Kia Troubleshooting Questions? Ask a Mechanic
There is no oil pressure relief valve in the vehicle. What is probably happening is the leak is on the lower part of the engine like the oil pan area or the crankshaft seal and when the vehicle is running the oil is getting sucked up by the oil pump into the rest of the engine, then when the vehicle is off the oil is all sitting in the lower part of the engine so it leaks. You should not add oil while the engine is running. Check the oil level with the vehicle off and add oil until it is between the lower and upper level indicators on the dip stick. The hissing sound is the crankcase ventilation system which is open with the oil cap off. Replacing the PCV valve will help the leak but not fix it, if you cannot see an obvious leak from the oil pan at the bottom of the engine I would bet that the crankshaft seal is leaking. You can confirm this by looking at the lower timing cover and seeing if oil is coming from there. If it is, you'll have to remove the covers and timing belt to get access to the seal, its a large job and fairly difficult on this engine. I have attached a picture to help.
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