View Full Version : Only overheats when the AC or the heat is on....

10-02-09, 04:32 PM
I have a 2000 STS. It overheats ONLY when the AC or the heater is on. I have had two block tests done, flushed the radiator and added the sealant tabs, the fans are all working like they are supposed to. Had the purge valve checked to see it if it was blocked and it's not. I never have a problem with overheat as long as I don't turn on the heat or the AC. It goes in to a different mechanic on Tuesday to be looked at. I have searched and searched the internet for this problem and can't find anything. Thoughts or suggestions?

10-02-09, 04:39 PM
Hmm, I'd start by flushing the radiator and forget the tabs. See if that helps.

10-03-09, 08:17 PM
I bet they did the block test wrong. Drive the car hard to the point it over heats. When you get home with it, carefully relieve the pressure by slightly loosening the radiator cap. The car will be belching out all the coolant. Once the pressure is gone take the cap off all the way and then block test. I bet it comes back positive this way.


10-17-09, 11:12 PM
Follow up on my car. Different mechanic did a block test and it showed NO head gasket problems. He said that I had a very strange car! He found out that it was the overflow tank. The top inlet was completely blocked. He didn't know what was in there that was blocking it. So that meant no coolant could go in or out. I asked for the old overflow tank and took it to my dad's shop. We wanted to take it apart and see what the heck was in there that had cost me upwards of $1200 by this time. First he tried to blow into the inlet and nothing, it was definitely blocked. He then tried to insert a tool into it and still nothing. We were able to cut a square around the inlet but it was a long tube and it proved to be too much effort by this time so we pitched it. So anyone having overheat problems like mine, check the overflow tank for blockage.

10-17-09, 11:37 PM
The surge tank floats on the cooling system. The top inlet is to allow purge flow from the water pump housing nipple in order to scavenge air and gasses from the coolant, preventing water pump cavitation. The lower fitting, about 3/4" or so, is the main in/out feed, as the tank merely acts as an air loaded "spring" to allow coolant expansion and contraction. Without the purge line, the surge tank is a slightly more elegant older "coolant recovery" type tank which does not require a two-way radiator cap and a hose to the tank.

Your experience with a plugged surge tank inlet nipple parallels others' experiences with the water pump housing nipple (actually a hollow bolt) being clogged with hard deposits.

10-18-09, 12:36 PM
Check that purge line and be sure it is clear and flows coolant. Not sure if the 2000 used the cooling system supplement (sealant tabs) or not, but if it did (and I think it did), that is likely what plugged it.