well, if im sitting at a stop light, it will be idling high, around 1200rpm, (i dunno why) and all of a sudden it just drops to like 700 and only revs up to 3500. I have no clue why. I did this swap in my 90 Civic hatch and it just started doing it a while ago. It ran fine for about a month.

 

common guys!

 

its most likely your idle motor if it does it at every light

 

check for:

-radiator leaks
-idle (throttle cable)
-fuel lines

that's all i can think of for now

 

Do you have any CEL'S?

If you do pull them

 

you might want to check vacuum lines

 

u might have a bad ecu cuz when my ecu was going it would put my car in safe mode and it would idle at 2k and my rev limit at 5k. it sucked very bad. i would also check ur fast idle control or ur idle control valve. are u throwing any codes?

 

do you have cel on?? check iac and vacuum

 

What is your fuel psi? check that if it is low it could cut out around 3500.... But if it is always exactly 3500 than most likely it is not