So far, the car has a hybrid turbo, bigger smic, 0.216 injectors, and a remap pushing the turbo to 1.75 bar.

 

however, i have found that now, the thermostat doesnt open, and the system is presurising so all the coolant comes out of the expansion tank via the overflow.

 

i have changed the thermostat, and tested it and its fine.  Head gasket has been replaced but issue still persists. (head was skimmed and pressure tested)

 

Im now wondering if the head is lifting off and i need more meaty head bolts - any opinions?

 

Kaz

Can't imagine it's a head gasket problem. Have you had the water pump changed lately? Original plastic ones disintegrate over time leaving no effective impeller to pump the water around your cooling system which may present itself in a similar manner as if the thermostat was stuck closed.

+1 for the water pump, seen it on most my MK1's and similar models VAG cars.

Does the car smell hot.... that old leaky radiator smell from days of old?

All sorts of reasons why this could be happening on a performance car. nearly running 26PSi will be creating huge exhaust gas temperatures. Have you invested in those temperature reducing gaskets for the manifolds? Heat wrapping or ceramic coating the hot side of the turbo and downpipe will help. You can also upgrade the thermo for a cooler running one? How about running waterless coolant? No water means the coolant can run at a much higher temp without expansion/compression therefore reducing the pressure in the entire coolant system.

Also big 1.8t engine tuners will advise you run water methanol to reduce intake and overall engine temps. Obs great for mapping regs ignition advancing and knock reduction.

Too many elements to consider but if I were you, I'd not be running/pushing that turbo with some if not all of the above mentioned mods carried out. Performance tuning is certainly a barrel of fun

Or..... It could just be the water pump lol

the waterpump was changed 4k ago.  I took the small pipe off and started the engine, and there is a nice flow of water coming out of there (I was advised this was the check for the water pump)

 

I haven't added any heat reducing stuff on there. I never actually thought of it.do you have the part number of a cooler running thermostat?  I think that might be the next step before I invest in waterless coolant

When the head gasket failed previously, or any other time, did anyone add a liquid sealer?

K-seal or something similar?

http://www.awesomegti.com/car/skoda/octavia1?subpart=Tuning

Neuspeed thermo and thermo gaskets can be found above. Apologies mods if I've broke the rules regards the above link.

Those gaskets appear to be for a 1.8 t, but the OP has a TDi,  which I believe those parts do not interchange with.

Sebring, who is the map from that the tuning company used on your car?. Do any of your hoses fell unduly soggy when you squeeze them that could be an indication of partial collapse inside?.

no k seal or steel seal added prior to the hg failing -

 

are the thermostats of the 1.8t's compatible with the tdi engines?  I cant find any cooler running thermostats for these :( 

 

the mapping is not from another car - hidden power is doing it for me

You will need to find someone on here with ETKA access to check what the stat options are for the TDi, I would think the engine code and vehicle year will be the bare minimum they would need to check for you. Do you know how they pressure tested the head at all?, Are there amy signs that the liners may be porous on the cylinders at all?.

 

I must admit, as you have already got the head off, I would be inclined to pull the water pump to check it and make sure nothing is blocking a cooling gallery myself.