Finding a Needle in a Haystack
When first diagnosing evaporative emissions system leaks on OBD-II vehicles, I have noted that many times the code P0442 or "small leak" actually turns out to be a leak so large that there should be no need for diagnostic equipment. In contrast, however, a P0455 or "gross leak" can often be like finding a needle in a haystack. The fact is, there are good reasons for the codes setting as they do. An explanation of why these codes set can be very useful in completing an accurate diagnosis.
Let's start with code P0442. This code does not necessarily mean it detects a small leak; it simply means that the evaporative emissions system is capable of pulling the correct vacuum on the system but it is not able to hold this vacuum for a long enough time. When the evaporative system does its own self-test, it tries to pull a vacuum on the fuel tank and uses the fuel tank pressure sensor voltage to see if it is affecting tank vacuum. It needs to be able to pull the fuel tank pressure sensor voltage down to, or below, 0.8 volt within one minute. Then it closes the purge valve and needs to be able to hold this vacuum within 0.4 volt for at least one minute. If it meets the 0.8 volt threshold, but does not hold the vacuum, this is referred to as a "small" leak.
The P0455 code does not actually mean a large leak is detected. This code simply states the evaporative system is not able to pull the fuel tank pressure down to 0.8 volt.
Therefore, since it could not meet its target vacuum, it considers this a "gross" (or large) leak.
So how does this affect the diagnosis? It has been my experience that small leaks are easier to diagnose using a vacuum system rather than smoke. One effective method is to block off one piece of the system at a time, such as a charcoal canister, fuel tank and sections of evap lines and then pull vacuum on the system and recheck its leak-down rate.
I have found the smoke machine does a better job on the large leaks. Be aware, however, that you may have to smoke the system for several minutes before the entire system is filled enough to see the leaks.
To confirm your repair, some scan tools have their own evaporative tests. But a simple way to do it yourself is to ground the purge solenoid driver wire and the vent solenoid driver wire. This will activate both solenoids. Then start the engine, watch the fuel tank pressure sensor voltage, and ensure that it drops to 0.8 volt or lower. Next, remove the ground from the driver wire at the purge valve and ensure the voltage stays within 0.4 volt for at least one minute. This test should confirm that you have correctly diagnosed and repaired the evaporative system.
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