This is probably a fuel management issue. The computer is getting feedabck and trying to keep the Air Fuel ratio consis ten and there is a faulty component making it hard to do that.
The easises thing to do is get a OBD code reader and see way diagnostic codes are being thrown. The fact that the engine has been swapped should not pose much issue as long as the sensors are the same and the injectors are the same impedence and the computer has no major differences. Usually, as long as everything plugs in it is fine.
If the car has run fine since the swap, I think there is not an issue. My first thought is a faulty Idle Air Control Valve. It senses the load on the motor and keep the idle steady as load changes. Sometimes the equivelent of a choke is also part of this vavle and it senses coolant temp and kicks the idle up till operating temp is reached.
Other thigs are the O2 sensor and service items such as ignition components and vacum leaks.
Let me know a little more about the cars behaviour after the swap and right before the problem started as well as its service history, (O2 sensore ever changed, etc.)
I will look for your reply and get back ASAP.
Monday, November 22nd, 2010 AT 11:49 PM