My 1977 240D with 260,000 miles, which ran wonderfully up to the point where I decided that I need get the green stuff out, now has overheating issues. I replaced old coolant with the Zerex GO-5 per numerous reference on this forum and other sources. While changing over I also back flush the cooling to confirm that all of the old coolant was completely removed. After purged the system of air through the bleed screw in the top of the thermostat housing and running the heater, I thought I was back on the road as good as ever. The car began gradually getting running hotter while sitting idle in traffic. So I thought it might be a good idea to do the citric acid flush. This did nothing to improve the temperature problem and now the car was running hotter than normal a freeway speeds. Since I wasn't sure of how to fix the problem I began throwing money at it by way of a new thermostat, radiator, and water pump. During the process of replacing these parts, I also adjusted the valves in the hope that that might have some positive effect. Now I can't purge all of the air out of the cooling system and it overheats within just a few minutes of driving.
The final time I tried to bleed the car, it was parked on a hill so that the radiator was above the engine. At which point I added coolant to fill the radiator. Then I opened the heater valve and pulled off one of the hose going to it to confirm that there was coolant in the heater core. Once again I topped of the radiator. With the engine running and the radiator cap off, I could see large burps of air began rising up to the neck of the radiator. I believed that this was an indication that the air was being purged from the system. The large size of the bubbles and the absence of froth lead me to initially think that the head gasket wasn't the problem and it just need to get all of the air out to cool properly. However the bubbles never quit coming and the the heater eventually stopped producing heat. All this happened while the car was park on the hill running at high idle. Although I had a new thermostat that was confirmed good by a hot water test, I decide to try a second time without the thermostat. I had no better luck without the thermostat.
It appears that I may have a leaking head gasket but I hope that isn't the case. There is no water in the oil or vice versa but the air/heat has to be coming from somewhere. I've checked the cooling system pressure and at 15psi it lost less than .5 psi in 1 hour. I'm planning to check the compression today as a final test of the head gasket. I assume that this should give me some indication of leakage from the combustion chamber into the water jacket.
Does anyone have any suggestions to what maybe causing this overheating besides the head gasket? If I do need to change the gasket I'll probably bite the bullet and get the heat reconditioned. I've read that Metric is good but appears to be expensive and Mark DeGroff's Cylinder Head Service was mentioned. What are your experiences with these and other shops in Southern California?