Hi guys so cold outside flushed water system out loads of ■■■ came out filled with 5 litres antifreeze and water…pipes blocked going into expansion tank all clean and flowing now.Went to put new thermostat in wrong one !! Ahhhhh… anyway got old in hot water works good so put all back together.Went for a drive top rad pipe went hard… lower soft and cool …hot then cool air into cabin temp on gauge went up stopped car released cap !..boom water shot out !! got home normal temp left running for nearly an hour temp normal fan cuts in and out temp normal half way mark now have hot air into cabin think and hope all is good now and both rad pipes soft and hot think this is worst car iv had to bleed water system…does this seem OK now …cheers for now
That’s is why I said the other day not to open the pressure cap straight after it’s been running 
Glad it’s sorted
Did you burp the system the way I said ?
Craig 
Easiest way to do it is to take the rad cap off, top it up, start the engine and leave it running until hot while topping it up as air burps out of it. Just make sure you leave it running long enough for the thermostat to open.
I too spent most the weekend troubleshooting the coolant system after replacing a coolant temp sensor on my 91 GTO!
With the air I introduced into the system by swapping out the sensor, I added 300 mls of coolant and took it for a drive. Then I noticed coolant leaking in the engine bay and under the car.
Initially I thought all that coolant spraying on the engine bay was caused by a leak in the sensor (the P.O.S VEMO brand sensor) wouldn’t thread all the way in like the old sensor would) so I tried re-installing, and tightening it further hoping the sensor hoping it wouldn’t leak.
Fast forward an hour (deep socket would not fit over the pattern part sensor so had to use an offset ring spanner), I got a check engine light, still coolant in the engine bay, and the car overheated. I then realised all the coolant was leaking from the overflow tank - so thought OK, it needs a proper burp.
Rad cap off, and let the engine run - it still over heated and erupted coolant all over the engine bay (the rad fan wouldn’t start!)
The last straw, so I removed the inlet hoses and swapped the old sensor back in, no more CEL and did another burp on Sunday morning and the fan kicked in as it should have.
All this over a sensor.
Hi guys thanks for all info,not used since Sunday had a lot of scummy stuff come through over flowing cap housing looked like aero bar bubbles !!! been out to check tonight level good in cap housing ill warm up and take for a drive this weekend just got worried about head gasket damage never had any check lights come up and when stopped temp went down quick so im sure OK ill let know later …
Had mine overheat a few times when i first got it due to wrong ecu fitted… takes a real good overheat to damage our cars… let’s hope its fine mate.
A good test is to go for a good drive with some boosting. Pull over and remove the overflow pipe from the expansion bottle (leave it connected to the filler neck). If it is spewing water out after a few minutes on a hot idle then head gasket would look likely. A small bit of steam is normal.
that’s the issue with mine headgasket is goosed 
fine driving off boost but when on boost it blows the rad cap pressurising full system . did a sniff test and its drawing gases through the heads
Like i said all was well till i had new water pump and cam belt changed so didn’t really let it get to hot i am sure its all good will let you know after weekend drive around…
Hi again had a small mess about today car runs great how hard are the 2 main big rad pipes meant to get.still getting hot air then cool air into cabin pipes get quite hard more so top one and very hot while holding lower rad one and squeeing top one can feel pipe move so thinking thermostat doing its job.Temp staying pretty well normal main fan kicks in then smaller second one kicks in is this all normal or not?do these cars run quite hot? Any more information would be great.
The main driver side fan on a GTO should come on when the sensor switch at the bottom senses warm/hot coolant and with the smaller passenger side one will turn when coolant temperature measured in the rad is hot or if you switch on the Air Con.
Hello again thanks for getting back ok been out car runs great but hot then cold into cabin …again!! ive let it run squeased all pipes, cap off level rises then falls a few bubbles.While is was out all good then temp went up so got back home top hose hard very hot… lower hose hot but not like top and semi hard top hose into bulk head hot but bottom one cool ? really dont know what to do now car was fine till went for cambelt and water pump change…as before main fan fan will kick in then other fan kicks on drive temp never rises only when on the road ???
Sorry I don’t quite understand the problem described here.
Are you saying the car is overheating? If so, does this happen when stationary or when moving?
Anh
Hi when moving ,on the drive cap off warm and cool ait into cabin on the road top rad pipe gets hard and hot lower one cool and soft same as both pipes into baulkhead …going back to garage wednesday let them sort it out was fine till cambelt and waterpump change.
When I had this problem with mine, the top hose getting solid, changed the thermostat and it fixed it.
Terry 
Hi cheers im thinking its a thermostat prob but i did take it out put it in boiling water and seemed to work opened about half an inch then out of water and closed up so thought it working could be wrong …
Hi been in garage today still same …going to change thermostat they too are baffeled …hot then cool air into cabin hot hard top pipe then soft car runs great no misfire idles all day long on drive… temp normal take out for a drive temp rises !! again only happened after cambelt change and water pump…
Sounds like you have an air blockage in the system.
Seem to remember my old mini doing this. Flushed he rad system through with a hose pipe and blasting water through
I don’t recall if its possible to do it on the GTO but I think these are symptoms of a thermostat fitted the wrong way around.
Worth a check.
Marty
Guide here with pics on replacing the thermostat.
Will help you check its correct.
http://www.supercar-engineering.com/rubberducky/3S/RnR/TT/WTStat/index.html
Marty