LDP EVAP SYSTEM
The LDP is a simple diaphragm pump which moves up-and-down
pushing air at the same time. A vacuum solenoid opens and closes at the top
of the LDP. The vacuum pulsation is what actually makes the diaphragm move
up and down. Also at the top of the LDP is a small reed switch that sends an
ON/OFF signal to the ECM. As the vacuum solenoid pulses and the LDP
diaphragm pumps air, the reed switch signal toggles to ON/OFF. The ECM uses
this signal to determine when the EVAP system is pressurized. Once the EVAP
system (fuel tank and related components) is pressurized, the LDP diaphragm
will stay in the up position due to the actual EVAP system pressure, which
make it very hard for the LDP diaphragm to move down. At this point the reed
signal will not toggle any more and the ECM will see this as a properly
pressurized EVAP system.
The longer it takes for the LDP reed switch to stop
toggling, the bigger the leak. If the leak is too large, the reed switch
will never stop toggling and the ECM will see it as a massive EVAP leak. The
whole principle behind this system is centered around the monitoring of the
LDP reed switch frequency. A fast toggling or switching (high frequency)
reed signal means that the LDP is pumping air into the fuel tank and
canister. A slow toggling or switching reed signal (low frequency) means
that the system is pressurizing. If the signal stops switching, then this
means that the system is fully pressurized. Picture the EVAP system as a
balloon, when it is fully filled or pressurized the LDP or air pump can not
pump any more air into it and the reed stops toggling.
SOLENOID EVAP SYSTEM
The solenoid based EVAP system has the same purpose as the
LDP, which is to detect EVAP leaks. The solenoid type, however, uses vacuum
switches, solenoids and pressure sensors to detect leaks. This system being
simpler in nature is also simpler to diagnose.
NOTE: By closing the vent solenoid the system is sealed, and
the signal is monitored for a pre-programmed amount of time for any signal
drop. A quick vacuum signal drop from the fuel tank pressure sensor
indicates a large leak.
Early systems from GM and others used a vacuum switch, also
called a purge flow sensor, between the purge valve and the canister. The
vacuum switch is normally closed to ground and would open as soon as the
purge valve went above a 25% duty cycle or 5” of water of vacuum. A switch
that remained closed after 25% of purge duty cycle indicated a massive EVAP
leak, which would cause the ECM to set an EVAP code. The system was simple
and easily detected the purge valve operation.
A more sensitive and tight leak detection system was needed
as OBD II progressed. To address this problem, a vent solenoid and a
fuel tank pressure sensor were added to the system. The vent solenoid
made it possible for the ECM to seal the EVAP system completely.
The system works as follows.
CONDITIONS THAT AFFECT OPERATION
Leaks are the main source of EVAP failures. By definition
the EVAP diagnostic system is meant to detect leaks. The source of problems
can vary from deteriorated and cracked vapor hoses, defective fuel tank cap,
faulty seals and sensors, broken fuel tank to a leaky purge valve. A
thorough visual inspection plays a major role in proper EVAP diagnostics.
The combination of careful observation and clever utilization of test
equipment is key to a fast repair.
COMPONENT TESTING (non OEM scanner)
The testing procedures presented here differ from the
standard way of testing. The standard way uses an EVAP smoke machine with a
calibrated flow tube. The EVAP machine works for some but it is not the only
way to test the EVAP system nor is it the fastest and the machine itself can
be expensive to purchase. Any reference to the scan tool, in this section,
is presumed to be an aftermarket unit. The next section deals with OEM scan
NOTE: Under no circumstance should you
introduce smoke from a smoke machine straight into the fuel tank as this
could cause a fire. Smoke machines use a glow plug as the heat source to
vaporize the smoke solution (oil). Always be aware that fuel vapors are
This procedure uses regular shop air or nitrogen to slightly
pressurize the system. It is a good idea when testing the EVAP system to
divide the system into four parts. By doing so, the leak can be traced to a
specific area. Start by isolating the four areas.
1. The fuel tank and associated components.
2. The charcoal canister and related solenoid and hoses.
3. The purge valve at the engine compartment.
4. The LDP (if present) in the vehicle being tested.
LDP type EVAP leak testing
1) “STEP 1 tests proper purge valve seal.” Apply
vacuum with a hand vacuum pump to the EVAP purge valve. Actuate or pulsate
the purge solenoid valve using a scan tool, power probe or an oscillator (pulser)
and check for proper purge valve sealing. The valve should hold the vacuum
when off and release the vacuum when on.
2) “STEP 2 tests the REED SWITCH.” Actuate (ON/OFF)
the LDP solenoid using a scan tool or power probe and look for a signal
toggle at the LDP reed switch. The test should be done while applying vacuum
to the LDP solenoid with a hand vacuum pump or using engine vacuum (engine
on and using a scanner).
3) “STEPS 3 to 6 test the LDP vent valve & canister for
leaks.” Disconnect the line to the canister purge solenoid (from
purge valve to canister). Apply about 2 to 5 PSI of air pressure (use
EVAP port if present).
4) Activate the LDP vacuum solenoid, using a scanner or
power probe (ground the solenoid).
5) Apply vacuum to the LDP solenoid to seal the vent valve.
The system should start to pressurize.
6) Check for air leaks at the LDP, fuel tank and the
canister as well. (If air leaks at the LDP vent hose, then the LDP vent
valve is defective). The LDP is replaced as a unit. It is also a good idea
to use an ultrasonic leak detector and listen for the high frequency hiss
caused by an air leak. These units are not costly and make for a great leak
detection tool. Also, by dividing the EVAP system into sections, the
different sections can be tested independently by crimping the particular
hose going to the section in question. Warning: Flammable vapors present.
Do not smoke while performing this operation.
7) Disconnect the vapor line going to the fuel tank from the
8) Attach a vacuum T to the tank side vapor line.
9) Attach a low pressure gauge to the remaining T port.
10) Using shop air or a small hand air pump, pressurize the
fuel tank to about 2 to 5 PSI, as previously mentioned. (remember that the
LDP only delivers ¼ PSI, 2 to 5 PSI is used because of the inherent
difficulty in measuring such a low pressure). Take pressure readings
different sections and determine which one is leaking.
11) Watch carefully for any pressure drop which would
indicate a leak. If a leak in found at the tank, then the it would have to
be lowered and inspected.
12) Finally once the system is repaired the entire system
can be pressurized with air and the system can be monitored for any pressure
drop. A sealed system should keep the pressure for 5 to 10 minutes or even
The last six steps check for leaks at the fuel tank. A
pressure technique is used here to avoid the introduction of smoke into the
fuel tank. Before running the test make sure that the fuel cap is working
NOTE: An electronic pressure sensor in preferable
to a mechanical one. A graphing multi-meter (like Snap-On’s Vantage or
Fluke) is a great solution for diagnosing these problems. A graphing meter
can be set to measure for the duration of the test and the pressure drop can
be plotted on the screen. It is also possible to use vacuum, instead of air
pressure, to perform the previous testing procedure. In which case, the
system should hold about 2 to 4 in Hg (inches of mercury) for at least 5
Following these steps and careful observation will lead you
in the right direction. Once the leak is traced to one of the before
mentioned areas, it will only be a simple matter to conduct further visual
checks to pinpoint the leak.
Solenoid type EVAP leak testing:
The solenoid type EVAP leak diagnosis is somewhat easier to
perform than the LDP system. The basic steps are as follows.
1) Tests proper purge valve seal exactly as with the LDP
type. Apply vacuum with a hand vacuum pump to the EVAP purge valve. While
commanding the purge solenoid valve shut using a scan tool or voltage
injector check for proper purge valve sealing. The valve should hold vacuum
when off and release the vacuum when on.
2) Test the vent solenoid. Disconnect the vent
solenoid vapor hose and apply vacuum with a hand vacuum pump. While
commanding the vent solenoid shut, monitor the vacuum gauge for any drops,
which would indicate a leak solenoid. When activated, the solenoid should
keep vacuum which indicates a sealed system.
3) Seal the EVAP system. Using a scan tool or a power probe,
ground or command the vent solenoid on.
4) Pressurize the EVAP system using shop air or a handheld
vacuum pump. If it is necessary, T off the purge hose otherwise use the EVAP
5) Using a low-vacuum/pressure gauge or an electronic
pressure sensor, monitor the air pressure drop from the EVAP system. A
sealed system should hold air pressure or vacuum for 5 minutes or more. If
not keep isolating the EVAP system sections until the leaky section is
found. Then perform final manual tests on the defective section.
This procedure uses either pressure or vacuum to test the
system. The EVAP system can hold either and an EVAP leak will show itself
with either (vacuum or pressure).
NOTE: On many late model OBD II and few OBD I
systems, there is a scan tool EVAP seal test that can be performed saving
valuable time. The scanner test menus are somewhat different from one
manufacturer to another.
SCAN TOOL TESTING
Testing the EVAP system with the scanner is by far the
fastest way to test this system. However, this is what separates the OEM
scanners from the lesser aftermarket ones. Most OEM scan tools have
extensive provisions for EVAP testing. In fact, even the EVAP monitor can
also be activated with the OEM tool. The ability to run the EVAP monitor and
to actuate the different solenoids (purge, vent, LDP, etc) is a great
diagnostics help. All of this simply translates to time saved and money in
• Seal the system by activating the vent solenoid (with
• Start the engine and purge the system, using the scanner.
By purging the system, all the fuel vapors are removed from the system.
• Keep monitoring the fuel tank pressure/vacuum sensor (if
available) or connect your own to the system. The purge line that goes to
the canister is a good place to tap into for pressure/vacuum reference and
so is the EVAP port (green cap).
• After a minute or so the vacuum in the EVAP system should
be down to 8 to 10 Hg. Do not let the vacuum go beyond 10 Hg to prevent
damage to the system. If the leak is too large this level of vacuum will
never be reached. Some scanners will not allow excessive vacuum to the EVAP
system; the test will simply be aborted.
• When low vacuum is achieved in the EVAP system, shut down
the purge valves. The system should hold vacuum for at least 5 minutes. If
not, there is leak and the a process of elimination should be applied. Keep
isolating the three main EVAP sections by crimping the vapor lines until the
leak is found.
This is basically the same procedure as previously
explained, but it is all done with the scan tool. In many cases the entire
test is done automatically, with a built-in