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EVAP
For Client ReviewOnly. All RightsReserved. Advanstar Communications Inc. 2005
CODE
BLUES
UNDERSTANDING HOW FORD, GM AND CHRYSLER ENHANCED
ON-BOARD EVAP MONITORS WORK IS JUST AS IMPORTANT AS USING
THE PROPER TOOLS TO TROUBLESHOOT THEM.
BY JACQUES GORDON, TECHNICAL EDITOR
IN THE NEWEST GENERATION OF ENHANCED EVAPORATIVE EMISSION CONTROLS, THE PCM CAN MONITOR
EVERY COMPONENT AND FUNCTION OF THE EVAPORATIVE EMISSION (EVAP) SYSTEM. THIS IS GOOD NEWS,
BECAUSE IT MEANS THE POWERTRAIN CONTROL MODULE (PCM) CAN PROVIDE USEFUL DIAGNOSTIC
INFORMATION FOR WHAT MAY BE THE MOST DIFFICULT SYSTEM TO TROUBLESHOOT. EVEN BETTER, NOW
THERE IS A SERVICE PORT THAT MAKES IT EASIER TO SEARCH FOR LEAKS USING SMOKE. BUT IN ADDITION
TO TOOLS AND TRAINING, TROUBLESHOOTING EVAP CODES ALSO REQUIRES AN UNDERSTANDING OF HOW THE
SYSTEM IS SUPPOSED TO WORK. SO HERE GOES . . .
BACKGROUND
The enhanced evaporative emission control system is a
result of California emissions regulations that went into
effect for the 2000 model year. Most new vehicles sold in all
50 states met those requirements that first year, and today
they all do. You can tell the difference by looking for a serv-
ice port cap under the hood, sometimes marked “EVAP.”
Like the earlier systems, the enhanced EVAP system cap-
tures fuel tank vapors and sends them to be burned in the
engine. The two “enhancements” refer to the on-board
diagnostic (OBD) system’s ability to detect leaks as small as
0.020 of an inch (0.5mm) and to monitor canister purge
flow. The former is required by law to monitor the health of
the EVAP system, and the later is needed by the PCM to
control tailpipe emissions and preserve driveability during
canister purge. There are several different ways to accom-
plish these tasks, and of course the automakers have used
them all. Fortunately, they’re not hard to understand.
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There are two different ways for an
OBD system to leak test an enhanced
EVAP system: pressurize it or put it
under vacuum. Each has its advan-
tages. Vacuum testing is mechanically
simpler, requiring only two solenoid
valves and a fuel tank absolute-pres-
sure sensor. But because the vacuum
comes from the intake manifold, the
drive cycle (the engine operating con-
ditions needed for the test) is very pre-
cise, so there’s no guarantee the test
will actually run during any given trip.
Leak-testing with pressure requires
a pump and extra plumbing, but the
drive cycle is less exacting. Ford
Motor Co., General Motors (GM) and
some Asian models use vacuum tests,
while DaimlerChrysler, some Asian
and most European models use pres-
sure tests.
In either case, leak testing an EVAP
system requires sealing the system at
each end. The purge control solenoid
valve is used to seal the system at the
intake manifold end, and another sole-
noid valve seals the canister’s fresh air
inlet. This canister vent valve is nor-
mally open, and on several models a
clogged fresh air inlet has caused problems that turn on the
malfunction indicator lamp (MIL), but more on that later.
VAPOR MANAGEMENT VALVE
(NORMALLY CLOSED)
CONTROL
VACUUM SOURCE
INTAKE
MANIFOLD
For Client ReviewOnly. All RightsReserved. Advanstar Communications Inc. 2005
ENGINE
PCM
WIRES
FROM PCM
FROM
ATMOSPHERE
FUEL TANK
PRESSURE SENSOR
EVAP
CANISTER
VENT SOLENOID
(NORMALLY OPEN)
FUEL TANK
EVAP CANISTER
Ford and General Motors EVAP systems have a similar layout. In this Ford
schematic, the purge valve is called the Vapor Management Valve.
the test if fuel tank pressure changes by only 1 inch of water
during the vapor check. It takes more pressure than that to
blow out a match. Testing for such small changes requires
very stable vehicle operating conditions, aka enabling con-
ditions, which is why the small-leak test (0.020 inch leak) is
so difficult for the PCM to run.
Although each model of car has very specific test require-
ments, the following enabling conditions will give you an
idea of how the PCM uses vacuum to leak-test the EVAP
system. These are general enabling criteria; they are not a
complete list of what is needed to actually run the test:
• The PCM needs to know
fuel temperature.
But unless
there is a fuel tank temperature sensor, the test runs after
cold-start when the coolant temperature is near ambient,
indicating the engine has not been running for a long period
of time and returning warm fuel to the tank.
• The test requires a
steady speed and load condition.
GM’s first small-leak test and Ford’s small-leak validation
(second) test require the engine to be at idle with zero or
near-zero vehicle speed. The idea is to test with steady man-
ifold vacuum and no fuel tank sloshing.
•
Ambient temperature
must be between 40°F and
100°F so the fuel tank vapor pressure remains stable during
the test. On some models, the upper limit is only 86°F.
•
Fuel level
must be between 15 percent and 85 percent,
and the PCM must be getting a stable signal from the tank
VACUUM TESTING
The vacuum leak test used by Ford and GM has two
phases – large-leak and small-leak detection. In both phases,
the PCM will close the vent valve, pulse the purge valve to
draw a vacuum on the system and monitor a pressure/vacu-
um sensor in the fuel tank. The PCM is looking for how
quickly the vacuum builds to a predetermined level, and
then with both valves closed, how quickly it decays. The
basic difference between the large- and small-leak tests is the
amount of time for the test, and, of course, different algo-
rithms are used to calculate the results.
Assuming all the enable criteria have been met, the first
step in the test is to calculate how much fuel vapor is being
generated in the tank. The PCM does this by closing both
the purge and vent valves and looking at the tank pressure
sensor. If the rate of vapor generation (tank pressure rise) is
too great, the PCM may draw a slight vacuum on the tank
and/or purge the system to “precondition” it, and then
check again. If the PCM still thinks fuel tank vapor pressure
is rising too quickly for a valid test, it will abort the test.
Along with fuel sloshing and heat, vacuum in the fuel
tank makes fuel evaporate, so the deepest vacuum draw is
rarely more than 10 inches of water. The PCM may abort
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Motor Age
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Illustration: Michael Stassus. Source: Ford Motor Co.
INTAKE AIR PLENUM
THROTTLE BODY
For Client ReviewOnly. All RightsReserved. Advanstar Communications Inc. 2005
level sender so it can calculate how long it
should take for vacuum to build and decay. On
some models, if fuel level increases between
engine starts or while the engine is running,
which indicates the vehicle has been refueled, it
may run a large-leak test right away to see if the
filler cap has been tightened properly.
•
Barometric pressure
must be above 22
in.Hg on Fords and 19 in.Hg on GM vehicles.
Put another way, altitude must be less than
8,000 feet or 10,000 feet respectively.
There’s one other thing needed for the EVAP
monitor to run: There must be no engine fault
codes. There are two reasons for this. First,
while the EVAP system is important for reduc-
ing overall vehicle emissions, controlling engine
emissions is more important, so engine malfunc-
tions have priority.
The second reason has to do with the way the
PCM detects and controls canister purge flow.
Earlier Fords use a sensor to detect temperature
drop in the hose between the purge control
valve and intake manifold when purge air is
flowing. The earlier GM system uses a vacuum
switch between the purge control valve and the
EVAP canister. Neither of these devices can
measure purge flow; they only confirm that flow
is or is not happening as commanded. But with
the enhanced systems, the PCM can calculate purge flow
using data from the MAF sensor or by comparing MAP
sensor readings with the tank pressure sensor reading.
The amount of fuel vapor in the purge flow can also be cal-
culated using the fuel trim correction data.
LEAK DETECTION
PUMP (LDP)
LDP SOLENOID
EVAPORATIVE
CANISTER
LDP
VENT
VALVE
EVAPORATIVE
SYSTEM AIR FILTER
SERVICE PORT
EVAPORATIVE
PURGE ORIFICE
(MODEL SPECIFIC)
TO FUEL TANK
EVAPORATIVE
PURGE SOLENOID
Chrysler’s EVAP system uses manifold vacuum to power the LDP,
controlled with the LDP solenoid.
LARGE-LEAK TEST
The large-leak test always runs first. With the vent valve
closed, the purge valve is pulsed to pull a target vacuum on
the system.
• If the target vacuum cannot be reached and fuel level
has not increased, the PCM will store a hard code and turn
on the MIL.
• If the target is reached but vacuum will not hold, the
PCM will store codes indicating a large leak, but it won’t
turn on the MIL until it fails the test a second time.
• If the vehicle has been refueled since the last engine-
start or during this drive cycle (in-flight refueling) and the
target vacuum cannot be reached, the PCM will assume the
cap has not been tightened properly. It will set a pending
code and turn on the MIL or a “Check Fuel Cap” light. If
the system passes the next large-leak test, it will clear the
code and turn off the light.
• If the target vacuum is surpassed quickly, the PCM will
assume a plugged vent hose or faulty vent valve. If this hap-
CHARCOAL CANISTER
Fuel tank vapors are stored in a charcoal canister, sometimes called a
carbon canister. Charcoal is made by heating wood to about 1,000°F in
a sealed container. With no oxygen, the wood can’t burn, but the volatile
organic compounds will be driven off, leaving nothing but pure carbon.
After it cools, the charcoal granules are sealed in another container
and pressurized with oxygen. This opens tiny pores in the carbon mole-
cules, creating what’s called “activated charcoal.”
Hydrocarbons of all kinds, especially vaporized gasoline, will stick to
activated charcoal like a balloon sticks to the wall after rubbing it on a
wool shirt. Air moving across the charcoal can strip the hydrocarbons
away again, just like blowing the balloon off the wall.
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Illustration: Michael Stassus. Source: DaimlerChrysler
For Client ReviewOnly. All RightsReserved. Advanstar Communications Inc. 2005
pens over two tests in a row, it will set a hard code and turn
on the MIL.
• If vacuum increases after the purge valve is com-
manded closed, the PCM will assume a leaking purge valve.
If this happens over two tests in a row, it will set a hard
code and turn on the MIL.
time all the other enable criteria are met.
• In either case, if the next test is passed, the pending
code will be erased. On some models, it must be passed
twice in a row. If the second test fails, the PCM will set a
hard code and turn on the MIL.
Steady conditions are needed for the small-leak test, and
most GM vehicles will only run it at idle. On all vehicles, if
vehicle speed, engine load or fuel tank pressure changes
too much during the test, indicating fuel slosh, the test will
be aborted. The PCM keeps track of the number of aborts,
and Ford’s Mode $06 data will display zeros in each Test
ID that did not complete due to an aborted test.
SMALL-LEAK TEST
The small-leak test takes longer. After completing the
large-leak test, the PCM monitors and records the time it
takes for vacuum to decay or bleed up to a second target
level. The acceptable bleed-up time is calculated anew for
each test to account for conditions that generate fuel tank
vapors, such as ambient temperature and fuel level.
• If measured bleed-up time is between the calculated
minimum and maximum, the test passes. The test must be
passed twice for the monitor status to indicate “complete.”
• If bleed-up time is shorter than the minimum allowed,
the test is failed. A pending code is stored and the test is
repeated at idle the next time all the other enable criteria
are met.
• If bleed-up takes too long, the PCM assumes a leaking
purge control valve or a blocked vent valve or tube. It will
store a pending code and repeat the test at idle the next
PRESSURE TESTING
Since 1996, Chrysler has been leak testing the EVAP sys-
tem with air pressure generated by an on-board pump.
This has several advantages, the greatest being that because
manifold vacuum plays a different role in the test, the drive
cycle is much simpler. Manifold vacuum is used only to
power the on-board Leak Detection Pump (LDP), and is
controlled with a solenoid valve. Here’s how it works:
• The PCM opens a vacuum solenoid valve inside the
LDP, admitting vacuum to the top of the LDP diaphragm.
• As the diaphragm is pulled up against a spring, filtered
air is drawn through a check valve and into the cham-
ber below the diaphragm.
• The PCM closes the solenoid valve, venting vacu-
um above the diaphragm and allowing the diaphragm
spring to push the diaphragm back down.
• The air under the diaphragm is pushed through
an exit check valve and into the EVAP system through
the canister.
• The PCM continues cycling the vacuum solenoid
valve to operate the pump, but the diaphragm stops
moving and is held in the full-up position when EVAP
system pressure equals diaphragm spring pressure.
The diaphragm spring exerts a pressure of only 7.5
in.H
2
O, about 0.25 psi, and that’s the pressure used to
leak-check the EVAP system. When this pressure is
reached, the PCM stops cycling the vacuum solenoid
valve. How does it know when the system is pressur-
ized? The key to the whole EVAP monitor system is
the reed switch inside the LDP.
Above the diaphragm is a simple on-off reed switch
that the PCM uses to monitor the position of the
diaphragm. The switch is open when the diaphragm is
up and closed when the diaphragm is down. When the
reed switch stays open, the PCM knows the system is
EVAP Codes
P0441:
INCORRECT OR UNCOMMANDED PURGE FLOW (PURGE VALVE LEAK)
P0442:
SMALL LEAK
P0443:
PURGE SOLENOID ELECTRICAL FAULT
P0446:
BLOCKED CANISTER VENT (HIGH SYSTEM VACUUM)
P0449:
CANISTER VENT SOLENOID ELECTRICAL FAULT
P0452:
TANK PRESSURE SENSOR VOLTAGE LOW
P0453:
TANK PRESSURE SENSOR VOLTAGE HIGH
P0454:
TANK PRESSURE SENSOR VOLTAGE NOISY
P0455:
LARGE LEAK (LOW SYSTEM VACUUM)
P0456:
VERY SMALL LEAK
P0457:
GROSS LEAK (CAP OFF)
P0460:
FUEL LEVEL CIRCUIT
P0461:
FUEL LEVEL SENSOR STUCK/NOISY
P0462:
FUEL LEVEL SENSOR VOLTAGE LOW
P0463:
FUEL LEVEL SENSOR VOLTAGE HIGH
P0464:
FUEL LEVEL SENSOR VOLTAGE NOISY AT IDLE
P1443 (FORD):
GROSS LEAK, NO FLOW
P1450 (FORD):
EXCESSIVE VACUUM
P1451 (FORD):
VENT VALVE CIRCUIT FAULT
P1486 (CHRYSLER):
PINCHED HOSE
P1494 (CHRYSLER):
LDP FAULT
P1495 (CHRYSLER):
LDP SOLENOID CIRCUIT
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For Client ReviewOnly. All RightsReserved. Advanstar Communications Inc. 2005
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pressurized and will stop cycling
the vacuum solenoid valve. At this
point, the PCM measures the
amount of time it takes for the
switch to close again, meaning the
system pressure has fallen below
diaphragm spring pressure. If the
system builds and holds pressure
in the correct amount of time, the
test is passed.
The PCM calculates the correct
amount of bleed-down time before
beginning each test, based on con-
ditions similar to those used by
Ford and GM. Enable conditions
for the leak test are:
• Cold start.
• Ambient temperature between
40°F and 86°F.
• Fuel level between 30 percent
and 85 percent for small-leak test;
15 percent to 85 percent for large-
leak test.
• Vehicle speed less than 35
mph.
• Engine run time greater than 10 seconds.
• Battery voltage between 10 and 15 volts.
• MAP (BARO) sensor reading 22 in.Hg or more.
As you can see, while the drive cycle is not as critical, it’s
still important to avoid generating fuel tank vapor pressure.
That’s why more and more vehicles are built with a return-
less fuel system.
There are two phases to the leak test. First the PCM
measures the amount of time needed to pressurize the sys-
tem. If pressure never builds, a gross leak is assumed. If
pressure builds within a few pump strokes, this indicates a
pinched or plugged hose between the LDP and the canis-
ter. Next, it will measure the size of the leak by measuring
the amount of time between turning off the vacuum sole-
noid valve and the reed switch closing as the diaphragm
falls. In general, it takes 1.2 seconds to pass the large-leak
test and six seconds to pass the small-leak test.
The Leak Detection Pump also includes the canister
vent valve. The diaphragm still has a bit of downward travel
left after the reed switch closes. If the PCM doesn’t open
the vacuum solenoid valve to pull the diaphragm up again,
the diaphragm will continue down and bottom out. At this
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The key to Chrysler’s LDP is the reed switch. When the diaphragm is all the way
down, the vent valve is pushed open.
1. To canister
2. One-way check valve outlet
3. Vacuum from intake manifold
4. Reed switch
5. Pressure/vacuum solenoid
6. Diaphragm spring
7. Pump cavity
8. Diaphragm
9. One-way check valve (inlet)
10. Vent valve
11. To air filter
point, a shaft attached to the bottom of the diaphragm
pushes the spring-loaded canister vent valve open. That
means the vent valve is closed only when the LDP
diaphragm is pulled up with vacuum.
It’s easy to see from the enabling criteria that cars driven
in extreme hot or cold weather may not run the EVAP
monitor for months, and cars that never leave the high
country might never test the system at all. With a bidirec-
tional scan tool, the purge and vent valves can be com-
manded closed for testing with smoke, and on any vehicle a
scope or graphing meter can be used to monitor the tank
pressure sensor or LDP reed switch during a drive cycle.
This can be helpful in states that use the vehicle’s own on-
board diagnostic system for emissions inspection. Just
remember that the PCM is looking for stable testing condi-
tions, correct operation of the solenoid valves and/or LDP,
and the time needed to build, hold and release system pres-
sure or vacuum.
READER INTEREST SURVEY
Circle the appropriate number on the Reader Service Card to indicate your level of
interest in this article.
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Illustration: DaimlerChrysler
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