# EVAP purge valve voltage and fuel tank pressure?



## Bigplanz

Having more fun with FORScan. I graphed the voltage of the EVAP purge valve solenoid with fuel tank pressure, to see if there is a relationship. Apparently, there is, as per the screen shot below.

I was curious about EVAP voltage changes under real driving conditions. Here’s what I found: 

EVAP purge valve solenoid voltage read the same as alternator voltage most of the time. On the EVAP graph you can see the highest voltage registered during the 6 mile trip was 14.62 V and the lowest was 5.95 V. I assume the voltage drop corresponds to the valve opening and purging the canister. 

Notice Fuel Tank Pressure. It read a consistent -0.1 PSI, except then the purge valve voltage went down. You can clearly see the relationship in the graph. Fuel Tank Pressure goes to -0.2 PSI when the voltage from the EVAP valve went down, then returned to -0.1 PSI when the voltage went back up.

I assume this is a normal relationship, since the SUV runs fine and no codes of any kind have ever been thrown. Thought some of you might find this information interesting. :smile:


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## Brainbucket

Having fun with your new toy I see:vs_karate:. Here is a piece I copied and pasted.:vs_cool:

Enhanced Evaporative
Emissions Systems
One of the most misunderstood systems
found on Ford’s OBD-II vehicles is the
Evaporative Emissions System. This is
generally due to the lack of available
information.
The Enhanced Evaporative Emissions
(EVAP) system uses two solenoids and a
pressure sensor mounted on or near the
fuel tank. The first solenoid is called the
Vapor Management Valve (VMV); the second
is the Canister Vent Solenoid (CV). The
pressure sensor is called a Fuel Tank
Pressure (FTP) sensor.
The VMV, a normally closed solenoid, has
three hoses and two wires going to it. Two of
the three hoses are larger in diameter,
usually 3/8”. The third hose is smaller,
usually 3/16”, and usually has manifold
vacuum on it. One of the wires has battery
voltage with the key on and the other is
grounded by the PCM to energize the
solenoid. For the valve to allow vacuum into
the EVAP system, the PCM has to energize
the solenoid by grounding the electrical part
of it, and also has to have vacuum applied to
the small control vacuum port.
The CV is a normally open solenoid, with
two wires going to it. One wire is battery
voltage; the second goes back to the PCM to
be grounded. Some systems also run a vent
hose from the solenoid up to a higher point
(the most common is up to the filler neck
area) to be open to atmosphere. Generally,
the solenoid itself is mounted directly to the
charcoal canister. When the solenoid is at
rest, the charcoal canister should be able to
vent to atmosphere. When the PCM wants to
check the system for leaks, it energizes the
CV solenoid, which should close off the
canister from venting and seal the system.
The FTP sensor works very much like a
typical Manifold Absolute Pressure (MAP)
sensor. It is mounted either in the top of the
fuel tank or in the vapor line going from the
VMV to the fuel tank and charcoal canister.
When the PCM commands a vacuum into the
system to check for leaks, the voltage
changes according to the amount of
pressure, or in this case, negative pressure
in the system.
An input found on some of the newer
enhanced systems is a fuel level input to the
PCM, which allows the PCM to adjust its
bleed off rate calculation based on the
amount of fuel in the tank.
For the PCM to test the system
,
certain criteria must be met: 1) Engine is
running; 2) Not overheating; 3) Is at idle or
part throttle; and 4) Is either in open or
closed loop (loop status will vary per
calibration). Once all these criteria have
been met, the PCM will start to duty cycle
the ground on the CV solenoid to close off
the system from atmosphere.
After the CV solenoid is energized and the
system should be sealed, the PCM will start
to duty cycle the ground on the vapor
management valve. If everything is working
correctly, this will create a negative
pressure, or vacuum, in the EVAP system.
The FTP sensor voltage should drop as the
pressure decreases. Once the PCM has
determined that there is enough vacuum in
the system (typically 7” H
2
0), it will
de-energize the vapor management valve
and look for the voltage on the FTP sensor
to change.
If the system is sealed properly, the
voltage should remain about the same
until the PCM de-energizes the canister
vent solenoid. If there is a leak, the voltage
will start to increase while the CV solenoid
is still grounded. The PCM knows how
large the leak is, depending on how much
and how fast the voltage changes. Leaks as
small as 0.040” will cause a code to set. A
code P0442 will set if there is more than
about 2.5” H
2
O change in 15 seconds at
approximately 75% fuel tank level.
Starting in model year 2000, some
calibrations actually started checking for
leaks as small as 0.020”. A code P0456
will set if there is more than 2.5” H
2
O
change in 45 seconds at approximately
75% fuel tank level. The amount of bleed-
up allowed will vary again based on how
much fuel is in the tank. A code P0455
will set if there is more than 7” H
2
O
change over 30 seconds. There are other
codes related to the EVAP system, but
these are the most common.
Hopefully this information will give you
a better understanding of the EVAP
systems and how to test them when they
fail.


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## Bigplanz

Brainbucket said:


> Having fun with your new toy I see.
> 
> I am totally geeking on it!
> 
> Did some more testing this morning. I ran the EGR duty cycle, Engine RPM and Engine Coolant Temperature. Here is a graph. Since it's blurry in the screenshot, I wrote the values in blue. The horizontal lines are the three sensor readings. The vertical line is the instant the reading was taken. Click anywhere on one of the horizontal lines and the vertical line comes up, so you can look at the graph, click a spot and see an instant comparison of the three variables.
> 
> EGR duty cycle: 52.05% (valve open by this percentage)
> RPM: 1961
> ECT: 196
> 
> This is really interesting and valuable information, if you know how to interpret it. The example above shows (I think) normal operation: engine at full temperature, throttle partly open, EGR venting exhaust gas back into the intake manifold to cool the heads and reduce NOX emissions. At idle, EGR closes, partial throttle, EGR opens.


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