I have a B20 swap in my 1997 civic and I've been having a horrible idle problem ever since the shop put it in and I cant figure it out. Its a constant surge like a vacuum leak, specially when its cold but once it gets hot it calms down but still surges a bit. It smells strongly like gas when I turn it on until it gets hot. I checked the timing, the intake manifold gasket, the PVC valve, I checked the MAP, The IAC valve, the ECU, I checked for vacuum leaks around the intake manifold but couldnt find any....Im out of ideas...anyone has any?

 

when you say it smells like gas, that reminds me of that coffee can, as we use to call it.
the canister - that is what it is.
sounds like a problem with that - but not real sure.

with that swap though, hard to tell.
take a spray bottle of water and spray around the intake at the engine.
see if you can detect a leak - or did you already do that?

i would probably replace all the vacuum hoses - cost 2 to 3 dollars. i did that.
did not replace the 2 big ones - off of the brake booster or off the iacv (or eacv).
should be able to look at them at tell if they are bad - those small hoses though, you cannot do that - at least i could not.