Fails the Loaded Emissions Test on NOx Only
A 2002 2.0 L four-cylinder with 165,000 miles fails a loaded steady-state emissions test. At 15 mph: NOx 420 ppm against a 600 limit, HC 40 ppm against 120, CO 0.12 percent against 0.70. At 25 mph: NOx 1,250 ppm against a 700 limit, HC 45 ppm, CO 0.14 percent, CO2 14.2 percent, O2 0.3 percent. Coolant is 195 F, base timing is 10 degrees BTDC against a 10-degree spec, and the cooling fan works. The EGR is vacuum-operated through a solenoid. At 25 mph the PCM commands 55 percent EGR duty, but manifold pressure and fuel trim do not change when the command steps up. There are no EGR codes stored and the solenoid coil measures 32 ohms against a 30 to 40 spec.
1. Interpret the gas readings. Why does this point to combustion temperature rather than a mixture problem?
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The four gases together tell the story:
- NOx is the only failure, and it gets worse as load rises (420 at 15 mph, 1,250 at 25 mph). NOx forms at high combustion temperature with excess oxygen available in the flame.
- HC and CO are low and CO2 is healthy at 14.2 percent. Combustion is complete and the mixture is not excessively rich or lean. A lean misfire would raise HC and O2 and usually hurt driveability; a rich mixture would raise CO.
- O2 at 0.3 percent says the exhaust is near stoichiometric, not lean. There is no leftover oxygen to blame.
- Timing is at spec and the fan works, removing two of the usual temperature causes.
With mixture and ignition essentially normal, the remaining lever on peak combustion temperature is dilution of the charge, which is the EGR system's job.
2. The EGR solenoid resistance is in spec and there are no EGR codes. How do you prove whether EGR is actually flowing?
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Resistance only proves the coil is intact. It does not prove the valve moves or that the passage is open. Use a command-and-observe test:
- Command EGR duty up and down with the scan tool and watch manifold pressure, fuel trim, and idle quality. A working EGR system should make MAP rise slightly and trim shift as exhaust dilutes the charge. Here nothing changes, which means no flow.
- Apply vacuum directly to the EGR valve at idle. If idle stumbles or stalls, the valve and intake passage are open and the problem is upstream in the solenoid, hoses, or control. If nothing happens, the valve is stuck or the passage is plugged.
- Check the EGR passage at the intake and the valve pintle for carbon. A plugged passage is common on a 165,000-mile engine and will not set a code on a system without a position or flow sensor.
- Verify the vacuum supply to the solenoid and the solenoid's ability to pass vacuum when commanded.
Do not replace the solenoid based on resistance. It tested good and the symptom is downstream.
3. If direct vacuum to the valve makes the engine stumble and EGR flow is confirmed, what else can produce this NOx pattern, and what is your next move?
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If EGR flows, the dilution is there and you look at the other temperature levers:
- Carbon deposits raising compression and creating hot spots. A 165,000-mile engine with normal HC and CO but high NOx is a candidate for a decarbon treatment or a borescope look at piston tops and valve faces.
- Intake air temperature high because of a missing or misrouted air duct, or a heat stove stuck on.
- Cooling system marginal under load even though the fan runs: a partially plugged radiator or a thermostat that runs hot under sustained load raises combustion temperature.
- EGR distribution uneven because only one intake runner is plugged, so total flow looks fine at the valve but one cylinder runs hot.
Next move: verify EGR actually reaches each runner, check IAT and coolant temperature under the same 25 mph load, and borescope for carbon. The repair must address the cause, not just clear the code, or the vehicle returns with the same failure.