NOx Failure Prompts Checking Reasons for High TempsPosted 10/15/2004
By Wayne Pasicznyk
Vehicle: 1992 Honda Accord.
Problem: Failing emissions for oxides of nitrogen (NOx). This vehicle would stall instantly at idle when the EGR valve was opened.
Background: Hondas are known to have restricted or blocked exhaust gas recirculation (EGR) passages. I looked at reasons for a NOx failure, including the inputs Honda uses to control EGR operation. High NOx emissions are caused by excessive heat due to high combustion chamber temperatures.
Causes of high combustion temp:
1) The cooling system. This system needs to be in proper working order or the engine can run hot, which can elevate NOx emissions. In this vehicle, the cooling system was working properly.
2) Incorrect timing or spark advance. I had the technician verify that timing and spark advance were correct. The distributor was in the center of its adjustment so I knew camshaft timing was also correct.
3) Carbon deposits in the combustion chamber. A compression test showed it was within specifications.
4) Incorrect spark plug heat range. Always verify that the plugs are the right application and heat range.
5) Incorrect vacuum to the EGR valve. First we checked codes in the engine control module (ECM). In most Hondas, if it has an EGR problem it will set a code. There were no codes in the ECM so we assumed everything was functioning normally. I had him tee a vacuum gauge into the vacuum line to the EGR valve and test-drive the vehicle, which indicated there was no vacuum signal to operate the EGR valve.
That was strange because there were no codes for EGR operation. Then we started to look at all the inputs the ECM uses to control the EGR system. Using a digital volt ohm meter (DVOM), I had the technician backprobe the coolant temperature sensor. It read 0.53 volts. Most Honda coolant temperature sensors read 0.5 to 0.6 volts when fully warm, so that was OK. The next inputs used for EGR operation are the manifold absolute pressure (MAP) sensor and the throttle position sensor (TPS). Using a DVOM and backprobing the MAP sensor and the TPS, both read within specifications. We also located the EGR solenoid and verified that it operated correctly when energized. The other inputs listed in Honda's factory service manual are the three signals from the distributor. The distributor tested OK.
The next step was to access the ECM and verify that if the EGR solenoid was energized, vacuum would be applied to the EGR valve. The ECM on this model is located under a plate on the passenger floor. A quick look at the ECM showed no corrosion and all the connections looked good. The tech located the EGR solenoid ground wire, started the engine and manually grounded the EGR solenoid. The solenoid operated and vacuum was applied to the EGR valve, indicating the EGR solenoid electrical circuit was good. I started to wonder if the ECM was faulty - Honda ECMs don't often fail. Next, I had the technician perform voltage drop tests of all the ECM power and ground wires with the engine running. There are three power inputs and four grounds, and all voltage dropped within specifications.
Before replacing the ECM, I kept thinking we were missing something. High combustion temperatures can be caused by a lean running engine or a faulty or rich biased oxygen (O2) sensor. I didn't check that first since the reading from the emission test station did not show a lean condition. But before replacing the ECM, it was worth a look. Since the ECM was accessed, I had the technician locate the White signal wire for the O2 sensor and backprobe it with his DVOM. It read over 3.0 volts, when it should have been between 0.1 and 0.9 volts in closed loop - never over 1.0 volt. The ECM had not set an O2 sensor code or a fuel delivery code.
Where was the excessive voltage coming from? I had the technician unplug the O2 sensor, turn the key on and check for bias voltage of 0.4 to 0.5 volts to the O2 sensor signal wire from the ECM - it tested OK. The tech inspected the O2 sensor connector for corrosion and it was OK, so we determined the excessive voltage must be coming from the O2 sensor heater circuit.
The technician installed a new O2 sensor and started the vehicle. Within a minute or two we had proper closed loop operation, and when he test drove the vehicle, he had vacuum to the EGR valve and the NOx failure was repaired.
Even though Honda does not list closed loop operation as an input for EGR operation, it turned out to be an important input. It pays to verify all the ECM inputs before replacing an $850 part!
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