It's also worth checking that the jubilee clips on the submarine tube are really tight. While I was getting air locks out after fitting my Laminova, I managed to pull the J-hose off completely (hot coolant everywhere)! I've now put two clips on each end.

 

JV

[Mike Bees]

I've been round once, but I'll do it again. At least I'm not the only one to have had this problem. I'll replace the expansion tank pressure cap as a matter of course, since these are known to be a common point of failure.

 

Mike

[R2D2]

I fitted an expansion tank that uses a Stant metal rad cap rather than a plastic cap.

 

I bought the Stant cap from DT and it has a lever to de-pressurise.

 

The bottle is plastic and was from an early TR7.