How Can I Lower NO on my Jeep Cherokee Laredo?

My Jeep Cherokee Laredo 4.0 Liter Engine failed the smog check because NO was too high. Everything else passed. CO at 15mph was 0.02 and at 25mph was 0.01, but NO at both speeds was above 700 ppm.

I have done a lot to this Jeep to try to get it to pass the smog test. The above results were from my third test. The Jeep has made great improvements from the first test, but can not get it to pass.

Most recently I repaired: EGR valve, valve cover hose (PCV valve), the oxygen sensor is new, the MAP sensor is new, plus I put in a new catalytic converter.

I recently also treated the fuel system with Sea Foam. I took it to a different smog test shop. They said that my #3 fuel injector was leaking, and my Jeep Cherokee failed the Fill Pipe Restrictor test. This did not happen on the first 2 tests at the other shop. I need some help. If I replace that injector, will that solve my problem, or is there something else?

Answer:

Sounds like you've replaced the main components on your Jeep Cherokee necessary to control (lower) NOx. We recommend two things in particular, conduct a fuel feedback test to ensure your Jeep Cherokee's ECU is commanding the fuel injectors to increase fuel delivery when an increase in MAP frequency is detected. Assuming the MAP is functioning properly and the signals are reaching the ECU the ECU should command "rich" with MAP frequency increases. Same applies to the oxygen sensor only in reverse; with lower O2 voltage the ECU should command more fuel delivery and visa versa. Assuming the new components are working we just want to make sure the new components are connected properly and their signals are reaching the ECU; and that the ECU is operating properly. Although your Cherokee hasn't failed for high CO, it is important that CO we well controlled. Your Jeep's CAT needs to run slightly rich in order to control and lower NO emissions.

As far as the "leaking" fuel injector, this would cause an increase in CO and decrease in NO. There might be an injector problem, but typically a sticking (or stuck closed) injector contributes to high NO. Remember your Jeep Cherokee will produce high NO when it is running lean or when the combustion chamber temperatures rise above 2500F. Things such as high engine compression and vacuum leaks should be diagnosed as well.

posted by SmogTips Support