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my 2008 2.4l ecotec has a rough idle sometimes. other times it idles fine.
I've changed the spark plugs, coil packs, o2 sensor and map sensor recently with no change.
I checked the trim and found the LTFT is about +1 to +2. when it is idling ok the STFT is between +5 and -5. when it starts idling poorng, the STFT goes high...as much as +28.
I'm guessing there could be a vacuum leak, but if that is the case, I can't find it. Is there a common place a vacuum leak occurs on the 2.4L?
Last edited by GoBison; October 18th, 2020 at 7:28 PM.
although the exhaust manifold has a crack, I suspect that isn't causing the issue. I unplugged the o2 sensor and the idle didn't change. if the crack in the exhaust manifold was causing the idle issue, unplugging the o2 sensor should have smoothed it out.
I replaced the cracked exhaust manifold and purge valve. Also cleaned the throttle body and checked torque on the intake bolts. idle is still poor but I never saw the STFT go above +15. here are some snaps:
try a throttle body clean and do a throttle relearn procedure with the scan tool.
signs of a vacuum leak are normal fuel trims at 2500rpm and high fuel trims at idle. with the throttle held open manifold vacuum is low so vacuum leaks have little effect.
try a throttle body clean and do a throttle relearn procedure with the scan tool.
signs of a vacuum leak are normal fuel trims at 2500rpm and high fuel trims at idle. with the throttle held open manifold vacuum is low so vacuum leaks have little effect.
good suggestion! I removed the throttle body and cleaned off the carbon. unfortunately, no difference in idle. I'm going to try smoke testing next.
that works
you can also try spraying brake cleaner over the manifold ( or other potential leak points) with the engine r unning. if the idle surges it may be the vacuum leak location. i also use a mechanic stethoscope to listen for vacuum leaks.
performed smoke test and compression test today. also installed new MAF. found no leaks with smoke test. smoked the dipstick, breather hole on the valve cover, intake through the EVAP port, EVAP line after the purge valve. compression test results:
October 18th, 2020, 6:56 PM
2.4l rough idle positive STFT
