overheating question

[aquariustiger]

hi im having a bit of situation here with my 1991 240sx automatic.

replaced new oem thermostat & water pump & the car is fine on the highway; but the moment im on the street during a red light it will start overheating rapidly. to control the heat i have to turn the heater full blast & its freaking me out!

i have a question concerning the large radiator hose that is connected to the top of the radiator & going towards the thermostat housing?

when the car is hot should i be able to easily squeez the hose with my bare hands; it feels like theres no pressure going through it.

also when carefully removing the radiator cap when its hot; very little pressure is realeased & the resevoir tank starts to fill up. is that normal?

& let say i ; just to be safe & all abouth the over heating; how could i be able to keep the secondary fan( u know the one that turns on sometimes) ON at all times? any quick way to do that???

[DRFT(kinda)]

It sounds to me like you have a lot of air in your system. Try jacking the front of the car up a little bit and let it run while you check the fluid level in the radiator. It should always be close to full.

[Vegascorbin]

I agree sounds like Air in the system.

The amount of pressure required to compress a hose will varry. The system does not run a lot of pressure (13 lbs I think but don't quote me) and if you have a strong grip then compressing the hose is easy.

BE VERY CARFULL REMOVING THE CAP FROM A HOT SYSTEM. 2ND AND 3RD DEGREE BURNS ARE NOT FUN AND CAN CAUSE PERMINENT DISFIGURMENT. And on top of that sitting in the ER for 3 hours for even lesser burns ruins your WHOLE day.

That said, if you release pressure from a hot system you should be able to hear the pressure releaseing and the coolent will escape to the overflow. That is normal. The amount of pressure and fluid released will vary depending on hte air inthe system, how hot it is etc.

[aquariustiger]

were is a good stable point to lift the front of the car; the front underside looks pretty fragile to me.

[aquariustiger]

ok then! ill be sure to check for trap air as u guys are stating

...p.s. I wasn't the one who replace my pump & thermostat, so i'm thinking it was my mechanic.

[Cone Junky]

ok then! ill be sure to check for trap air as u guys are stating

...p.s. I wasn't the one who replace my pump & thermostat, so i'm thinking it was my mechanic.

In that case, your mechanic should be troubleshooting it

[Rucca]

Also - to get the other fan on - turn on your A/C

[aquariustiger]

problem is the AC don't work! ive notice that my power steering pump was leaking down the AC but thought that would affect it that much...maybe i was wrong.also when ever i turn the heater full blast; the air at first is hot but a few minutes later it comes down drastically to a very comfortable temperature.

THE MAIN PROBLEM IS STILL THE OVERHEATING.

forgot to mention that the motor was swapped by a better one just two weeks ago; but everything seems to be running good to me with the engine.

today i was driving in town & again it overheated as expected. i stopped , pop the hood & saw that allot of the coolant was purged to the resevoir tank; even after i replaced the radiator cap with a new one!I had to refill the radiator with coolant again & drove home. while driving home noticed that uphill the temperature went down fast, when going down hill the temperture went up but not that fast.

OK! what if the mechanic stupidly placed the thermostat inside out; is that even possible?; would that cause the coolant to be purged out into the resevoir tank?

[aquariustiger]

thing is i got to get to work during the weekdays; weekends are the days of thunder for my sweet SX !

[Rucca]

Did you bleed the system thoroughly?

It is possible to install a thermostat backwards - which will cause overheating and can cause the overflow tank to fill. Basically the overflow tank will fill up if the coolant boils and expands - which is commonly caused by air in the system not allowing the coolant to build sufficient pressure (air can be compressed, fluid cannot - pressure lowers boiling point).

You can try replacing the radiator cap if that doesn't work.

I suppose if your problem persists after thorough bleeding, you should check your system for leaks thoroughly and perform a pressure test.

[aquariustiger]

Did you bleed the system thoroughly?

It is possible to install a thermostat backwards - which will cause overheating and can cause the overflow tank to fill. Basically the overflow tank will fill up if the coolant boils and expands - which is commonly caused by air in the system not allowing the coolant to build sufficient pressure (air can be compressed, fluid cannot - pressure lowers boiling point).

You can try replacing the radiator cap if that doesn't work.

I suppose if your problem persists after thorough bleeding, you should check your system for leaks thoroughly and perform a pressure test.

hey ill be doing that after i give it good flush; one thing the radiator plug is not like the 89sxi'm having trouble getting that sucker free; i don't want to break it...any suggestions. is this plug replaceable???

[pwrofdrm]

Sounds like air in the system, make sure all bleeder valves are closed, run the car with the cap open and heat on, see if any air bubbles up. You might try getting a new radiator cap while you're at it, because over heating or not, you should be releasing some pressure when you take the rad cap off a hot car.

[aquariustiger]

well replaced the radiator just because the old one had many bent fins around; & still overheats. i asked my mechanic about air stuck in the system but assures me that he bled the system through.

the shop is near the highway so the mechanic tested it there. i'm thinking that wasn't a very complete test because that only test it on high speeds & not on a stop & go situation.

so here are the new results after the radiator install & me giving it a test spin:

i noticed today after the radiator install that while driving it through town; after maybe 10-15 minutes; it begins to over heat really fast all the way to H.

i stopped to check the upper & lower radiator hose & found out that the upper hose was hot & the lower one was cool. also noticed that when i have a constant speed like 40mph; it begins to cool down very quickly but starts to heat up again on a red traffic light even with the heater ON full blast .

i decided to return to the shop right after the over heat & checked the hoses again. after about 5 minutes while idling the lower hose began to warm up & the car began to cool down almost to normal level.

i'm pretty confident on my mechanic because he has a lot of experience with this type of car. he has like seven 240's at his shop just waiting for him.

well thats pretty much the symptoms now! I would of like to troubleshoot it myself but just don't have the time; i'm a family guy who needs to show up to work. hopefully we will find the fault & save some poor soul the trouble with dealing with this specific situation.

[aquariustiger]

Sounds like air in the system, make sure all bleeder valves are closed, run the car with the cap open and heat on, see if any air bubbles up. You might try getting a new radiator cap while you're at it, because over heating or not, you should be releasing some pressure when you take the rad cap off a hot car.

on the 89 model i believe theres just two bleeder valves; one the driver side on the block & one on top of the engine next to the coolant temperature sensor.is this correct for the 91 model?

also whats the deal with the coolant temperature sensor & the small sensor next to it. do those just communicate with the computer & not the dash???what sends the engines temperature readings to the dash board( my dash is not digital)???

[assassin7420]

Change the T-stat by your self, Get a nismo unit wile your at it. It sounds like it was installed the wrong way or is defective.

[vancouverbc]

unplug the coolant temp sensor to see if that helps

park the car on steep hill and run engine with rad cap off. its quite hard to get the air out. if this doesnt do it, take the stat out.

look up coolant temp sensor and thermostat in tutorial link in my signature. you can take stat out without removing anything else. just have the right tools. and some high temp silicone to seal it up again.

dont bother feeling the hoses or rad. doesnt help with diagnosis