I have a 2006 cobalt SS that’s been r titled and gone through multiple owners so it’s a little rough. The radiator and ac condenser were hit at some point before I bought it months ago. During hot days it’ll overheat sitting in traffic, that’s the only time. However when it gets hot I hear a loud high pitched whine from the rear of the car, and my brakes will get spongy as well as my clutch. However the coolant reading is 210. I was wondering if the engine bay gets so hot the brake fluid somehow overheats? I’m also wondering if the coolant reading is wrong? I plan to replace the radiator but I wasn’t sure how that would ever affect my brakes. If I pull over and open the hood in the shade for half an hour the issues go away. How do I go about diagnosing this?

 

I would suggest replacing the t-stat, and coolant temp sensor while you have the rad and condenser removed.

Do your radiator fans work? (Turn on the AC to find out as shortly after turning the AC on the fans should kick on)

Have you tried burping the coolant system by having the cap off the overflow and starting the car and allowing it to get to operating temp?

As far as the soft brakes, no clue what could cause it related to the higher temps. I'd start with getting the temps closer to "normal" operation temps of 185-200F and then address the brake pedal feel. That said it's never a bad idea to bleed your brakes every 2-3 years or when you buy a new used car.

 

If it only gets hot when stopped, first thing to check is the radiator fan. High pitched whine from the back is likely the fuel pump. Common GM issue. Replace it now, before it strands you.

 

That’s what I was thinking it was when it happened, the fuel pump. It would happen when I turned the key on sometimes, and idling and driving but never with the key off.

As for checking the fans, I will do that like the other guy said too but I don’t think that’s it if my coolant reading never gets higher than like 210 before I let it cool off due to poor performance. Don’t the fans kick on at a higher temp?

I’ve noticed when my jeep overheated the coolant reading was just a little over but in reality the engine was so hot it was struggling to run still. Perhaps similar issue? I’ll just replace all my cooling system parts and give it a flush.

 

I’ll go through my cooling system and give it all some attention. I didn’t know you could burp the system like that, I’ll try that.
As for the brakes, wasn’t so much as having soft brakes but it was just very rough. I’ll get it up on a lift and inspect everything soon however and I’ll be back with questions I’m sure.

 

Thanks for the help. This car was counted as a loss though and it’s a parts vehicle now. Transmission went.