I have a '96 S500 w140 that has a blockage somewhere in the coolant circulation that causes it to run hot. A shop of ill repute did a head gasket job on it before I got it, (~one year ago), and told the previous owner that the heat/climate control was somehow causing the car to run hot. The shop bypassed the heater hoses on the back of the engine at that time, (connected the L/R together from rear of engine), and the car did not overheat but ran hotter than normal still.

When I got it, I suspected a bad radiator and changed it. No real difference. Items that have been changed or otherwise ruled-out are the thermostat, water pump, fan clutch and electric fans. The fans all come on when they should depending on temp and/or A/C activated.

It is 100% sure a circulation issue. I have the heat connected again and it works as it should. No leaks, L/R heat operable and aux. pump as well as monovalves are good. The car will overheat on a cool day w/ heat connected but shut-off and it will not OH if heat is on inside car. (But it will still eventually climb higher than it should).

I am at my wits end. I know that the heat is a bypass system on all liquid-cooled engines, i.e. it should have nothing to do w/ coolant circulation even if it's completely plugged. I am pretty sure that the reason the car runs cooler w/ heat on is because there is a blockage somewhere between the left and right side of engine on the back and the heater cores are allowing the flow when on, even though it's still not a 100% flow rate. The main radiator and fan are working efficiently, when the car overheats you can put it in Park and rev the engine and watch the temp gauge come down pretty quickly.

HELP!