240sx Overheating

[aochoa24]

I recently bought a 1991 240sx coupe and the car overheats about after 10-15 min of driving. The needle rises to "H" then randomly go back down(a little above 1/2). I replaced the radiator and the thermostat and bled it. The car still overheats...i did a compression test and the they were about 120psi give or take 5psi across. So I i bought a engine gasket rebuilt kit and was planning to changing it. But someone told me it might be the pistons...i guess my next step is a leak down test?? should i continue to fix this engine or swap a new ka24de? please help!

[apex760]

check ur oil make sure there no water in there is check eng. light on?

[bigperk11]

mine did the same thing and it was air in the system. i would first try bleeding it agian.

[aochoa24]

nah theres no water in the oil and the check engine is not on and ive bled the system quite a few times...yesterday i took out the thermostat out and it was running fine on the freeway( i guess cuz more air) and when i drove on the street the temp gauge started to rise i checked the car and it felt pretty hot im pretty sure it was overheating....ok heres what ive done-new radiator-new thermostat(currently running w/o one)-flushed the cooling system like 7 times-ive bled it-compressing test (120 give or take 5psi across) the engine runs strong-cylinder leak down test(no leakage if i did it right)head gasket? when i first bought the car white smoke was coming out of the tailpipe but then stopped...the only time white smoke came out again was when i was doing the timing.
Modified by aochoa24 at 11:11 PM 9/25/2008

[grecco28]

you have to use a special tool to take out all the air of the coolant systemis like a vacum device in fact sucks all the air in the engine but you have to take out the coolant firts and the refill the system with this tool you should go to a shop o buy it is not too much expensive and it works great...

[SHIFTVAX]

Yeah sounds like there is an air pocket in there; 240's are very hard to get all th air pockets out of the cooling system.

[NISSAN240SX12]

the way i bleed a coolant system is to pop off the radiator cap and start the car and let it run for about 15-30 min. Make sure that the radiator is full and have a some spare coolant to fill the radiator cause you will have some fluid spill out during the bleeding.

[leonelrr]

wtf i think the car is not even overheating, it might be just your coolant temperature sensor, or the temp sensor that sends the signal to the gauge..

Well actually and someone feel free to correct me if i am wrong, but the coolant temperature sensor, i think sends a signal to the ecu, and the other sensor to the gauge and thermostat?

Anyways it's a good idea to change both, they're not expensive ..

[NISSAN240SX12]

you can replace both if you like. but i dont think that the temp sensor for the ecu is bad. all the sensor does is as the engine heats up it tells the ecu how hot the engine is running and how much fuel is needed. as far as the other sensor goes it just hooks up to the gauge. you may have a weak connection somewhere. run a diag on your temp sensor using a ohm meter. check this link out it will help you diag the sensor. zerothread?id=254375

[aochoa24]

damn thanks for all the advice peoples...when i bled the system i had the radiator cap and the bleeder screw off i kept topping it off and i ran the car for 1HR...oh i forgot to mention i already changed the temp. senor and temp. switch cuz they looked carotid. i havent changed the the plug tho...everytime the gauge says its overheating i think it actually is overheating it gets pretty hot. on the freeway the needle stay at 1/2 and on the streets it slowly creeps up... ok update on what ive done....-new radiator-new thermostat(currently running w/o one)-flushed the cooling system like 7 times-ive bled it( radiator cap and bleeder screw off for 1HR.)-compressing test (120 give or take 5psi across) the engine runs strong-cylinder leak down test(no leakage if i did it right)-temp switch-temp sensor-tylenol( for all the headaches)

[madrift]

what about the radiator cap???

[aochoa24]

yeah its a new radiator cap...ok today on my way to work the check engine light went on...on my way home the light turned off.

[madrift]

well the only thing i can think of is the calcium deposits built up in the block

[aochoa24]

calcium deposits build up in block ey?...this there anything i can i do to remove or fix that?

[Terrorinc17]

I had the same problem. I left the cap on and jacked up the front of the car after driving it a while to get it up to temp. I then opened the bleeder on the water outlet to bleed it. I think the cap should be on to keep the pressure up. Be carful not to remove the screw all the way out. Just let it trickle for a few minutes. Make sure your heater is on hot also. It worked for me.

The other option is check your water pump. The impeller maybe spinning on the shaft at higher rpms. Might not be a bad idea to replace it anyways.

Good luck.

[jemini91]

Just fill the radiator up to the top until it tops of then leave the radiator cap off start the car and see if the coolant lowers in the radiator keep doing that until it doesnt go lower then put the cap on and take off the overflow cap fill it up to the max. If it overheats still then turn off the car and coolant will go the overflow turn the car back on immediately rev it high for a couple of seconds and the air will come out of the system through the overflow and you will have to add more coolant because the air took up space. I bet this will solve your problem