1993 Nissan failed smog check for high HC

We found that the vacuum hose to the EGR valve had a hole in it. We replaced the hose and the numbers changed. Now it fails in both 15mph and 25mph but only the HC fails, the numbers are lower but still not low enough. Also, I don't know if this is a clue or not but the NO before the repair was 1236 at 15mph and 86 at 25mph. After the repair the NO is 112 at 15mph and 0 at 25mph. Any ideas?

Answer:

It's great that the NOx numbers decreased significantly. Now you have to work on the HC problem. If CO is normal (between 0.00 and 0.07) you should look into your Nissan ignition system... the sparkplugs, wires, distributor cap and so on. High HC faults, if not related to fuel control, are usually caused by worn ignition parts.

If CO is not normal, too high or too low... then you want to look into the fuel control system. High CO (rich fuel mixture) or low CO (lean fuel mixture) will cause high HC as well.

posted by SmogTips Support