Hope I'm in the right section here.

First some background info. I have a '98 Civic with a B18B1 motor, this is not a engine swap, they came this way new from the factory (local derivative). Two weeks ago the car went in to have rust removed on the lower body between the front wheel arches and where the subframe assembly mounts to the body, cam belt, discs and pads were also replaced at the same time.

Car came back with a really rough/lumpy idle and it also idles high, 1500-200rpm. Driving below 3000rpm there is some jerkiness. When warmed up idle will drop to about 400-500rpm or it would just cut out completely and die.

Are there any sensors wires/connectors they could have disturbed underneath the car that would lead to this? I know they had to pull the interior carpets back in the front and I also know the ecu is located in the left side in the front by the footwell and on the right hand side there are some wires & connectors as well. I removed the panelling in those locations and the ecu, wire& connectors look fine. Looks like it was moved out of place before any work was done.

Their diagnostic system reported a idle actuator issue (dunno if this is the same as the IAC valve) but they did not buy this as it was running fine before.

Because the car was standing for such a long time without the battery connected they suggested I run it for a while, replace the fuel filter & add a bottle of valve cleaner to the the fuel tank and see how it goes. I've replaced the fuel filter and added the valve cleaner yesterday but so far no difference.

Any ideas on how to diagnose this, where to start looking and tests I perform myself?

Any help would be greatly appreciated.