I have an issue with my 2003 Hyundai Tiburon 2.7L I own. This problem has stumped 2 Hyundai dealers, 3 mechanic shops, and a back yard mechanic already. Im ready to rip my hair out because of this car.
This all started when my Compustar command start shorted out and the sensor wire that ran to fuel injector 6 to let the command start know when the car was running, It grounded out and over fueled cylinder 6, when the engine was started the next day it was full of raw fuel and put a rod through the block. I got a used Tiburon 05 engine from a car recycler and installed it, the only changes I had to make was the intake manifold and one fuel rail because the 03 had a fuel return line and the 05 did not. At this point the car would start for a split second and die. I tried many things from checking the timing, replacing the spark plugs and go as far as swap all the engine sensors from my 03 engine to the 05 engine. At one point it would idle but was randomly misfiring one different cylinder every few seconds. Also at one point I was able to get it to idle normally for one hour. When I tried to give it gas it stalled out instantly. I was able to get it to idle a few times but every time I touched the gas it stalled out.

With no luck I took it to a Hyundai dealer, they tried a few things, they also contacted Hyundai Canada for help and they said there’s a good chance the 05 would not work because it’s a different engine, even tho the dealer was told this, they were convinced that it was the ECU/ECM, the recycler that I got the engine wanted to try to get the car running themselves before they would consider getting me a different engine, the reason I asked them for a different engine was because I talked to the parts guy from Hyundai and we figured out that the 05 2.7Ls were different part numbers, but I would be able to match a 01 2.7 Tiburon engine to a 04 Tiburon engine. The recycler called the dealer and the dealer told them it was the ECU/ECM, and they told me that and would not help till I replaced it. Well $1,300 later and a new ECU/ECM which I cannot return, the car did the exact same thing. Finally the recycler got me a 03 Tiburon 2.7L engine. I installed this engine and guess what same thing. So now I know it’s something with the car itself, Frustrating. The BCM was also replaced. The other dealer and mechanics could not figure any thing out, and that cost me well over another $1000 in labor with no results.
One day I was playing around with sensors and found that if I unplugged the IAC (Idle Air Control Valve) the car would start first time and idle almost normal, the when I plugged it in it would Idle completely normal. I tried to give it gas and it would die. I tried this a few times, even with the IAC plugged in and unplugged, same thing. So I’m out of ideas, the car gives no codes when it idles or stalls. I also completely removed the command start and wiring. Please any ideas would be great

As well fuel pressure was checked by hyundai and its good, MAF sensor and o2 sensors have been checked.

the only other thing that i did that i thought was weird is i bypassed the EVAP Canister Purge Solenoid Valve with a vacuum line and the car reved up to 3000 RPM and sat there until i disconnected the vacum line