Detecting a Leak with the Aid of a Diagnostic OBD2 Scanner


The OBD2 system, especially when used in conjunction with a Diagnostic OBD2 Scanner can help detect initial faults which the human eye will not be capable seeing until the fault becomes more evident. This system is a very sophisticated and intelligent system which prevents most faults from building up unnoticed. A typical example of such is fault code P0456. This fault code when deciphered means Minor Leak detected in Evaporative Emissions System. A detailed explanation and how to decipher a fault code from a reader or scanner which only gives the code can be found on my other previous posts. This minor fault is not only very difficult to trace unless there are obvious signs like damp or wet parts, cracked pipes and loose pipe fittings, to name a few but it takes ages tracing the whole fuel system.

Conventional vehicles without any PCM (Powertrain Control Module) or as known by most Europeans ECU (Electronic - Engine Control Unit) which had everything mechanical and electrical, with no electronic circuits might have been simpler in their own form and were easier at times to work on, but such a fault would surely go unnoticed for ages before the smell of fuel vapor becomes too evident. Even then, if there is no visual indication there will be a long diagnosing mission to try and find where the vapors are escaping from. The method of a smoke machine or color dye is still used today as it was used then but as we can see we only need to check visually, then with a smoke machine around the EVAP system only, not the whole fuel system, starting from either end (fuel tank or carburetor) then moving forwards, till the culprit is found.
Trouble here is that the fault will display on the instrumentation panel on the dash as ‘Check Engine’ or ‘MIL’ (Malfunction Illumination Light). Unless you have access to at least a code reader or better still a diagnostic OBD2 scanner with CAN interface compliancy (for vehicles manufactured after end of 2007 in US – mandatory), then there is no way of telling what the fault is and how bad the situation is. It is always bad to assume that nothing major has occurred if the engine seems to run fine. It is also illegal to drive a vehicle with that warning light on in many parts of the world.

The Evaporative Emission System
The EVAP test is not a test that is constantly being tested like other constant tests. This test is only applied if certain criteria have to be met - otherwise this test will be kept on hold. The main criteria for the test of the EVAP to perform are that the fuel tank has to contain between ¼ - ¾% fuel, it hasn’t been used overnight and vehicle has to be driven for a certain distance. It might not run on certain extreme hot or cold environmental conditions too.

The main reason for the EVAP is to keep any fuel vapor from entering the atmosphere and re-circulates it back to the engine to be burnt off. These tests are conducted from the PCM, monitoring any fuel pressure drop in the system. The EVAP pressure is monitored from the PCM through the Fuel Tank Pressure sensor (FTP) and if there is a drop or difference in pressure it is recorded. Once the tolerance is exceeded it flags up a fault code on the PCM and the ‘MIL’ or ‘Check Engine’ Light is illuminated on the instrument panel. When the system is checked using a diagnostic OBD2 scanner the fault code P0456 emerges on the scanner’s display. With such a fault the engine should keep on running smoothly as engine is not becoming lean on fuel, the EVAP circuit is designed as a close circuit so it doesn’t have any effect on the rest of the program. Possible causes may be a faulty or badly engaged fuel cap, refueling with the engine on (unsafe practice and bad for fuel injection pump), a leaky or worn out hose, loose hose fixtures, miniature purge valve or vent valve leak or tiny leak on EVAP canister.

The best possible way to detect the fault is by breaking down the diagnosis into a small section. Using a diagnostic OBD2 scanner activate the vent solenoid, taking care to seal the rest of the system. Any pressure and any difference are monitored through the FTP and are recorded on the PCM, then entering the scanner. If the pressure reading remains the same, entering the OBD2 scanner screen as constant (same pressure reading) then that test has passed, meaning no leaks. If any pressure difference is recorded it is transferred onto the OBD2 scanner screen and you know there is a vapor or miniature leak somewhere across the EVAP system. If nothing can be seen visually, then the best option is to use a smoke detector. When smoke is seen the leak is uncovered. Never attempt to pressurize the EVAP system with compressed air. This can damage the delicate and accurate the valves in the system.

It is also a good idea after finding and replacing the faulty component to check all hose conditions. If they seem to have any hairline cracks or serious color discoloration, just change them with new ones, using the appropriate fuel hoses and the proper clamping hose devices. Re-check for any leaks with a smoke test. This way there is much less chances that the same code will appear again for a different part, which was overlooked the first time. If you are interested in OBD diagnostic techniques and would like to acquire more tips and information visit Automotive Diagnostic OBD2 Scanner.

Monday, September 14, 2009

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