why is my car idling after i got the engine changed?

[milaan]

After getting my entire engine changed the car started idling at 1,800 rpm and it did not accelerate and even the check engine light is on. the mechanic didnt know the reason

please advise

[taakir]

my doubt will be that the ECU may not be compatible. check with the guy who has installed the engine in your car.

[Ozimandius]

It would help if you could tell us the year/model of your Miata. I had this problem and it was fixed by adjusting the idle valve, which is fairly easy. I get the CEL once in a while though.