1979-1993 & 94 Conv [Subscribe to Daily Digest] |
Very interested to hear final fix as have had recent failure with 82 900t, and will also have to pass with 87 900t and 92 900t within next 3 months. On another unrelated problem I just bench tested 3 005 AMM's. Powered units with 12V and ground and took output readings at between 200 and 600 ohms every 50 ohms with still air and moving air. Do not understand results as they were identical for all 3 AMM's - 1.55 volts still air and 3.66 volts moving air...ohms seem to make no difference. Don't see how this can be correct - do I have 3 good AMM or 3 bad with exactly same fault? Comments welcome.
Link below has good info even if it is for Volvo - NOX info from aircare.ca site
http://www.brickboard.com/FAQ/700-900/EnginePerformanceSymptoms.htm
Harder to diagnose are elevated oxides of nitrogen (NOX) emissions. NOX levels are affected by engine combustion temperatures. When the temperature inside the combustion chambers exceeds 2500°F, nitrogen combines with oxygen to form oxides of nitrogen, or NOX. Many engines rely on EGR to lower combustion chamber temperatures and reduce NOX formation to an acceptable level. When the engine is running at stoichiometric level (mixture at 14.7:1), NOX production ranges between 1700 and 2500 parts per million (ppm). Since NOX formation is a temperature-related reaction, lean mixtures cause higher NOX production. Mixtures leaner than 14.7:1 (stoichiometry) increase combustion temperatures and cause NOX production to increase. When the mixture reaches 16:1, NOX production drops off. To find the cause of NOX problems, you'll need to determine what's causing the engine to run too hot or too lean, or both. Causes here may include air intake or vacuum leaks; defective EGR valve, EGR vacuum solenoid or motor, or plugged EGR ports in the manifold; a failing or sluggish oxygen sensor; carbon deposits in cylinders or on plugs; over-advanced ignition timing; fuel pressure too low due to plugged filter or failing pump; a failing catalytic converter; or engine overheating due to cooling system malfunction or overheated intake air due to air intake box thermostat failure.
This vehicle was booked in for diagnostics following a customer complaint. The repair shop had replaced the catalytic converter, but this had not corrected the high NOx emissions. The first clue this was a lean air/fuel ratio problem was the final test result, which was very similar to the retest result of .1056 g/km [0.17 g/mi] HC, 1.6447 g/km [2. 65 g/mi] of carbon monoxide and 2.5314 g/km [4.08 g/mi] of NOx. The clue here is that the NOx was actually higher than the carbon monoxide!
The second clue was the Diagnostic Trace Report (figure 4). If you look at the sections of the driving trace where accelerations occur, you will see the NOx goes high and carbon monoxide remains low. This is just the opposite of what you should expect! There will usually be some degree of enrichment and high carbon monoxide during accelerations to improve throttle response and reduce NOx emissions.
The third clue was Long Term Fuel Trim (LTFT) at high idle. I hooked up my scanner and checked fuel trim and airflow at exactly 2500 RPM while also observing the O2 sensor on my lab scope. I noted the indicated airflow for future reference, which was 14.1 g/second.
The scope showed the vehicle was maintaining closed loop, but the scanner indicated the control system was compensating for an under-prediction in airflow or a lean condition. This would serve as a baseline test to see if things improved after cleaning the sensor.
At this point, it was time to stop checking and start repairing. Removing the MAF sensor was easy. Just remove three screws and the sensor module is in your hand. A close visual inspection showed there was a fuzzy build-up on the sensing element. This required a bit of scrubbing with a soft toothbrush and some throttle body cleaner. Just spraying the sensor wire did not remove the fuzzy build-up that appeared to be small fibers from the air filter. I followed this with some brake cleaner and compressed air to make sure there was no residue on the sensor that would attract more dirt. With the sensor module reinstalled, it was time to see if the cleaning improved the transfer function of the sensor. Reproducing the test conditions from my baseline, I took the RPM to exactly 2500 again. The indicated airflow was 17.6 g/second, an increase of almost 25 percent. The long-term fuel trim values were now close to the nominal 128 value with a reading of 124. A second test at the Technical Center confirmed the NOx had been reduced by 40 percent and the vehicle fast-passed when taken through the inspection lanes for an official I/M240 test.
Ron Leavitt is an Emissions Technical Advisor, British Columbia AirCare Program.
posted by 138.88.170...
, Fri, 28 Jan 2005 11:12:14
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