I have an 01 hyundai accent gl 4 door 1.6 dohc. timing belt broke at 106,000 miles and bent some valves. got an engine out of an 03 accent 1.6 same everything with 35,000 miles. pulled old and put in 03 engine. ran great for 100 mi. but had check engine light on. obd2 scanner said o2 sensors bad. we checked it out and it was bent. we also noticed a ground wire by the coil packs was not connected. hooked it up and cleared the computer and went on my way. made it a mile down the road and the engine light came on again only this time the car was running very rough so i went back and hooked up the scanner. this time the scanner said missfire on cylinders 1 and 2. after we scanned it we cleared the computer again, unhooked the ground, and started it and it ran fine again. i took the car around the block 5 times and no problems so i drove it 20 miles home with no problems. started the car 2 hours later and it was fine so i started down the road. made it a mile before it started to run rough again so i headed home. i talked to a mechanic and he said to change the plugs and wires from the old engine so i did and no change still missfire. he said try the coil packs and i did and still no change. i then changed the o2 sensor i had from the old engine and still missfire. so he came over. we decided to try the injectors changing the entire fuel rail and still ran rough. i then talked to a honda specialist that i know. took the car to him. he hooked it to his 10,000 dollar snap on computer and got the same thing, missfire on cylinders 1 and 2. as he ran his tests and wiggled the plug wires and messed with things it began to run smooth. he said i did nothing to it but its running well so he cleared the computer and i went on my way. i drove it about 8 miles and it went back to the missfire. limped it home 4 miles and by this time it was dark so i looked under the hood and saw a little bit of an arc at the coil. asked the honda specialist his opinion and he said plugs, wires, or computer. so i put new plugs and no change, wires and no change, and used computer and no change. he said last thing i can try is throttle position sensor. changed it and no change. called him and he said bring it back over so he can check the fuel pressure.he hooked it to his scanner and within 10 minutes it had gone back to running smooth again. he said he was baffled but checked the fuel pressure anyways. it was at 50 and it calls for 49.8 but by this time it was running smooth but as soon as you touch the gas the idle goes crazy idling up and down for a couple seconds and then levels out. so he said i guess drive it home and e will talk to someone who knows hyundais and get back to me.