Overheating so far i have fitted a new thermostat, new fan switch , water pump impeller and shaft ,had the radiator cleaned and pressure tested at a local radiator specialist ,renewed the head gasket all though there was no sign of water in the oil ,the head checked for warping, flushed the entire cooling system several times and bled the system according to the manual at a loss of what to try next i think it must be air in the cooling system but i`ve tried every thing i know.everything seems ok when i run it up to temperature in the garage. The water circulates ,the fan cuts in, at the right temperature all seems ok then after a short run the temp gauge goes into the red the fan doesn't cut in, one side of the radiator is stone cold while the other side is red hot and all the water has boiled over forcing steam to come out of the header tank overflow pipe. Its seems that there is a blockage or air in the system i've tried to refill the water several times and bleeding it according to the haynes manual but each time the same result. Don`t know what to do next i might be missing something but at the moment i`m stumped any advice would be appreciated.

Perhaps you have a thermostat that is stuck closed or filled with trash or Did you actually test the thermostat to see if it is working? Even if you replaced it with new? It should open when the water temp is 180F to 160 depending on it's rating. How do you know the radiator is clean? But did you test it to see if you had flow through it?
How do you know the Water pump is working? Sometimes the impeller slips on the shaft, or the drive pulley slips on the drive shaft. Does it leak at the water pump? That may indicate you need to change the water pump anyway.

First pressure test the radiator for leakage throughout the cooling system (or Blockage) then you should have it checked for flow percentage. If that all checks out do a complete flush making sure to use the correct anti freeze. (providing you have checked rad fan switch, cool temp sensor, and cap.) If there is no water showing in the oil then check for bubbles in rad while car is running. Just make sure by vin # that you use the correct antifreeze because adding or mixing the green and red can cause gelling in the coolant system.

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did you bleed the water system of air ?
does the radiator build up lots of pressure ? have radiator pressure tested as the cylinder head gasket could be leaking !

did you recon the rad it may be corroded all ways replace rad when you recon an engine but a handy way to see if the pump is working is to see if the water is returning into the header tank

could very well be the water pump. they dont always leak if they are bad. sometimes the impeller that moves the water through the system fails.

Right start by running engine and testing for the radiator getting warm all over and not just on the outside tubes ,then clog up in the centre where the fan is first so if its warm on the outside and cold in the middle where the fan is then this is your problem ,Now assuming this is ok and checks out then the main cause of failure for overheating is the water pump ,it may not leak but what happens is a plastic impeller on a metal shaft and the plastic impeller comes loose on the shaft so the shaft turns but the impeller doesnt or very slowly causing the engine to overheat ,also try bleeding off the top heater hose at highest point but i doubt if this is your problem as most cases i get are where the impeller comes loose

Believe it or not, new thermostats can be bad. Did you maybe get some adhesive on the thermostat preventing it to open fully? Is the thermostat installed with the crown up? The only other thing that I can think of, if it's a closed cooling system and the wrong radiator cap is installed, it won't hold the pressure, therefor overheating. The other thing may be the water pump. Many times the impellers will wear down internally causing to low of water flow.

The thermostat is behind the upper radiator hose...remove the upper radiator hose and loosen and then remove the two bolts on the outlet (where the upper radiator hose ends) The thermostat is inside that outlet...Another thing to check is if there is water in the oil...If there is water in the oil then it could have a head gasket problem..Did you ever replace the thermostat? If it is stuck then it would also cause overheating...Excessive exhaust backpressure because of a clogged catalytic converter could also cause overheating..Also check belt tension and condition. A loose belt that slips may prevent the water pump from circulating coolant fast enough and/or the fan from turning fast for proper cooling...Another thing it could be is a faulty Water pump -- Any wobble in the pump shaft or seepage would call for replacement. In some instances, a pump can cause an engine to overheat if the impeller vanes are badly eroded due to corrosion or if the impeller has come loose from the shaft. The wrong pump may also cause an engine to overheat. Some engines with serpentine drive belts require a special water pump that turns in the opposite direction of those used on the same engine with ordinary V-belts... Also check the Fan -- With mechanical fans, most overheating problems are caused by a faulty fan clutch, though a missing fan shroud can reduce the fan's cooling effectiveness by as much as 50% (depending on the fan's distance from the radiator) which may be enough to cause the engine to overheat in hot weather or when working hard.

overheating has a few possibilities.
thermostat.
low volume from water pump.
air lock caused by a small leak
cooling fan not coming on. from radiator fan switch or relays.

have the system pressure tested,
check the fan operation,electric or clutch fan.

fan will only run when engine is hot,check its working ???if not check the relay connections.if and when you get the fan working see what happens,if it persists then bypass and run fan continuosly ,,see what happens if it still happens with fan on all the time then two possibilitys,,,,,head gasket or plastic impeller on water pump sliding on shaft and not turning ....