Help! Surging hot idle stabilizes when front O2 sensors are disconnected

[mark96]

I'm troubleshooting my 1996 with surging idle in closed loop operation. It's an LT1 with 50k miles. The idle is smooth and normal when cold and in open loop mode. When the engine is hot, the idle will run at about 600 rpm and surge intermittently to about 900 or 1000 rpm and then come back down. During driving, the engine runs smooth and strong with no missing or hesitation, even at WOT. No codes are present.

Things I've already checked or tried:

IAC Valve - replaced.
EGR Valve - free movement with vacuum changes.
TPS - stable 0.61 VDC at idle, increases smoothly to 5 VDC at WOT.
ECT Sensor - disconnected wiring from sensor and connected 200 ohm test resistor across the wiring to simulate steady 200 deg F (approximately).
O2 Sensors - connected DMM to outputs and saw voltage varying up down.
Vacuum lines - checked and re-checked and re-checked again.
Plug wires - sprayed water on wires in dark garage and saw no arcing. (I thought maybe I had a wire breaking down in the heat.)
MAF Sensor and throttle body have been cleaned with MAF cleaner.
Checked for possible broken wires within insulation at all connectors.
All the wiring ground connections I could find have been cleaned and wire brushed.

Since the front O2 sensors provide the feedback to the computer in closed loop mode, I was still suspicious of them, even after seeing their output voltages vary. Today I disconnected both front O2 sensors and let the engine heat up through several cooling fan cycles. I got the expected Service Engine light, but the engine idled fine with no surging whatsoever. I then shut the engine off, reconnected the O2 sensors, and started it back up. The surging idle returned immediately. At this point, I think I need to replace the front O2 sensors. A lot of guys will change them at 50k 'just 'cause' anyway. What do you think?

Mark

 

[Hib Halverson]

I think you need the FSM and a scan tester to find out why the idle is unstable.

It's unlikely both O2s would fail at the same time.

 

[mark96]

Have a FSM. Doing the best I can without a scan tester. I agree it's probably not two bad sensors.

 

[Hib Halverson]

Since you've read the sensor outputs with a DMM and seen the voltages rapidly vary, let's assume for now, the sensors are ok.

To solve the problem you may need to view the fuel trim data in the serial data stream.

You mentioned the EGR valve...are you sure it's not leaking at idle?

Also, I'd go over the engine again for vacuum leaks and I'd check for exhaust leaks near the front sensors.

Beyond that, I'd study the FSM's discussions of unstable idle.

 

[mark96]

Thanks. I think I'm going to give up and take it to a local repair shop.

 

[boomdriver]

Thanks. I think I'm going to give up and take it to a local repair shop.

Where's the fun in doing that ?

There are limited ways this symptom can appear...
EGR leak
vac leaks/ misc air leaks
Poorly seated IAC / dirty seat
Throttle body leak/shaft worn loose
bad injectors
damaged wire harness, dirty connections, loose or dirty harness grounds. (G-104 especially)

 

[Hib Halverson]

Thanks. I think I'm going to give up and take it to a local repair shop.

I see you're in IL. If you're near Chicago, try Haibeck Automotive Technology.
ZR-1 Specialist

He does a lot of LT5 stuff but he's good with diagnostics on C4s in general.