I am trying to diagnose a hunting idle on my 1991 Honda Civic. I have bled the coolant system thoroughly. Going to check for vaccum hose leaks next. However, I was thinking the next likely suspect would be the Idle Air Control Valve… I took it off and cleaned it with carb cleaner. I noticed that when I unplug the IAC’s power plug the idle of the car slows a little bit, but the vehicle doesn’t die.
If the IAC was functioning properly would one be able to expect it to close off all air flow and the engine, resulting in the engine dying when IAC is disconnected?
The IAC is a valve that acts as a low rpm throttle and the passages into and out of the valve must be free flowing as well as the throttle in order for the IAC to properly control idle speed. The throttle is set to allow enough air to pass so that the engine will run in neutral under ideal conditions and the IAC adds to that flow enough air to bring the idle up to the programmed speed. Everything in the throttle body must be clean in order for everything to operate properly.
We can’t see what needs to be done. All we can do is inform you as to where to start in correcting the problem. You may have a bad computer. But why start by replacing that expensive part when much simpler maintenance will likely clear up the problem? If not then work up to the big bucks.
I don t know the answer to your question, but I d clean inside anyway, check for vacuum leaks check the air filter and clean the throttle body and see if that helps first. and check pcv system
An engine can run with the IAC disconnected if everything else is right.Sometimes if a base idle speed adjustment is needed the IAC is unplugged, the base idle set with the throttle plate screw, and the IAC then reconnected.
Are you certain you bled the air out of the cooling system and that you dont have a blown head gasket that is pumping air into the cooling system? Did you use the bleed nipple on the thermostat housing? That particular Honda is VERY SENSITIVE to air in the system…as the air pocket will be right where the IAC resides and will cause the idle to Wildly Hunt…it will rev up n down up n down…all day long…until the system is purged.
If the system is purged properly…then the next thing to target is the IAC itself…just as you are doing now.
If it is of any relevance, I replaced the radiator and hoses recently. After replacing the radiator and hoses, I goofed and improperly filled the car with coolant the first time. There was definitely some air in the system… I drove it for two shorter drives ~40mi each with absolutely no problems. Right before getting home, the car started surging at idle. I realized that I had forgotten to bleed the air and open the heater valve, so then I bled the system properly.
I turned the heater all the way up with fan off, filled the radiator with bleed valve open until coolant ran free of bubbles… topped off. Started the car and ran until the upper radiator hose warmed up and I knew fluid was circulating, and then topped off again. Ran the car with radiator cap off for about 5 minutes and squeezed the upper radiator hose to expel bubbles until I could no longer see new bubbles surfacing.
Is it possible that driving with air in the system for 100 miles or so could cause the IAC to overcompensate even after I have bled air out of the system? Do I need to reset the ECU somehow? I will run the car at rough idle for awhile and see if more air gets into the system to rule out head gasket.
If it is of any significance, when I first start the car it will barely fire and idle. I have to give it a little gas and then once I get it above 2k rpm for about 5 seconds it will turn to a “stable” hunting idle 200 rpm - up to 1200 - down to 200 - etc.
I will spray carb. cleaner to look for vaccum leaks today and clean the EGR valve. I will attempt to clean inside where the IAC valve attaches. If this doesn’t fix it I am going to be tempted to take it to a mechanic for a professional flush of the coolant system and diagnostics… but since the car is so old and little value it may be time to think about replacing her… I just hate to give up on such a great car… just turned over 300k.
I checked for vacuum hose leaks, nothing. PCV valve is irrelevant as the car has cold air intake on it, crank case fumes are redirected into the air filter assembly directly.
The only real clue I got was that there is a hose sucking air off of the right side of the air cleaner assembly. When I plug that with my finger the car idles much better. It would appear the hose connects to the intake manifold directly below the ICV valve. What this means I have no idea.
I feel like with this issue there are just so many things it could be I am going to take it to a mechanic and have it looked at. Thanks a lot for trying to help guys.