i have a '98 sc300. i changed radiator hoses, flushed the system 3 times, changed the thermostat, changed the oil, radiator cap, etc. filled with the red coolant.

recently changed fan and fan clutch which was obviously bad since the new one pulls a lot more air. the old one could be stopped with your hand easily.

drove the car about 800 miles with no problem but it was cooler temps outside. now it runs hot in this 100 degree heat and air conditioning running. once i turn it off it goes back to normal as if i had turned on the heat. overheats on hwy and city driving.


guess next is radiator then water pump. no smoke/coolant in exhaust and coolant was clean as can be.


i read somewhere it could be a/c compressor but that doesnt sound right to me unless it was going out and slowing the fan down but it seems to be turning ok.

 

maybe radiator has a leak.. check it out..
a/c compressor dont think soo..

 

overheating on highway and fast city driving would indicate a blown head gasket. hopefully i am wrong. have you ever overheated your car before for a long period of time?

 

Well yeah, but only when his AC is on... Apparently if the AC is turned off it doesn't overheat.

Jonny

 

yes but its not losing coolant, there are no leaks and it overheats at idle with a/c on.


if not for those things id agree. no i never had it go more than 3/4s much less in the red.


also it never got hot a couple weeks ago when i drove it 10 hrs straight with a/c on.


rather confusing.

 

this is correct and the engine runs fine. quiet as a mouse, accels great and no misses. if it was water pump id think its be leaking though or id hear a bearing going out. leads me to think its the radiator.


i was hoping it was the wp so i could go ahead and change all that out since i was going to have it done anyhow. now i gotta wait to find the cause of this first.

 

hope its not the dreadful headgasket, these motors(JZ) dont usually go out like that, but it can happen. and i know from an experienced friend that the heads on these motors run really hot, especially with more load i.e A/C. let the car run with the radiator cap off, A/C on and see if there are any bubbles, you may have air in the system

 

Also, depending on whether the WP is aftermarket or OEM, the blades inside, if aftermarket are plastic and not metal like OEM. If one of those plastic blades somehow broke or got damaged, then the waterpump would be running less efficiently/productively... thus when you are putting more strain on the car (AC running) the waterpump can't keep up... Just throwing possible options out there...

Jonny

 

yeah i checked that about 10 times already. no bubbles. guess ill have it pressure tested and do a combustion leak test to see. i was planning on dropping another engine in it anyhow but wanted to wait a bit.

yeah ive seen something similar to that before. im just working my way from piece to piece.


running out of pieces though.

and i know if i bring it to my guy hes gonna keep it a damned week and have the radiator checked out. if i can find one for 100 bucks i just as soon change it altogether. he surely is going to charge me more just to check it.

 

When you kick the A/C on the engine is supposed to idle up, which will run the fan quicker (on cars with electric fans it just kicks the aux fan on). You need to pay attention to if the car is clearly and audibly idling up when you hear the A/C clutch engage. There is an idle up switch/relay for this issue.

 

Headgasket is probably gone. Also, double check the rotation of your electric aux fan. It might be spinning in the wrong direction and pushing air out of the engine vs. in.

 

Yeah I was thinking about that earlier and actually was wondering why I didnt hear any difference when I was turning ac on and off.


Huh? what aux fan.

 

Tabaka is probably thinking of a MKIV supra, SCs dont have a second aux fan, well as far as i know, or maybe a sc400 does in which case wouldnt apply to magneti, but i know my car doesnt have a second aux fan

 

how could it be a headgasket and only cause trouble when the a/c is on?

 

Is the clutch fan working properly? it sound like a fan clutch to me.