Troubleshooting Jeep Cherokee Emissionsby Derek Odom
One common area that causes Jeep Cherokees in particular to fail emissions tests is in the vacuum system. There is a great deal of vacuum on a Jeep Cherokee, from the engine to the four-wheel drive system to the heating and cooling system. What the mechanic at the emissions testing center will do is open the hood and perform a visual inspection, looking specifically for vacuum hoses that are disconnected, broken or not present. One safe way to ensure your Jeep Cherokee passes this portion of the test is to buy a manual and check your engine bay against the vacuum routing schematic in the book. Also, restoring good vacuum to the engine will result in cooler temperatures and more power.
This is the unit on your Jeep Cherokee's exhaust system located just before the muffler, closest to the engine. What it does is super heat the exhaust fumes so that harmful vapors and emissions are literally burnt up before they hit the tailpipe. The trouble is that they fail easily, especially if you take your Jeep off the road. There is a honeycomb-looking baffler in them that is prone to failure, catching debris and getting clogged or simply cracking into pieces. If the catalytic converter is not working properly, there is little chance of your Jeep Cherokee passing an emissions test. Unless you have welding experience, it is recommended to have this component changed out by a professional if your unit is bad.
One key component of current emissions tests is to inspect the fuel tank for leaks. There are now two fuel tank tests in California, and your Jeep must pass both to receive a smog certificate. The first test they do is a performance test on the gas cap itself. They pressurize the tank, throw the cap back on it and then measure the rate of leak down. If it is in the acceptable range, they perform a leak down test on the actual tank itself by pressurizing it with no cap on, but leaving a hose on the inlet nozzle. In this manner, they can see if the tank or any components other than the gas cap is allowing harmful emissions to leak into the air. If your Jeep fails this portion of the emissions test, pay particular attention to the rubber hose on the filler inlet. Many times, these will break down over the years and require replacement.