Flite

Buddy at work has a son with a high mileage 98 Civic. He recently put exhaust on the car with a test pipe. Shortly after doing this, the car developed a bad idle. He put a new Honda OEM IACV in it and it seemed to go away. After about 40 miles, it got the bad idle again. ECU throws 3 codes, 2 for idle air control and one for secondary O2. If the codes are reset, it idles fine again. However, within about 40 miles the bad idle returns with the CEL/codes.

From what he describes, it isn't a vacuum leak. The idle "surge" is much quicker than what I remember vacuum leaks causing. It does have some sort of aftermarket cold air intake.

MAP?
Bad IACV?
Vacuum leak?

Dough_Boy.

I don't know to much about honda's but alot that come in to my shop are low coolant. I doubt that is it but worth a check.

Laserjock

Normal indicators when the ECU is reset. It may take as little as 5 minutes for the codes to re-appear. This is OBDII so if the test pipe is causing the 2nd O2 sensor to show a code, fix the probable cause of the problem 1st. There are plenty of high flowing cats out there. No need to screw up the air quality for a 1-5 HP gain by running no cat

Fabrik8

Don't worry about the idle codes until the O2 sensor is fixed.. The idle codes are probably related to the O2 sensor malfunction. Why does he need a test pipe on a high mileage 98 Civic?

dirty c

exactly needs both oxygen sensors working.

99blackcivicsi

the rear o2 won't make the car idle funky, that o2 is only a tattle tale on the catalyst thats all its function is nothing more, thats why the code came to light when the test pipe was installed since there is no cat. the front o2 tells the pcm to go richer or leaner quicky, but won't effect the idle, the sensors to look for would be map and coolant temp sensor for idle related issues on cold start since that is the pecking order for inputs on the ecm for how much fuel to give the motor off its predetermined parameters. if he put a new honda iacv on it and didn't clean out the intake mani before installing it theres another issue, i don't know what model it is so i can't tell you what to look for wiring wise if its a 2 or 3 wire IACV (96-00 has both kinds). make sure he didn't swap plugs on something and make sure its plugged in correcty and the pins are in good shape and he didn't break a wire when he unplugged the IACV to install the new one or rip it when he was on the bottom getting the exhaust off wierder things have happened.

sorry thats one big sentence

Flite

As it reads below, secondary O2 shouldn't affect idle. I don't know why he needs a test pipe... I know it's stupid... I'm just the messenger here.

Hall Honda put the new IACV on the car... from what he said, they didn't sound very savvy though. Why would putting a new IACV without cleaning the throttle body/intake manifold out cause an erradic idle? It idled fine with the old stuff on there and I'm kind of thinking the exhaust is unrelated to the idle.

Is there a way to check the collant sensor without just putting a new one on the car?

Flite

Ok, talked to him today again...

It seems the exhaust was on the car for a couple months before the idle issue started. When the idle issue started, he brought the car to a Honda dealer and they replaced the IACV because of the codes the ECU was throwing. He left the dealer and everything was good for about 40-50 miles. Then it started acting up again.... same problem, same codes. He reset the ECU and the car idled fine again. After another 40-50 miles, the iddle issue returned. It is NOT a cold idle thing. He has checked the coolant and bled the coolant system to make sure there were no bubbles. Once the car gets 40-50 miles (or just a shit load of time at idle), the codes come back and the idle goes to shit.

Dough_Boy.

To check the ect switch let the car warm up and use a meter. There should be .2k ohms at 212 degrees. That is straight from a program we use at my shop for procedures. It also says there is a engine load sensor and the description kind of sounds the same. Then there is also a crankshaft fluctuation sensor.

crxmadness

Tell him the clean the engine ground wires and the battery ground wire. If these wires are not making good contact, then it will sometimes cause idle surges. Im sure there will be people who will call bs on this, but I removed my ground wire to test it out on my civic years ago and it caused a my car to not shift smoothly and the idle started surging.

Also check all fuses to make sure nothing is blown

O and a spark plug non fouler from your local auto part store will take care of his secondary O2 sensor check engine light