Looping Idle? whats wrong? 91 EF
 

Please help. I have a 91 EF hatch DX with a 91 Si MPFI/wiring harness/computer. The car runs good but when it warms up it starts to loop idle from around 1500rpm to 1000rpm, I am pretty sure there are no vaccumm leaks and everything seems to be working alright. The only thing i can think of is that i didn't change the fuel pump to the Si pump. would that make a difference? any help is greatly appreciated.

try unplugging the IACV (or EACV) and see if the idle smooths out. If it does i would suspect it to be the culprit.

could be iac vavle, id try intake manifold gasket, also make sure your coolant is full

When I did my multipoint swap I had the very same problem, it was very easy to fix, just a quick tweak of the idle screw. Try it, it might work for you, though I would check the IACV first as they do fail quite a bit.

That was happening to my dad's car because he left it for a couple days with almost an empty tank of gas and his mechanic told him that condensation must have got into the tank. Cause you know how in the morning sometimes there is water on the outside of your car? Supposedly that happens in the gas tank as well :shrugs: and is more of a problem when the tank is empty.
He put in a fresh tank of gas and a fuel line additive (gas-line anit-freeze) and after driving for about 20mins. it went back to normal.
But my dad's car isn't a Civic so it might not help you out, but worth a try if the things other people posted don't work for you.