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1992 Mazda Navajo 4.0 L 6 cyl. Auto trans. Inquiry on different RPMs for 15 mph on smog check retest.

My initial test results 15mph RPM 1894 CO2 14.7 O2 0.1 HC 10 CO 0.21 NO 1104 25 mph RPM 1808 CO2 14.8 O2 0.0 HC 9 CO 0.19 NO 897 Failed NO at both speeds. Changed out O2 Sensor. Noticeable improvement in engine acceleration. Took back for retest. Results 15 mph RPM 1426 CO2 14.9 O2 0.0 HC 8 CO 0.07 NO 1056 25 mph RPM 1944 CO2 14.9 O2 0.0 HC 7 CO 0.07 NO 553 Passed all except NO at 15 mph. Max @ 15 mph is 699 & max @ 25 mph is 601. 1st thru 4th transmission gear ratios are 2.47 ,1.47. 1.0 , and 0.75. At 3200 rpm in 3rd gear w/ full converter lockup vehicle speed is 65 mph. My calculations show that on the first 15 mph test my vehicle must have been in first gear. On the second 15 mph test it appears the vehicle must have started in 1st then shifted to 2nd as 1426 rpm is too low for only 1st and too high for only 2nd gear.My calcs show that both 25 mph tests were in 2nd gear. It seems to me that since 1900 RPM @ 25 mph in 2nd gear showed a definite improvement that it would follow that 1900 RPM @ 15 mph in 1st gear would show a similar improvement.Am I correct in this assumption? I read that the state board selects rpm to mph ratios on a model to model basis.If so how they accomplish that? The gear ratios are fixed as is the mph speeds they specify. Are they varying the load on a model to model basis? Another thing if my model of vehicle is so suceptible to NOx increases with a different torque / speed ratio does the tech have to experiment to get it within tolerance? I know you would only be speculating but why would the tech run the first 15 mph in 1st gear and the on the second 15 mph test allow it to shift into 2nd? Also how much leeway is there in the rpm and /or speed? Is it the techs discretion ? Besides going to another test center what recourse does the customer have? Your response will be greatly appreciated. Thank you.
Answer:

The need to switch between 1st and 2nd gear will be indicated by the smog machine during the smog test. The technician should needs to keep the vehicle which ever gear will produce RPMS which fall between the limits set by the BAR. Based on the VIR numbers you've provided we have to assume your vehicle's fault is not so much related to which gear it was tested in, rather a malfunction with an emissions component controlling NOx... such as the EGR valve/system. If your Mazda has high mileage, we'd recommend conducting a cylinder compression test as well to ensure compression is within limits. posted by SmogTips Support

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