Why a 1997 honda civic engine surges at idle - 1997 Toyota Tacoma

There are two coolant lines that connect to this Honda IAC to help quiet the operation of the valve. The part number for a OBDI 1992-1995 Honda Civic Idle Air Controller is 36450-P28-A01. While the OBDII version of this valve is 36450-P2J-J01, and the two units are very similar in design and execution. The EACV or Electronic Air Control Valve is a big part of maintaining your Honda Civic idle. If you've checked your IAC and your Honda still suffers from a surging idle, check your FITV. This Honda Idle Air Controller or IAC is under the throttle body and incorporates into the throttle body passages o locate your Fast Idle Thermo Valve, remove the intake snorkel and locate the FITV beneath your throttle body opening. If your SOHC engine is still having problems, check our guides above on how to check your FITV as well as your IAC. To check this operation, remove your air intake pipe and use your fingers to plug the passages in the lower left hand corner of your throttle body opening. If your idle surge does not go away, unplug your Idle Air Control valve and if that stops your idle surge, your IACV must be replaced. Follow our instructions on how to replace your Honda Civic Idle Air Control Valve. Need to raise your Honda Civic idle or having problems maintaining idle? You can adjust the base idle by unscrewing or screwing in the Idle screw on your throttle body. This screw allows air to bleed into the passages on the backside of your throttle body, as shown above. To learn how to adjust Honda Civic idle speed, you will not need to remove anything. With a standard screwdriver, simply locate your throttle body and Idle screw.

Idle air control. Its what meters the air at idle. They just get worn out and can't find where they want to be for the specific rpm the computer is looking for. Replace it.

hot engine or cold, or both???????????????????
surging or hunting idle is complex but for sure the ECU has lost control of idle speed , there are 2 macro reasons, (vast causes)
1: ISC is stuck (in non SAE terms, the idle speed control valve ) is stuck.
2: engine is missing cause the ISC to GO NUTS trying to control
idle and can not.. the 50 reasons, , have list ask.
we do tests to find out if either or both the above are bad.
do you want tests, or guessing>???

sounds like the idle air control valve. does the check engine lite come on? if so have it read and repaired.

The surging is related to your Idle Air Control Valve (IACV), it adjusts idle based on coolant temperature, your coolant could be low or your IACV could be bad.

Possibly a Throttle position sensor. If it surges in park and it won't idle correctly this could be a solution.

check the iac valve

Consider replacing the rotary idle air control valve, and clean your throttle body.

Check and make sure your coolant level is up. If it's up, your idle air control valve is faulty or your cold start solenoid isn't working anymore. Both are activated by the engine water temp.

if the check engine light is on it probably the egr valve but if the light is not on the idle control solinoid is dirty or broken