QuestionNeed help to diagnose high NOx. 2.0L SOHC engine - very good condition on this very clean car/engine.
Failed emissions tests multiple times this spring due to high NOx. Code P0401 (EGR System Failure) for the past couple of years, I bought the car in 2008.
Last year replaced entire EGR assembly (valve+pipe+solenoid) due to engine code 32. Engine code 32 continued.
This spring failed emission test ... replaced catalytic converter (twice! originally because mechanic said O2 sensors showed converter wasn't working, second time the supplier provided a new converter), failed the emissions test again. Put in new oxygen sensors, failed again. Always with high NOx, HC reading was fine.
Recently monitored engine vacuum (about 20) and exhaust manifold vacuum (about 5) while driving ... then stopped the car and disconnected the EGR solenoid wiring connector - manifold vacuum behaved differently (the solenoid must be doing SOMETHING).
If I physically push the metal disk (inside the EGR valve) inward toward the solenoid body, the engine hesitates or stalls. Let go, engine runs okay. If the solenoid wiring connector is unplugged, the engine starts to run a little rough until the connector is plugged in again.
Performed EGR System Test as per 1995 Neon Service Manual - the engine stalls when vacuum is applied as described, and the EGR solenoid moves ("wiggles", less than 1/16 inch - is that right?) when engine is abruptly accelerated.
I have a Digimoto ODBC scan tool which shows that MAP and O2 sensors are working.
I have found other wiring in the car with splices "twisted but not soldered" and indeed found the EGR solenoid wire was like this - but after cleaning and soldering the wires, and also checking continuity to the PCM via a large engine connector - the problem persists.
No problems otherwise. Gas mileage is usually around 6.4 L/100km (not bad, I wish it was better). Compression good in all cylinders except #2 is low. Fuel pressure rail was tested 48 psi.
AnswerHi Mike
The high NOX and EGR code are consistent. In checking the wiring did you verify that dark green/white shows 12v in the start and run positions, and the gray/yellow is not shorted to ground, even though connected to pin 40 of the pcm?
Other than that, the manual shows a series of vacuum tests to sort out the possibilities:
Warm the engine,Disconnect the egr solenoid then:
Disconnect the vacuum hose at the solenoid, does it read at least 10" of vacuum
Reconnect the vacuum hose to solenoid, disconnect the vacuum hose to the egr and connect a vacuum gauge to the disconnected hose. Does it pulsate? if yes
Mometarily raise rpm to 2800. Did vacuum staibilize?
if no
Reconnect hose to egr.
Disconnect backpressure hose from transducer. Connect gauge to hose.
Does it pulsate? If so replace egr. If not check for sizeable exhaust leak. If none found check egr back mounting, valve tightness, check egr gasket. If none found replace egr.
Let me know if any of this produces anomolies.
Thanks for all the details.
Roland
