my buddy did a mini me swap y8 head on a d15 block and now the car is overheating, after driving it for about 5-10 hard it will overheat, but the weird thing is when its almost at the top and he comes to a complete stop the needle will go right back down to normal(sometimes when he accelerates the needle will go up , sometimes down) now the car wont overheat while idling so i dont think the head/block is cracked (or is it?) i dont think the headgasket blew because there is no water/coolant in the oil, or the coolant is not turning brown, and theres no smoke out of the exhaust or sweet smell. i took out the thermostat because i thought it was stuck, ran it and it still overheated (after driving), i bled the system too many times to count... my friend suggested cam/crank timing was off, but the car runs really good, so i dont think its that... anyone have any ideas ?

 

hmmm.... maybe an obstruction in a cooling passage?? It shouldn't overheat at all with no thermostat. something must be really wrong

 

it really soundsn like your waterpump is starting to malfunction.