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Forum > Emissions Component > Question

1998 Toyota 4Runner SR5 Almost Failed HC. Why?

Vehicle has 135 K miles. Well-maintained. Almost failed smog test recently. At 15 mph, elevated levels of HC = 50 ppm (max allowed 51ppm). Also elevated levels of CO = 0.2ppm (max = 0.65) and NO = 229 (max = 512). All levels are AVERAGE or BELOW FOR 25 MPH speed. What are possible causes for elevated levels only at slower speed? (New plug wires, 02 sensors, air filter OK, recent plug wires.
Answer:

The reason your Toyota 4Runner produced high HC emissions only during the 15 mph part of the smog test and not the 25 mph part is because the smog test is designed to place a predetermined load on your Toyota's engine while it is being driven on the dynamometer. At 15mph this load is twice (50%) as high as during the 25mph test (25%). The added load on your 4Runner's engine during the 15mph part of the smog check will cause it to work harder and produce more emissions. The smog inspection is set up this way in an effort to simulate real word conditions. Since your Toyota came very close to failing during the 15mph portion, and it produced marginal emission levels during the 25 mph portion we'd have to recommend you have it inspected by a smog check repair station and have them diagnose these faults, incluing the almost failing HC results. It is not normal for a vehicle to produce HC close to the smog check cutscore. In fact if you look at your Toyota's smog check VIR, you'll probably notice HC average results closer to 10 ppm. Your Toyota's CO and NO are high as well. posted by SmogTips Support

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