I am working on a 2000 Tiburon, it has 256,000 miles on it. It came in with misfire codes and we found that the motor was shot. Replaced the motor with a used 2000 Elantra motor of the same spec with about 17,000 miles on it from a wrecking yard. We did compression testing and got about 215 across the board. It all went together fine and fired right up. It came back a couple times with some starting issues and we found that the plugs had a lot of carbon build up on them for only being used for a couple hundred miles. We went out to try and start it, and it started to turn over and then it would stop, sounding like it had detonated. It had a few times where it would start and get to about 300 rpm and then would die. We finally got it to start and then it ran fine. We had to replace the starter due to it being burnt out. After doing that we thought that would fix it, turns out it didn't. Long story short it keeps coming back with the issue and still having it. We have replaced the ECU, Intake Air Temp sensor, Coil Packs, Wires, Plugs, Battery, Fuel Pump, Had injectors cleaned and flow tested came back fine, Also found an aftermarket alarm/remote start wiring and pulled it all out (was a T Harness so we know the wires are all correct now) We are still having an issue with it starting. It seems to happen after it has sat all night in the cold, once you get it started in the morning, any where from 1-15 mins to do so it will run and start fine all day with no codes ever, it only seems to act up after it has sat all night. Has anyone had anything similar with this? Any ideas would be great, an entire Hyundai dealer is out of ideas.