P303 Misfire

So the car overheated a few weeks ago. It dumped the coolant from the lower Rad hose and I had it towed home. I put the hose back on, filled it with coolant, and drove for two more weeks, about 1,000 miles. The other day it started misfiring on the way home under pretty normal driving conditions. It threw a p301 and p303. I took the plugs out, cleaned em up a little as they didn’t look too bad, put em back in and now only p303. The coil on cylinder 3 had some rust and separating layers. I replaced with an oem coil. Still misfiring. It’s worst at idle, and if driven runs almost perfectly fine above 2700 rpm. Any thoughts?

possibly warped head and valves wont seal. I would suggest doing a cylinder leakage test. From my experience misfire at idle/low load is usually a compression issue. I've seen a couple cars overheat so bad that the valve seats actually fell out of the head. but since your misfire goes away at higher rpm I doubt that's the case for you and a resurface and valve job should cure your problem if what im thinking is true

Ugghhh. That’s what I was afraid of. Now to add to the predicament, I found a complete engine an hour and a half away for 2600, with less than 100k. Do I rebuild this thing with almost 250k on it, or just swap in a used one

I would fix what you have so you know what you got vs rolling the dice on a pretty expensive engine. I had exhaust valves fail on mine with 134k on it, if that helps at all

Well I had a head gasket go at 60k and a rod bearing at 90k so you never know what you’re gonna get in regards to mileage. That said 250k is a good run and I’d do an extensive refresh if you’re committed to keeping it.

Unless you can drive the car as is until the other engine is ready to drop in I would either get a new short block or rebuild what you have.

I bought a Nissan Altima from my dad who just upgraded to a new truck. It’s a good daily for now. I’m going to rebuild the lgt in the spring.