mark96
Member
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
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