Hi guys, 1985 Corvette, at warm idle, my car sometimes has an idle surge, and while poking around today with tuner pro, I noticed that when the car went into closed loop, it didn't seem to run as smooth as it did in open loop.

My AFR tends to try to stay around 14.7:1 in closed loop, but I notice that during my idle surging, the o2 sensor reading will sometimes be 800mV, 700mV, 600mV, 500mV, and even down to 300mV. I think I read somewhere that it should be at 600mV during warm idle?

This is a heated o2 sensor also that I installed.

Do any of you guys suspect anything? My idle surge only a difference of 100 RPMs. The car seems to idle around 750 RPMs.

It doesn't seem to surge in open loop as I've noticed.

Thanks for any help!

 

In closed loop in constantly crosses over from rich to lean, so it is normal.

 

But if I hold the throttle open, it stays at a steady like 800mV.

 

At wot it defaults to an open loop map. A narrowband o2 is not accurate at such rich mixtures.

 

Well not WOT but like maybe 10-15% throttle.

 

If it is not crossing over there is a fault. Could be too rich due to oversized or leaking injectors, but if it functions properly at idle it should function the same at part throttle.

 

But it can't be too rich because my AFR gauge shows it trying to float around 14.7:1

Let me ask you this, I have Hedman headers, which have a common problem, (as I've read), of leaking at the collector. Mine have a leak.

Would this cause an idle surge? My o2 sensor and Wideband sensor are on the mid pipe, right after the collector.

The car was showing a little lean when cold. Like 16-17:1 AFR.
Could it be that it is leaking air into the exhaust and giving the wideband a false reading? And when it warms up is it possible that the header is expanding into the collector and giving a less false reading?

Thank you for helping me.

 

Of course it would create a false reading.

 

First off let me say that I am an ASE master certified mechanic. I may not know a whole lot about the performance side of the auto industry but I know my fair share about the repair side. Trying to diagnose a problem without seeing the car is always a crap-shoot, so I will give you a few causes for your symptoms, but know that without seeing the car and having the right equipment do diagnose the issue, everyone is just guessing. So take advice with a grain of salt.

No car's ECM is going to force a rich to lean to rich air fuel ratio off idle. The only reason for the lean/rich switching is to make the catalytic converter more efficient to produce less emissions at idle. Off idle it is going to try its best to stay at the desired air/fuel ratio. The only thing that a narrow band O2 sensor is going to do off idle is tell the ECU one of three things, too rich, too lean, or just right. Then the ECU will react accordingly.

While the engine is below operating temp, the O2 sensor is pretty much ignored, however I have never seen an O2 sensor cause an idle surge. If the O2 is bad, you will either see the engine running too rich or too lean and there will be other symptoms showing up because of a fuel mix issue before an idle surge shows up because of it.

Your best bet is to look at the throttle body, I can almost assure you your issue is there, as it usually is with most idle surging issues. Either your throttle body is extremely dirty, your idle air control motor is bad, or your throttle position sensor is bad. Most likely either a dirty throttle body or a bad idle air control motor, if the throttle position sensor is bad, there are usually other symptoms.

The idle air control motor is a stepper motor with a plunger attached to it, which over time will burn out as it has to fight more and more carbon, so it will start sticking and then eventually overcoming the resistance of the carbon buildup and opening too far, causing a surging idle.