Dyno89

I have a cel on with the p0455 code, it's something with the evap system they said but they aren't sure of an exact fix.
I read that it could be anything from the leak detection pump to bad fuel pressure regulator above the gas tank or simply a bad gas cap.
Before I start throwing money at parts I was hoping to get some input.
I do not notice a fuel smell anywhere outside or inside the cabin but I have noticed a popping or clicking sound from the black canister in the engine bay near the passenger firewall which I think is part of the evap system.
One odd thing I have noticed is when the jeep sits and I go to start it there's full voltage / power but when I turn the ignition sometimes randomly there is nothing, it won't crank or turn over kind of like when the battery is dead but like I said I'm getting full voltage. Then I'll take the key out put it back in and try again and it starts up. Other times it will start 5-10 times in a row without this issue. It seems to be random.

I need to resolve this and clear the cel for inspection and emissions asap so any help is appreciated.

Radi

Automatic transmission? Full power but no crank is often the neutral safety switch (NSS).
Jiggling the shifter around sometimes gets it to start. You can pull and clean the switch.

The ticking is normal, that's the purge solenoid.

P0455 is a large evap leak and you are correct, there are quite a few options. Got a locking gas cap by chance? (they leak)
The system assumes there is a leak when the LDP cant' pull a small vacuum. First thing to check is the LDP itself (use a vacuum gauge) and associated tubing. From there you can put a hand vacuum pump on each section of line, the charcoal canister, etc to see if it holds. If not, start pinching lines off until it does.

Dyno89

Yes, 99 4.0L automatic. No locking gas cap, as far as I can tell its the original gas cap, its definitely an OEM Mopar one with the plastic strap (I can tell bc I looked up replacements online already). My gas cap feels tight and clicks when I tighten it down and I always click it at least 3-4 clicks after getting gas. I guess I could replace it and see if it makes a difference but it looks good to me and the rubber o ring looks soft and flexible still and not dry rotted or cracked in any way. Could it still be a bad cap?

If I get a vacuum guage tester what exactly do I want to do to check this and what should it read on the guage if good or bad?

Dyno89

Ive been reading threads related to this code all night and I noticed one of the lines coming out of the top of the gas tank (The metal one that goes to the EVAP canister) is extremely rusted and corroded. It looks like I can just pull it apart with my fingers if I tried. I read a couple posts that this was the source of a leak so I am going to try to replace this rusted metal line with a fuel hose line and see if that makes a difference before I get into more technical things to check.