| Author | Message | ||
     Brad Noviski (Bradnoviski) Member Username: Bradnoviski Post Number: 95 Registered: 02-2003 |
After installing a new O2 sensor, this fault code occured. P0448 Evaporative Emission Control System Vent Control Circuit Shorted. Anyone know what this means and what is the cause? Thanks | ||
| michael a. kerr (Ethanrover98) New Member Username: Ethanrover98 Post Number: 17 Registered: 11-2003 |
Brad, P0448 - Evaporative emmision system vent control circuit shorted. Axel posted the link to the codes on 1/3/04..... | ||
| Brad Noviski (Bradnoviski) Member Username: Bradnoviski Post Number: 96 Registered: 02-2003 |
Michael Thanks for the link. It is very usefull, but I know what the code stands for/is but I want to know why it happened and what caused it. I am not sure it the new O2 sensor and this fault are related. | ||
| michael a. kerr (Ethanrover98) New Member Username: Ethanrover98 Post Number: 18 Registered: 11-2003 |
Brad, I hope that this article helps............ sorry for the last post. I didn't read your post "fully" and thought that you were just looking for the descripition of the code..... I read several other articles and none of them mentioned O2 sensors as a possible cause of a P0448 code..... Evaporative emission control systems have been around for almost 30 years, and for most of that time the basic design has evolved slowly as emissions regulations have tightened and equipment has improved. However, like so many other emission control systems, OBD II has required radical changes that are apparently causing lots of headaches for drivers and techs alike. To help you understand this new system, we'll describe how it's supposed to work, some of the reasons it might fail, and some clues about how to deal with it. The EVAP charcoal canister was first introduced in California for the 1970 model year. Its purpose was to capture raw fuel vapors (hydrocarbons) from the tank and carburetor bowl before they could escape into the air. It's been known for some time that activated charcoal readily adsorbs hydrocarbon vapor, holding it on the surface of each granule like a magnet. This is different from absorb, which sucks it in like a sponge. Since the HC is only on the surface of the charcoal, it can be easily blown off with a blast of fresh air. On earlier carbureted engines, a non-venting gas cap and a simple vacuum operated check valve were used to keep the system closed with the engine off or at idle. With the engine running above idle, the valve opens and a hose between the canister and the throttle body allows fresh air to be pulled through the charcoal and mixed with the intake air. There is a calibrated orifice in this circuit to limit the total air/vapor flow, making this a calibrated vacuum leak. This simple system worked just fine when tailpipe HC emissions were regulated to 1.5 grams/mile. But according to the Bosch automotive handbook, if 1% of the total intake air is composed of fuel vapors from the charcoal canister, the air/fuel ratio can change by as much as 20 percent. Dealing with such a large variable forced the design of a closed-loop fuel trim system with a large range. Finally, total control of the EVAP system was turned over to the Powertrain Control Module (PCM). Not only could it easily make all the decisions about when to purge the canister, in many systems it also could control the flow volume with a single pulse width modulated solenoid valve. This had some obvious advantages: the PCM could maintain the right compromise between tight HC control and good driveability precisely under all operating conditions. It also greatly reduced the number of parts in the system, reducing the opportunities for leaks and malfunctions. But this simplicity was not very long lived. The EPA has an official list of components that defines the evaporative emission control system: Fuel tank Canister vent solenoid valve Fuel tank vacuum or pressure sensor Fuel lines Vapor lines Fuel tank cap Charcoal (EVAP) canister Purge lines Purge solenoid valve.(Vapor Management Valve) Failure of any of these components will compromise the overall system, causing hydrocarbon emissions to increase beyond the limit, sometimes even if the car is parked. On-board diagnostics must be able to verify proper operation of the system, record any failure in DTC memory and alert the driver when the system is not functioning properly. Two parameters define proper EVAP function: system integrity, as in leaks or blocked lines, and proper purge airflow from the canister to the engine. In laboratory settings, it has been determined that an EVAP hose with a leak only 0.020 in. in diameter is capable of increasing HC emissions enough to fail the FTP certification. Those tests also show it is possible to detect a leak that small using on-board diagnostic equipment. However, real-world testing at that level showed extremely high failure rates, so the EPA set a 0.040 in. leak as the standard for illuminating the MIL for leak detection. At this time there are two basic strategies for checking system integrity. Some manufacturers use a pressure sensor; either built into the fuel tank unit or mounted somewhere else in the fuel vapor control system. At the appropriate point in a drive cycle, the PCM will close the canister vent solenoid valve, use the engine to pull a vacuum of about 14 inches of water on the system, close the purge valve and measure how long it takes for the vacuum to decay to a specified level. If it decays too fast, there is a leak. If it doesn't decay fast enough, there is a blocked line. The same test can be performed in the service bay, basically a leak-down test using a hand operated device like the Waekon Model 46568 EVAP System Master Diagnostic Kit. |
