EVAP some more, what are you talking about?

The EVAP system is in charge of retrieving the fuel vapors from the tank and rest of the fuel system. These vapors are then re-introduced into the engine and burned. However, a leak to the EVAP will render it useless, so modern fuel control and emissions systems have the requisite of proving the viability of the EVAP system. So it all works here...



Fuel vapors are responsible for a great deal of the HC (hydrocarbon) pollution expelled by an internal combustion engine. It is estimated that as much as 40% of the HC is from leaks at the fuel tank and supporting fuel vapor lines. Back in the 70’s, legislation was passed to try and reduced some or all of these HC or fuel vapor leaks. The EVAP system was the answer to this problem.

The simplest EVAP system is composed of a charcoal canister, supporting fuel vapor hoses, and EVAP purge valve. The system works by absorbing the fuel vapors from the fuel tank after the engine has been shut down. The fuel vapors are actually absorbed by the charcoal canister. Charcoal is an absorbent material used in many filtering applications, including water filtration. It has the ability to absorb contaminants and pollutants. When an internal combustion engine is shut down, a great deal of fuel vapor is left inside fuel tank. The fuel tank is a big source of fuel vapor due to the fact that the fuel inside the tank vaporizes and expands, especially during hotter days. All these unburned fuel vapors need a place to go after the engine has been shut down. The charcoal canister is such a place. By directing all fuel vapors to the charcoal canister they are localized and stored for future reintroduction into the engine, after the vehicle is re-started. After the engine is re-started, the EVAP purge valve lets all the stored fuel vapors in the canister into the intake manifold to be burned as part of the combustion process.

NOTE: Newer fuel injection systems use a returnless (no return hose) fuel delivery system. With regular returned systems, the hot fuel goes back to the fuel tank. The hotter fuel inside the tank causes excessive fuel vapors that expand causing possible leaks. The returnless system addresses this problem by eliminating the return hose and therefore preventing any fuel from returning back to the tank. In essence the returnless fuel delivery system was created to address the ever-tighter EVAP emission laws.

The modern EVAP system however has total integration with the ECM. Newer vehicles actually have the EVAP operation characteristics already pre-programmed into the ECM’s memory. Every time the EVAP purge valve opens and closes it creates a small vacuum leak. The ECM takes such a lean condition into account. The result is that the ECM adds injection ON time to compensate. Every percentage of duty cycle increase of ON time for the EVAP purge valve is taken into account. If the canister is full of gas a rich condition on the other hand would create a fuel mixture control problem. This is because the ECM increases purge time but the O2 sensor goes high (rich) because of the extra fuel accumulated in the canister. As long as the extra fuel introduced to the engine from the canister falls within a pre-programmed limit no codes are set. If this condition then falls outside the maximum limit it will inevitably set a FUEL TRIM MAX LIMIT code With the newer OBD II compliant vehicles the need for a more precise control of fuel vapors has led to a more complex computer controlled EVAP system. The complexity arises from the need to monitor the system for very small leaks. In 1996 when ODB II started throughout the country the EVAP needed to detect a 0.040” fuel leak. Then a couple of years after that the specifications went down to 0.020” leak. Now some late model vehicles will need to detect a 0.010” leak. Such a small leak in the EVAP system calls for very tightly sealed components and difficult leak detection techniques. But with proper knowledge a correct diagnostic can be made faster and more precise. for the rich condition.

To present there are two basic EVAP diagnostic systems. The solenoid type, which uses solenoids to apply vacuum to the fuel tank and check the fuel tank pressure sensor for a signal drop (leak). And the LDP type, which uses an air pump operated by vacuum to pump air into the fuel tank and check for leaks by monitoring a reed switch.


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.


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.

• The ECM first looks for the proper conditions to be present before an EVAP test is performed. It checks that NO VSS, TPS, MAP, O2 sensor and MAF codes are not present.

• Engine cold but not bellow freezing (cold start).

• ECT and IAT are the same at start-up.

• Fuel level between 15% and 85%.

When all of these conditions are met, the test runs as follows.

1. The VENT solenoid is closed sealing the system. This prevents outside air front entering into the EVAP system.

2. The purge solenoid starts operating and draws a vacuum to the EVAP system.

3. After a pre-programmed vacuum signal is sent to the ECM by the fuel tank pressure/vacuum sensor, the ECM turns off the purge valve.

4. Since the system is totally sealed and vacuum is present, the ECM looks for a vacuum level drop over a preprogrammed period of time. A leak in the system will cause a faster than normal vacuum signal drop and therefore set an EVAP code. If the pre-programmed vacuum level is never achieved because of a large leak, a faulty code indicating a large leak is set.

In this system, a blocked vent solenoid or hose will always keep the EVAP system in constant vacuum and causes a faulty code to set. Diagnosing this system is also easier, since by simply activating the vent solenoid the system is totally sealed. Once a sealed EVAP system is obtained, further diagnostics can be performed to determine the nature of the leak.


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.


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 tool testing.

NOTEUnder 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 flammable.

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 canister.

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 longer.

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 properly.

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 minutes.

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 service port.

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.


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 your pocket.

• Seal the system by activating the vent solenoid (with scanner).

• 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 “run EVAP monitor” menu choice. Regardless of the system being checked, it is always a good idea to divide the system into sections. Once a particular section has been identified as having a leak, further tests can be conducted on that specific area to determine the exact location of the leak. The nature of EVAP system diagnosis involves a keen eye and very careful observation. The manufacturers mostly place the EVAP systems in odd and exposed places. This cause the hoses and seals to become hard and cracked due to moisture damage and so forth. Therefore a careful visual observation of the entire system should be the first step.

In the event that an OEM scanner with the ability to run the EVAP monitor is used, simply keep isolating each section (crimping the vapor hoses) and re-running the EVAP monitor. Every time the monitor is executed and a leak is found the ECM will set a code. When the leaky section is found and excluded (crimped off) off the system, no further codes will be set by the ECM. Warning: Remember to erase any codes set during the previous monitor execution. Some OEM scanners will not set a code, but will signal the EVAP leak on the test display screen itself. The important fact to remember is to keep isolating the different sections until the leak is found.

Important: When running the EVAP monitor on Chrysler vehicles, it is not possible to simply crimp off the fuel tank vapor hose, in order to eliminate the fuel tank volume. If this is done, the ECM will detect a rapid pressurization of the EVAP system. The reason for this is that since the fuel tank has been eliminated or crimped off, this also eliminates a great deal of EVAP system volume to be filled. The ECM will sense a crimped off fuel tank vapor hose as a kinked vapor hose and will abort the test. The solution is to disconnect the fuel tank vapor hose and re-route it to a sealed tank/container. A RED plastic fuel dispensing tank sold at any hardware store works quite good, so long as steps are taken to seal it. By doing so, the ECM is tricked into sensing the sealed tank/container volume as the normal fuel tank, which allows the EVAP monitor to run completely. Since the container is already known to be sealed, other sections of the EVAP system can be further isolated until the leak is found.


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