I just bought a 98 accord coupe v6 with 248000kms on it, It idles bad, I had the codes read and it said I needed to change the IAC valve, so I did, and the car still is idling badly, I took it to my mechanic and he said that maybe my head gasket was bad he said it was common in these v6 models, and he said a way to tell is if you take the rad cap off that the coolant should be sucked down, and mine is pushing it out.
What should I do ?

Thanks in advance

 

P0505 just means something doesn't work with the idle-control system. It can't tell if the IACV itself is actually bad, or some other reason it can't hold a good idle.

Beyond reading the code, what diagnosis was done to determine the IACV was actually bad?

 

do a compression and leakage test,,,,,,pull the plugs out and see if you notice any wetness ( coolant, water ) ..
i would not let a mechanic work on my car that assume that something is bad,, they need to DIAGNOSE to properly identify the problem