Idling problems

i just changed the engine in my 96 Honda civic, 1.6 base model. the change over went well and had no problems. when i have the car idle and it gets up to temp, the RPM’s go from 2000 down to 500 then jumps to 2000 then down 500 again. when i put it into gear, drive or reverse, it stops jumping. what did i do wrong?

Make sure it has the right thermostat.

Well, the new (used) engine has the thermostat that it came with. from what i can tell it hadn’t been changed. if its bad, is this one of the signs of being bad?

i was told that because of the change in engines, the computer in my car may remember the idle censer from the old engine and that i should take and switch the two. dose that sound right?

What you were told is the zero or idle position on the throttle position sensor (TPS). This tell the computer that your foot is off the gas and it is supposed to control the idle. from you description, I don’t think that is the issue here.

What may be the problem is the idle air control (IAC) motor. Instead of swapping that, the best bet (and easiest) probably would be to swap the whole throttle body from the old engine to the new. Get a new gasket for the throttle body, they don’t cost much.

When you changed the engine, the battery should have been disconnected. For that reason, the computer does not remember anything from the old engine. Once the battery is disconnected, it takes about ten minutes for the computer to learn the new engine, or relearn the old engine as the case may be, so if you don’t have 10 minutes of idle time yet, be patient.