Hello ive been Working on Honda's for many years and this one has me stumped need help. Have a 01 civic ex with d17. Originally bought it had over heating problem so ended up changing motor. The car at called start will rev about 1300 RPMs then drop to under 500 RPMs 3 times. Drives normal but every so often it will have loss of power or stall out. I've cleaned or changed iacv, map, and throttle body. Also currently not throwing in codes Cel. Please HELP!!!

 

How many miles are on the drivetrain? Have you changed the exhaust components? You may have a clogged catalytic converter.

 

The New motor has less than 30k. No exhaust components.have been changed

 

I would check fuel pressure. while it's acting up and report back. It should be around 45 PSI and it shouldn't drop much on a snap throttle. The fuel pump on my 05 Civic went bad and caused an intermittent loss of power, stalling and rough idle problems. You can also try removing the upstream o2 sensor and see if it runs better. I have seen o2 sensors intermittently freak out and cats melt or clog and cause problems like this. Good luck