Rhythmic engine surge 1996 LT1
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Rhythmic engine surge 1996 LT1
Hi folks,
I'm trying to diagnose a rough off-idle which has developed into a strange engine surge that seems to be around 2000 to 2200 rpm. Appears to be only in closed loop, but I'm not 100% sure. I was thinking EGR, so I had the EGR hose off the valve and was holding the rpm around 2000, and the engine seems to misfire/run rough very slightly every 2 seconds or so, very rhythmically, and very repeatably. You can feel it in the car, especially in drive (3rd). Engine was warm from driving previously, but not sure I was totally in closed loop since no vacuum yet at the EGR valve. Could the O2 sensors cycling rich to lean and back again cause this?? My scanner shows the sensors fluctuating voltage low to high, and I have no trouble codes, so I'm assuming they are at least mostly functional (although 87k miles, don't know if original). Was chasing this last year and cleaned the EGR valve, but didn't impact it. EGR valve seems to work (hold valve open, engine runs rough).
As usual, any help on this would be greatly appreciated!!
Regards,
Rich
I'm trying to diagnose a rough off-idle which has developed into a strange engine surge that seems to be around 2000 to 2200 rpm. Appears to be only in closed loop, but I'm not 100% sure. I was thinking EGR, so I had the EGR hose off the valve and was holding the rpm around 2000, and the engine seems to misfire/run rough very slightly every 2 seconds or so, very rhythmically, and very repeatably. You can feel it in the car, especially in drive (3rd). Engine was warm from driving previously, but not sure I was totally in closed loop since no vacuum yet at the EGR valve. Could the O2 sensors cycling rich to lean and back again cause this?? My scanner shows the sensors fluctuating voltage low to high, and I have no trouble codes, so I'm assuming they are at least mostly functional (although 87k miles, don't know if original). Was chasing this last year and cleaned the EGR valve, but didn't impact it. EGR valve seems to work (hold valve open, engine runs rough).
As usual, any help on this would be greatly appreciated!!
Regards,
Rich
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Fuel pump and regulator checked previously, no issues. Also checked throttle position sensor, thinking maybe there was a dead spot which corresponded to the throttle opening at 2000 rpm in idle, and it was fine, no issues. Thanks!
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Update - ran car today; O2 voltages lean (.1xx) when first started in open loop at idle, then acted normally once in closed loop after that. Ran engine to 2000 to 2200 rpm or so in Park, to the point where the surging occurs about every 2 seconds. Watched O2 sensor voltages and MAP sensor pressure to see if there was a correlation with the cycling; nothing apparent. When you're driving, the surging/vibration doesn't really show up in any gear except drive (3rd), when trans is 1:1 and converter locked up, around 48 - 50 mph.
