I had same problem; replaced the coolant temp sensor, thermostat, and reservoir cap. Didn't help. Turned out that the plastic reservoir itself was slightly warped/out-of-round where the cap screws in, so that even with cap on tightly it would not hold pressure. To test this, I put the black overflow hose that comes out of the reservoir (near the cap) into a clear plastic water bottle. With engine hot and a little water in the bottle, I could squeeze the radiator hose and see bubbles in the water bottle. No air or coolant is supposed to get through that cap until it's over 15PSI. Got a new reservoir ($35/RockAuto) and now system holds pressure. I'm leaving the plastic bottle in place, the overflow hose sort of holds it in place, doesn't interfere with nearby moving parts, and will show me if there's any more overflow (haven't seen any in 3 days)
Wednesday, July 8th, 2009 AT 9:28 AM