I have a 1999 Civic Lx and I believe I have a coolant leak. I was original having issues overheating while idling but over the last month Ive replaced my thermostat, my radiator and cap, all of my heater hoses, my bypass and radiator hoses, and all of my hose clamps. I started overheating again and I noticed it was because I was running low on coolant. I drove it around and it seemed to act ok when I filled it back up, but I had it pressure tested and it didn't hold pressure. I tried running the coolant pressure tester at 12 and at 16 psi (couldn't find how much I was supposed use anywhere) and after about 5 minutes the pressure would drop very slightly.

The only things I can think of that I havnt touched are the head gasket, the water pump, and the heater core. I don't think its my heater. If the head gasket or water pump were busted, would that cause a leak in the system? Would there be anywhere else to check for a leak? Thank you.

 

Check the head gasket by starting the engine cold with the radiator cap off. If bubbles or coolant blow out when you rev it, the gasket is leaking. There shouldn't be any action in the radiator until the engine has actually warmed up some.

Overheating only while idling is usually because the fan doesn't work. Sometimes the switch is intermittent and it works sometimes but not every time. You should jump the switch to make the fan run all the time then test drive like that. The switch is on the thermostat housing.

Finding the coolant gone AFTER an overheating incident, you can't really tell if lack of coolant was a cause or an effect.