I have a 1997 Honda Civic. I just Replaced the engine and trans. to another stock engine and trans. The problem I'm having is when I'm driving (usually under 40mph) The rpm's drop like the engine is braking and jumps back up. For example 2200 rpm down to 1000 and back up to 2200. Now since I just replaced the engine and trans I don't think its a problem there. I'm guessing it has to be something electronic. But since I'm not a car guy I have no idea what. Any advice would be much appreciated.

 

Is the transmission manual or automatic?
Did the "new" engine come with a distributor on it or did you re-use your old one?
Is the Check Engine light on?

 

Its an automatic. And its technically not new. I got it at a salvage yard. It had 80,000 miles on it. and I used the distributor from the newer motor. No the check engine light isn't on. I've put nearly 2000 miles on the motor since then with no lights or problems. Could it be a shift control sensor.

 

Engine losing power abruptly could be an ignition failure. Put the other distributor back on.

 

But the other engine and trans did this too