IN ENGLISH, PLEASE! I'm using Ford for this example because their Mode $06 information is very thorough. Using that information, I found out that
all the TIDs starting with "$2" are part of the EVAP system monitor for that model-year. Scrolling through the tests, I found
a few where the test values were near, but not over, their test limits. See Figure 4. Let's put all the tests into English and see if it's telling us anything. See Figure 5. To convert the values, subtract 32,768
from the test values and multiply the result by 0.00195. The resulting number is a pressure measurement in inches of water
(in./H2O). As you can see from the table, some of the conversions will be a negative number. That's OK. Now that I have "real" numbers, I can tell that this system is tested using a "vacuum decay" method. Reading up on the monitor's
operation confirmed this. Briefly, the system is closed and a vacuum is applied. This vacuum is allowed to stabilize, and
it is monitored over time for loss. If excessive loss (pressure rise) is detected, a leak in the system is indicated. Finally,
the system is again closed and monitored for the rise in pressure that would normally be expected due to fuel vapor build
up in the tank. This last phase is used as a confirmation if a leak was suspected in the first half of testing. Now the intermittent MIL makes sense. Both of the "leak checks" indicated vacuum loss. We started with an initial tank vacuum
of -7.56 in./H2O (TID $26) and rose to +1.51 in./H2O (TID $27). This, however, was still in limits, so the PCM did not record the possibility of a leak on this run. Look at
TID $2B. It shows that pressure rise caused by fuel vapor from the tank only hit +2.42 in./H2O, less than expected and this test failed. Had either TID $27 or TID $28 gone a little higher, a DTC would have been set.
Mode $06 can be helpful in finding intermittent problems. Many Mode $06 tests are electrical component tests that not only
check for opens and shorts, but current draw as well. These, too, can be useful in finding intermittent problems. BACK TO THE FORD I decided to pressure-test the EVAP system and see what I could find. Our test machine has a ball that will drop all the way
to the bottom of the flow rate indicator on a tight system. One extra step I like to take is to wait about 30 seconds and
hit the pump again. If the system is truly tight, the indicator ball will drop right back to the bottom. If it hesitates,
then settles, I know there is a leak. And that's what happened on the Ford. Gas caps are a common issue. I removed the cap and sealed off the filler neck. After retesting, I got all the indications
of a sealed system. I installed the cap again properly and tested one more time. Again, all tested well. Now, I have heard that some techs have comebacks on EVAP repairs, even though they had pressure-tested the system. That made
some sense. After all, we're using pressure to test a system that uses vacuum to test itself. Here's another use for Mode $06: repair verification. If we really want to verify a repair, we have to satisfy the demands
of our customer, in this case, the ECM. Our repair must meet the ECM's testing requirements or it will come back. I cleared the monitors and asked my customer to return in a few days for a quick recheck. You could also run the monitors
yourself by following the "drive-cycle" description found in the service information.
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