I recently purchased a used 1999 Saturn Ion and every now and then it overheats. The tempgauge is not showing hot but it will smoke and over flow in the coolant reserve. I had it in to a mechanic and he could not find anything wrong. We keep the reserve filled to the line. Any suggestions?
Try replacing the thermostat and bleeding the cooling system not it do the following below: Have it block and pressure tested-do the block first to pinpoint a combustion leaking into the cooling system or a gas analyzer to sniff for hydrocarbons at the radiator fill neck.
Pressure test: do not do a pressure test if there's leakage at the headgasket this might cause coolant into the cylinders and lock it up or bend a connecting rod if cranked thereafter. The headgasket should be repaired before doing the pressure test.
If the block and pressure test passes check the following: Thermostat, Pressure test the Rad. Cap, Clogged radiator, Fan clutch, Radiator electrical fan, collaspe hoses, water pump