AZ2000XJ

Hey all, got a funky one.

Bit of background on my XJ, it's a beautiful clean 00 freedom edition, completely stock. Bought it with a cracked head for cheap. Looks like a previous owner had swapped the engine with an engine that had sat for a while and accumulated rust in the block. Seems they didn't bother to clean the block of rust which caused all kinds of cooling issues that they chased down (almost the entire cooling system was replaced). At some point they must have overheated it and cracked the head. It wasn't that bad of a crack at all, it was on the exhaust outlet so not much coolant made it into the oil at all. But it did cause misfires and a poorly running engine.

I rebuilt the engine myself with a rebuilt head and new hardware. Threw in some extras as well: ignition coil pack, fuel rail and injector heat wrap (hot AZ summers), new 4 port injectors (note, these could be suspect as they were cheap...), new gaskets all around, new thermostat and thermostat housing (the only old cooling components on the jeep), thermocure to remove the rust, a new rad fan to replace the one that was falling apart, and new fluids all around.

It ran great for a while, has great compression in each cylinder at or slightly above spec, then I started having random stalling at startup issues about 1.5 years back. I changed the Idle air control valve and that seemed to do the trick. Again, used another cheap part so this could also be a failure point to consider.

Fast forward to now and I've been having some random issues in this AZ heat. First it started with odd behavior during/after throttle. I'll feather the gas pedal and then let off but the engine seems to rev further as if it's getting more fuel and speeding up on its own. It comes back down to idle when I give it some brakes or after about 5-10 seconds with my foot off the gas. But there's a distinct and concerning delay where the engine just speeds up on its own after I've let off. And then the starting issues started. Seems that at cold start it'll fire up just fine, but after driving it up to temp, then letting it sit for a bit (an hour or so) it'll have issues starting at all. I can hear the fuel pump prime at key turn, or at least I hear it prime for like 1 second before the noise stops. First time it happened we were at dinner and were going to leave, it cranked for a while, no start, tried it 3 more times and it eventually caught and ran perfectly fine once it did catch. Second time was just 2 days later, I went grocery shopping, came back about an hour later and it wouldn't start at all. Cranked multiple times and not even an attempt to catch, as if spark or fuel was non existent. Fortunately, someone who happened to have just bought a can of starting fluid came up and lent a hand, popped the airbox cover off and sprayed a few shots towards the throttle body and it started up on the 3rd try. Ran perfectly fine once it started up.

So, any ideas of what this could be? I'm thinking crankshaft position sensor, but it never stalls while driving. Just has that bizarre throttle surge after letting off and doesn't start when hot. Seems to be fuel related considering the starting fluid is what got it to catch. It could be the cheap reman 4 port injectors are failing (maybe they're overspraying fuel or staying open, causing the surge?) or the idle air control valve I replaced is bad?

Would love any and all input to point me in the right direction, thanks