This is a customer vehicle my lab partner and I worked on recently while going to school at BYU-Idaho. 2003 Chevrolet Trailblazer 4wd 4.2L The customer complained of low power and running rough. We attempted to test drive the vehicle and found it would not go over 10mph. The engine would rev high but there was a lack of power. We checked for codes and found several codes: P0495, P0172, P0420, P0756, P1484 The code I am going to focus on for this post is the P0420 and how to verify that the catalytic converter is actually bad. I am going to be focusing on advanced diagnosis procedures. For this post we are going to focus on examining the amount of back pressure in the cylinder using the PicoScope Automotive Oscilloscope, the WPS500X Pressure Transducer and a common laptop loaded with PicoScope 6 software. The advantage of using this pressure transducer to determine the cylinder back pressure is that there is no cutting, drilling, crawling, or welding required. Normally to read the exhaust back pressure it is required to drill a hole in the exhaust before the catalytic converter and hook up a vacuum/pressure gauge to read the back pressure. Then after taking the reading it is needful to either weld the hole or put a screw in it to plug the hole. The pressure transducer hooks up just like a compression gauge. We removed the spark plug from the #2 cylinder and attached the pressure transducer then started the car up. Then we recorded the wave forms and found the following results: At idle we had about 9.4 psi of exhaust back pressure in the cylinder and at 1500-2000 rpm we had about 22 psi. We could not find an in cylinder exhaust back pressure specification. The diagnostic chart for checking for a restricted exhaust specifies taking out the HO2S and installing a pressure gauge in its place. The specifications of the back pressure for this vehicle at the HO2S are 1.25 psi at idle and 3 psi at 2000 rpm. So even taking into account that these pressures are not at the O2 sensor we determined we had excessive amounts of back pressure. To verify our reasoning and the restriction in the exhaust we unbolted the exhaust pipe from the exhaust manifold and ran the same test again and found the following results: At idle we had about 1.8 psi of back pressure and at 1500-2000 rpm we had about 5.3 psi of back pressure with the exhaust pipe unbolted from the manifold. These readings are a lot closer to the specification. Our readings after unbolting the exhaust form the manifold are still a little high but remember these readings are in the cylinder and the specifications are for where the O2 sensor is placed in the end of the manifold. We are not sure what exactly is the cause of these slightly higher pressures. They may be normal pressures in the cylinder but we have nothing to compare them to at this time. It could be that by the time the exhaust has reached the O2 sensor the pressure has lost some of its energy and has expanded after going through the manifold therefore lowering its pressure. We will have to do further testing/experimenting on future vehicles to confirm these hypothesis. This test did however verify that there was a restriction was in the exhaust of about 6 psi at idle which is definitely not normal. So we removed the catalytic converter to preform a visual inspection and found that no light was able to be seen through the converter material and there were chunks of material that came out of the converter when we tipped it on its end. This was a bad converter. We replaced the catalytic converter and verified the correct operation. The plugged converter was the source of the P0420 problem. In future posts we will cover the other problems of this car and what may have caused the converter to go bad in the first place.
Stay tuned (pun intended).
0 Comments
Your comment will be posted after it is approved.
Leave a Reply. |
AuthorWalker Crystal Archives
May 2019
Categories |