I can't address the issue of the bent pipe unless you upload a picture. The sound you liken to pressurized gas could be a few different things. It could just be the sound of air passing through the idle air screw passage, it could be a vacuum leak someplace, it could be an A/C leak, or it could be pressurized coolant or coolant vapor escaping from a tiny pinprick size hole in one of your coolant hoses.
There are a few reasons your upper radiator hose might be hot while the radiator cap and lower hose are cold. The effect of the problem is that coolant is not circulating through the radiator. Possible causes:
- As already mentioned, you could have a thermostat which is stuck closed.
- The radiator itself could be clogged either with gelled coolant, rust particles, aluminum oxide or any combination thereof.
- There could be insufficient coolant. This would cause the coolant to start vaporizing at a certain point, so that even though there isn't enough coolant to circulate and warm the radiator and lower hose, the vapor could heat up the upper hose.
- A potential cause of #3 is that you could have a coolant leak. As noted above, this could be a potential cause of the hissing sound.
- You could have a bad water pump.
I would recommend the following:
- When the engine is cold, check the coolant level. If the coolant level is low, top it off, then run the engine until it reaches normal operating temperature. Once warm, rev the engine for a few minutes at 3000 rpm, while watching to make sure it's not overheating. Then quickly go over to the engine compartment to see if you can locate any leaks. Many leaks will only show up once the cooling system is hot and under pressure. Another possibility is that it could be boiling out through the overflow bottle. If you find a leak, replace the leaking hose.
- If the coolant level is OK, then you'll need to drain the coolant and check the thermostat. Make sure to drain into a container. You can check it by placing it in a pot of boiling water to see how much and how quickly it opens.
- The next thing is to check that coolant you drained. Slowly, decant the coolant from the container you drained it into into another container. Do you see any crud in the bottom of the container, maybe something like this: If so, you may have a clog as mentioned above. Try back-flushing the radiator. Reinstall the thermostat, then stick a water hose in the fill neck, while opening the drain plug. Make sure to put a clean container underneath to catch any crud that might come out. You reinstall the thermostat to make sure the waters only path is backwards through the radiator. If no water flows, or very little, it may be so badly clogged that it will need to be taken to a shop to be removed and manually cleaned, or replaced. If water flows, and a bunch of crud is flushed out, that might solve your problem.
- The last thing to check is the water pump. You can follow the link above on how to do that.
Good luck solving your problem.