Rep Power: 0

I have a 2001 Honda civic only starts when the engine is cold. When it does start it idles fine at first, but as soon as I press the gas pedal it starts surging from 1500-2500 rpm.
Where would be the best place to start diagnostics? My mechanic had the car in his lot for a while but never got around to fixing it. He said he looked at it once and cleaned out the throttle body with no luck. Also the engine temp gauge stopped working a few days before I started having issues.
Are we dealing with two separate problems here or is the no warm start related?

 
Rep Power: 224

Check/clean/replace the IACV.

 
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I think that is what he did already.

 
Rep Power: 224

You only mentioned the throttle body. If the mechanic replaced the IACV, it needs to be a genuine part.

 
Rep Power: 222

+1 for genuine parts