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1999 Mazda 626 PO171 and PO420: Engine Shakes at Idle, Smoke from Intake?

Model: 99 mazda 626 Fault Code: P0421 Posted: 2007-09-10 09:50

I recently encountered a persistent issue with my 1999 Mazda 626 2.0L engine. The OBD-II scanner consistently detects fault codes PO171 (System Lean Malfunction) and PO421 (Diesel Particulate Filter or Catalyst Efficiency Below Threshold - though applicable to older models). The vehicle shakes noticeably during idle, especially at stoplights. On a few occasions, I observed smoke or fumes emerging from the air intake system—appearing as if air was being pushed out rather than drawn in. After turning off the engine and restarting it, the smoke disappeared immediately. This behavior is puzzling: how could exhaust fumes be entering the intake? Given that my vehicle has 120K miles, I'm concerned about potential vacuum leaks or manifold issues. Any insights into possible causes would be greatly appreciated.

Related fault codes

P0421

Comments (3)

Anonymous 2007-09-10 11:05

A vacuum leak is a likely culprit. The presence of smoke from the intake suggests exhaust fumes are being drawn into the engine via an intake manifold or hose leak. This can cause lean misfires and trigger PO171 and PO421 codes. I strongly recommend taking it to a professional mechanic—this issue may be beyond DIY repair.

Anonymous 2007-09-10 16:05

Moved

Anonymous 2007-09-11 19:56

The fumes you're seeing are likely due to an exhaust leak in the intake manifold or gasket. If exhaust gases are being pulled into the intake, it creates a lean condition that causes the O2 sensors to register excessively low fuel levels—triggering PO171. This also affects catalytic converter efficiency, leading to PO421. Check for any cracks or loose connections in the intake manifold and associated hoses. Are these fumes consistent? Does your engine make unusual noises or emit a strong odor when idling?