I replaced my old thermostat after flushing my cooling system due to it being very corroded and old when I took off the T-head. However, now I keep having coolant expelled through the overflow hose from the expansion tank (I just replaced that as well with a brand new OEM tank). I bought the thermostat from Autozone and am wondering if I need to get an OEM thermostat or did I not do something right when I refilled coolant and installed new thermostat (I checked again and it can only fit one way). The engine temp never gets too hot (stays below 92c) and today was running at about 87c. Before that, never had this problem. No leaks in expansion tank or at T-head. Hope that's enough info. '91 300E.