Coolant leak from high pressure after engine cool
 

Background: I bought this car a while back but have only put about 120 miles on it. It had a bad coolant reservoir with basically water for coolant (didn't know this until my first oil change). It's always had a really minor oil leak coming from the back of the engine... the oil change guy said he thought it was the rear main seal. I recently replaced timing belt, water pump, did a coolant flush, replaced breather box, and cleaned intake manifold.

I drove the car a bit with no leaks (other than the minor oil leak), but once it cooled I had a decent sized coolant leak happen over night. After refilling and driving again I noticed I had a coolant leak after about an hour and a half. I narrowed the leak down to the upper coolant hose that connects the thermostat and the radiator. I re-seated the hose and tightened down the hose and had no leak after a 10 mile drive. No leaks in the morning either. This was a week ago.

After driving for a bout 30 min the other night I woke up to the same coolant leak :(. There's no way I hadn't clamped it down enough! When I loosened the hose clamp (after 2 days of letting the car sit) I could hear pressure releasing from the hose. It's worth noting that coolant is not making it back into the reservoir. I've been searching for "coolant system still pressurized after engine cooling" with mixed results. Some are saying head gasket, some are saying check hoses for leaks, and others say to check out the thermostat.

When I drive, the engine warms up properly and doesn't overheat. It just seems to be unable to relieve the pressure once it sits. I'm worried I have a bad head gasket, especially since I have a small oil leak, and the previous owner most likely let the engine overheat (judging from the water coolant and bad reservoir). I talked to a friend and will be doing the following:

1. Check dipstick and oil cap for cross contamination between coolant and oil.
2. Check reservoir for oil.
3. Run car and see if I can see bubbles in the coolant (meaning head gasket problems).
4. Check spark plugs for any noticeable differences in color.

After these things, should I do a pressure test or a leak down test? Where do I get those and are they expensive? I'm up for doing the head gasket if I need to and I know i'd need to resurface the head, but I'd really like to narrow down anything else causing a problem like this. Sorry for the long post, you guys are the best!

A friend pointed out that I may be an idiot and I think he is right :( I ordered an upper radiator hose to be sure I don't have a bad one, and I actually think the hissing sound was suction and not positive pressure. I guess I panicked a little and went worst case in my head :p I think this will fix my coolant issue, but I'm going to do a compression test anyway to check out my internals.

Please let us know how it comes out.

Good news! The new hose fixed the leak! Should I remove this whole post since the whole top part was basically a description of a non-existent problem?