What can be wrong if you swapped a 4L engine from my 1994 jeep Sahara with a 4L 1990 jeep larado its seems to be working fine except its idling at 2 grand and no codes are linking to it?

Check throttle linkage is not too tight: Throttle linkage usually is 'slightly' slack to the touch. Check the idle screw (if any): Unless it has a carb or throttle body this (probably) wont apply (computer control) Check for vacuum leaks: With a can of spray lubricant, use the small tube provided to spray a quick small burst on vacuum connection with the engine running, listen for subtle changes in the engine RPM. If RPM changes, you probably have a leak there. Check all vacuum connections in this way. Replace hoses as needed. Check ignition timing: Consult the service manual for procedure. Make sure (all) temp sensors are working properly: If the computer thinks the engine is cooler than it is, it will idle higher. Consult the service manual for procedure. With engine fully warm no throttle solenoids (if any) should be engaged on the throttle linkage. I dont know how similar or compatible these engines are to each other, so, some other thoughts I had were mostly computer related: Data from coil or distributor not being reported properly to computer. Or sensor and computer incompatible. (timing) Data from temperature, or vehicle emission sensor not being reported to computer properly. Or sensor and computer incompatible.(temp, timing or mixture) Data from automatic transmission incorrect (mechanical, vacuum, or electronic sensor) make sure automatic transmission fluid is at proper level. There may also be a PCM (powertrain control module), erroneous data from or to the PCM could cause strange behavior. once again, how compatible are the engines? While your at it make sure coolant is full AND it is actually circulating when engine is warm. there is a idle speed motor somewhere on the throttle body they might have a different step counter that the pcm uses try swapping the origanal to the replaced t.b. also might want to swap throttle possition sensor. sounds like a vacuum leak condition so check for leak. map sensor may be no good as well. lots of possible causes.