Recurring Air lock - Sigma Cooling

[krisfl]

Howdo Hive mind. Hope you're all well as can be.

I have a recurring airlock on my Ti-VCT sigma. Basically after a good run the expansion tank is full to the brim and the engine temps climb. Cracking the bleed bolt on top of the Rad with the tank cap off sorts everything out. Till the next run... It does this with the heater on or off. Makes no difference...

So far I have:

 - Bled x3 myself.
 - System checked and bled by a Seven Specialist.
 - Checked all hoses and connections everywhere, inc heater. All look and feel new.
 - Checked the infamous blue T piece

Next
 - I'm going to swap the expansion tank cap (it looks new, but its cheap so...)
 - Swap out the rad (possible pin hole and some suspicious foil tape from the previous owner over the top and bottom bolts leading me and the specialist to be suspicious)

But anything else I should be checking first? Or after? What could I be missing?

Ta muchly.

[K7 VCT]

Get it pressure tested at any garage would be the first thing I would do.

My sigma level changes a little bit only when hot or cold .

Raising the front of the car so the radiator is higher than the heater to bleed properly.

[Dave connors]

My sigma expansion tank used to fill when running changing the cap cured it.

[krisfl]

Thank you @K7 VCT. Yes, a pressure test before ripping the radiator out is a good shout. Though it won't be at my cost - warranty thankfully, but no point if there is no need.

I had read about tilting the sevens at various angles from various ends while bleeding too. But I couldn't find any consensus as to which end, then if to bleed at the heater or rad. A lot of the posts where for K series cars too. So I didn't at all. Which might be it. But the aforementioned specialist had just left caterham factory after 15yrs as a senior tech there. So assume he would know and have deployed any tricks, even if I didn't do a proper job!

Onwards! Its only a cooling system - how hard can it be?! 

[K7 VCT]

Any 7 with a heater can give air bleeding issues, I even jacked up the front of the car then undid the expansion tank and lifted as high as hoses allowed to get all air out.

It can.take quite a few attempts to be air free.

[ScottR400D]

Is the Sigma that much different from the Duratec in this respect? With mine I just fill the expansion bottle, bleed through the top rad bolt, run the engine for a few minutes and that's it.

I thought all the stories of tilting the car this way and that etc were K Series things?

In any case if it keeps doing it, a leak is likely and IME the rad is the first suspect. When I bought my kit the first 3 radiators I was sent all had small leaks. I ended up getting the third repaired myself rather than continue with replacements. 

[Tom_Arundel]

Sounds a bit like a head gasket problem......Sigmas are usually reliable gasket wise...Does the coolant show any signs of contamination?

[krisfl]

Head gasket  I Hope not. 
 

no coolant is clean as a whistle. I think it's more likely the rad or the bottle cap. 

[krisfl]

Yes most of the stuff I read was K series related and involved various jaunty angles. 
 

certainly the radiator is the same on the sigma and the duratec...

pressure test me thinks. 

[SM25T]

Expansion caps can fail. Easy to replace and remove from the list of suspects.

[pgm]

Although unusual we have come across a Sigma engined Supersport that had a jammed thermostat (in the closed position). Have only seen it once and it managed a couple of laps of Silverstone before it overheated. Worth running up to temperature and confirming that the coolant is flowing correctly. Agree the pressure caps can fail though. Also head gasket issues are extremely rare.

Kind regards PGM

[krisfl]

Thanks @PGM. I'll keep the thermostat in mind then. The flow all looks good through the tank. All the pipes look and feel good too, coolant is flowing. 
 

today she's had a new bottle cap, and since the radiator had silicone round the bottom bolt (last owner) it's had a new radiator too under warranty. Hopefully that's cured it! 
 

appreciate the hints and tips as ever. Thank you all. 

[Toughie]

Thank for that PGM.  Always good to get the professionals' advice.

[Tom_Arundel]

Did you manage to fix it?

[alcal]

The coolant level in the expansion tank of my 140 Sigma has always gone up and down depending on use. If I use it as intended it goes up and driven gently it goes down. The system has been pressure tested and is fine. It's a mystery to me and my best guess is that air is somehow being entrained at high revs, trapped somewhere in the system then released upon more gentle use. If anyone has a better idea I'd be interested to hear it.