Ever since I bought my GTA I noticed that the passenger side fan runs constantly as long as the ignition is on, it has never run hot though. Well, I figured this may be a defective fan switch so I unplugged the switch on the pass side head and sure enough the fan didn't come on when I turned the key on. I went to Advance and bought a new switch and installed it. I started the car and let it idle waiting for it to hit the 238 mark and the fan to start. It started boiling over before the fan ever came on. The gauge never made it to the 3/4 mark before I had to shut it off. I used 50/50 anti-freeze. Why would it boil over so soon? I hate to have to put that old switch back in it.

 

It just dawned on me that it could be the radiator cap. Would a bad cap cause this?

 

If the cap is weak, it'll lower the boiling point. What temp did it boil over at? If the fan never came on at 220*+ maybe the relay is cooked from running the fan constantly. Other thing is maybe there is a bad connection at the temp sender plug.

 

It was boiling over before the gauge hit 3/4 mark, I'm assuming around 230-235. It wasn't in the "red zone". The fan was working fine with the old switch yesterday (other than staying on). From reading around TGO I gathered the switch doesn't turn on the fan until 238. I get the feeling it was almost at the point of turning the fan on when it boiled over.

 

I'd try out a new cap for sure, Its cheap. I never was comfortable with my fan coming on so hot, I bought the 180* fan switch from hypertech. It helped.

 

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I just ordered AC Delco part # D1855B , it's supposed to turn the fan on at 215*. I found a reference to it on the Corvette forum. I will post an update when it gets here.