You are now probably getting symptoms/issues as a result of the K-Seal, which will be complicating the matter.
The symptoms (and temporary workaround of air release) were happening (and temporarily resolving) before I bought K-Seal, but I take your point, thank you. It was a calculated risk on my part. KSeal seems to have resolved the loss of coolant, but not the build of of air pressure which is why I think maybe its a blockage in the water system somewhere. Unless it had a gasket issue before I inherited it and the person before used something like Kseal to "fix" it?
I'd start off by doing a pressure test on each cylinder. Do you have one?
Unfortunately no, but I have a local garage who I guess can probably do it.
What is the temperature gauge reading when its starts to "play up"
85 degrees but i dont think its a temperature related trigger because it runs OK for 3 or 4 miles at that same temperature before I have to release the pressure again. So, time wise:
Starting from cold in the morning, with the engine and water dead cold, I drive about a mile and a half and the needle rises to 80degrees.
If I am stuck in traffic or go up hill it goes to 85. Then on straight runs, drops to 80. After about 3 or 4 miles you can feel the engine running rough / vibrating.
At which point I hop out, lift the bonnet, release the pressure from the water bottle until it stops hissing, tighten the top of the bottle and she is good at 80 or 85 degrees for another 3 or 4 mile after which I repeat the process. I drove from Manchester to Preston the other day, doing the above (so it took me a while, stopping in petrol stations and pub car parks but I got threwre in the end, and back, without any other issue, engine runs fine until it vibrates, then I release pressure and I'm good for a few short miles more.
I've never seen it go above 88 degrees or so.
It is as if something is preventing the water circulating somewhere around the system. Heater in the car is still nice and warm. Someone at work today said it might be a stuck theromostat that could have caused the head gasket to go and this could still be an issue as being the root cause. Is this right? Or could it be something else common fault?
If its hard to diagnose, do you think its worth taking it to local garage for a pressure test? If so, how much do you think they will charge me on average? Thank you.