Another nissan 240sx overheating problem.

[DatCookBoy]

when i drive my 1991 240sx at about 30 or so mph the temperature needle will shoot up but when i go around 50 mph or higher the needle will go back down.So its clear to see that the car needs more air than it already has.I have installed a Koyo oem radiator, and thermostat,.when i cut the car off water will be backed up into the overflow tank.I have done test so it is not a blown head gasket.Im thinkin that it could be the fan clutch has anybody ever had this problem?If so or you just know whats going on then please tell me....Thank you

[Chezedik]

Yes, I had the problem on my last S13, you have air in your cooling system. I am going to take a stab at this and say that you drive it for a while and it starts to overheat, then the temp just shoots down out of the blue. You have the heater on and it is very hot for a second, then starts blowing cold air, etc.. The fix is that little screw on the intake manifold, with the car warm, take off the radiator cap, I would suggest getting a safety vented cap so you do not hurt yourself. Then you take the screw out of the intake, be very careful not to burn yourself here either (in fact, you will notice a warning under the screw). With the bolt out, add coolant. Continue until coolant comes out of the bolt hole. See if your problem goes away. This problem will get worse every time you get in the cooling system (water pump, thermostat, radiator, etc), but that is the fix. Good Luck!

[Chezedik]

BTW if a clutch fails in your fan it stays locked up, insuring you do not overheat. The downside is it eats gas and horsepower, and destroys water pump bearings.