theres far less info than required to make any accurate diagnosis ,
ENGINES GENERATE HEAT,WITH FRICTION AND BURNING FUEL/AIR MIX, OIL AND COOLANT FLOW IN THE ENGINE TRANSFER HEAT, AIR FLOW THRU THE RADIATOR, DISSIPATES, TRANSFERRED HEAT IN THE COOLANT AND OIL TO HEAT AIR FLOW, you simply need to find where the process is failing or if its a mechanical failure in the engine or cooling system
Id get a new gauge and sensor installed , then pull the plugs and post CLEAR pictures with the cylinder numbers indicated and do a leak down test, theres something wrong,possioably a blown head gasket or defective water pump, Id also verify the ignition timing curve and TDC on the engine, personally Id also pull down the water pump and inspect the impeller or replace the water pump,a few tests should reveal the problems source
if your moving coolant at a good rate and the radiators of the correct size for the application, Id be looking at either fans or ducting if the engine testing doesn,t show any issues.
btw, back years ago I remember a friend who installed the wrong under size fan blades on his water pump that were easily 2" smaller in diam. and set back 3" from the fan shroud rear edge, the result was over heating until the correct fan and spacers were re-installed
any trouble codes reading with a scanner?
is he loosing coolant?
are you sure its not a defective temperature gauge ?
what does the spark plugs look like? (post pictures please)
is he low on oil?
has he had a tune up lately?
any vacume leaks?
ANY COOLANT IN THE OIL?
DOES HE NEED TO CONSTANTLY FILL COOLANT?
and INDICATION of a missing cylinder, or drop in performance?
do you see coolant flow in the radiator when the radiator caps removed?,
does coolant flow rates change when the engine rpms increase?
whats the ignition timing set at?
whats the ignition advance curve?
are the fans turning on,and running correctly,?
whats the intake vacuum reading?
are all the sensors functioning?
are the radiator fins clogged with debris?
whats he using as coolant?
whats the engine compression test show for each cylinder?
is the water pump functioning correctly?
any chance the lower radiator hose is collapsing at higher reves due to water pump suction?
has the radiator been flushed and inspected?
any fan belts loose?
has he tried running with out a thermostat to see if it changes his results?
if its got an automatic trans is there any chance the transmission is over heating?
any chance the transmissions low on fluid?
what the battery voltage reading?
whats the oil pressure?
any chance its got a clogged catalytic converter?
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