1990 Accord with surging idle

First look at the coolant level in the vehicle. Then look at purging air from the system by cracking open a bleeder. If this does not rectify the problem you might try taking off the IAC valve and clean it, put it back on and then repurge the air from the system.

EGR valve can cause this as well.

I don’t understand how low coolant or air in the coolant would cause a surging idle, but the coolant level is fine and the coolant recovery system is at the proper level. Does the 1990 Accord have an IAC?

I’ll check the EGR value this evening.

More detail: The car idle goes from 800 to 2000 rpm as if someone was stomping on the gas and quickly pulling their foot back off. As the car warms up, it will “only” surge to about 1600 rpm, then 1400 rpm and slowly settle down at about 900rpm. This process take about five minutes, remember I live in Texas, so it never gets real cold here.

You need to purge the air from the system. Air bubbles can sit at a sensor, IAC or temp sensor, and give the computer a false reading. The computer tries to dump in more gas to compensate if it thinks the coolant temp is cold but then the vehicle will surge up and the computer tries to correct again and again and again.

Yes there is an IAC vavle. It is toward the back of the engine. It has two small coolant lines that let coolant go through the valve.