2012 Honda Civic - P0171 Lean Condition - Rough Idle

Question

My Civic is throwing a P0171 System Too Lean code. At idle, my Short Term Fuel Trim is hitting +25% and Long Term is around +20%. If I rev the engine to 2500 RPM, the trims drop back down toward zero.

I've checked for vacuum leaks using soapy water but didn't find anything obvious. The car feels down on power when taking off from a stop. The MAF sensor looks clean. Any common spots where these 9th gens leak air?

MackMarty · Answer

Check the rubber air intake boot between the airbox and the throttle body. They are notorious for cracking in the bellows (the accordion part). Since the crack is after the MAF, it sucks in unmetered air at idle. When you rev it, the engine tilts and helps seal the crack or the volume of air masks the leak.

GearheadGus · Answer

✅ Confirmed fix: Found a massive crack on the underside of the intake manifold gasket. On these 1.8L R-series engines, the plastic manifold can occasionally warp or the gasket flattens out. I used a smoke machine and smoke started billowing out from the Cylinder 4 intake runner. Replaced the gasket and trims are back to +/- 3%.

2012 Honda Civic - P0171 Lean Condition - Rough Idle

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