Understanding This System, and Testing Tips
It can be overwhelming to keep up with the many different methods auto manufacturers use to detect EVAP system leaks. It seems like every time I really start to understand one of these EVAP systems, the manufacturer completely changes it. While manufacturers may use the same system for several years, if you work on every make and model you will see different systems every day. Keeping up and keeping them all straight is a challenge. Hopefully, you can stash this article in your toolbox and review it the next time a Chrysler ESIM (evaporative system integrity monitor) system comes into your bay.
Chrysler has used relatively few EVAP leak detection methods. Starting in 1996 with OBDII, Chrysler used a Leak Detection Pump or LDP. These were different from most system leak test strategies as they used pressure instead of vacuum. The pump was not something most of us think of when we hear the word pump. They used a solenoid to pulse manifold vacuum to one side of a diaphragm. On the other side of the diaphragm were two check valves, one allowing air into a chamber but not out and the other allowing the air out to the EVAP system which was then pressurized.
Starting in 2002 Chrysler switched to an NVLD or Natural Vacuum Leak Detection. The change was made in an attempt to reduce the number of parts used and simplify operation. The NVLD systems were normally sealed with a solenoid that would allow the system to vent when controlled. They also contained pressure and vacuum relief valves that opened at very low pressures. Inside this unit was a switch that made contact when a vacuum was present in the system allowing the PCM to detect leaks.
Then, again in an effort to reduce the number of parts and simplify operation, in 2007, Chrysler switched to an Evaporative System Integrity Monitor or ESIM which is still being used today.
If you work on Chryslers, you have likely come across these ESIM systems. After all, they have been around for more than 14 years. When it comes time to diagnose an EVAP system leak or other EVAP monitoring faults, you will need to understand the ESIM operation. When referring to the description and operation in service information, you might think you are working on a spaceship. It almost seems like the intent of the author was to confuse you, not help you. Over the years, I have been asked by many technicians to help them with diagnosing failures in these systems. When I ask them if they understand the operation of the ESIM, it becomes apparent that they do not. Some almost believe the ESIM is a small plastic box containing magical parts and top-secret advanced level technology. But, at a cost of around $20, how sophisticated or complex can these really be? After looking inside an ESIM then reviewing the description and operation, we can start to understand just how simple this little component really is. Let’s take a look.
Basics Of EVAP System Operation
First, it might be helpful to quickly review how an EVAP system functions and what the purpose of the ESIM is.
Looking back at early EVAP systems operation helps me to understand modern systems. The goal of all EVAP systems is to prevent Hydrocarbons (HCs) from being released into the atmosphere. The problem is we can’t just seal off the fuel tank. We have all seen a gas can expand on a hot day or suck in when cold. Also, since we are going to be pulling fuel from the tank into the engine, air will need to be let in to displace it.
Early EVAP systems are now referred to as “non-enhanced;” this is to say they have no leak detection capabilities. If there is a fault in the system, for example, a broken charcoal canister or split hose, the driver will likely never know or have any reason (i.e. check engine light) to have the system repaired. These systems had a hose from the sealed fuel tank that connected to the charcoal canister. Fresh air was allowed in through the charcoal canister when fuel was being consumed or when fuel temperature dropped in order to avoid negative pressure (vacuum) in the fuel tank. When filling the fuel tank or when the fuel tank temperature increased, the vapors in the tank were allowed to escape through the charcoal canister which would “trap” the HCs. (See Figure 1) Then, a hose connected to a purge valve leading to the intake manifold would allow the manifold vacuum to pull fresh air through the charcoal canister along with the stored HCs into the engine to be burned. This prevented HCs from being released into the atmosphere. (See Figure 2) Many carbureted models used valving inside the carburetor as opposed to a purge valve.
The next iteration of EVAP systems are referred to as “enhanced systems.” This means they have some sort of leak detection capabilities.
Moving forward to Chrysler’s ESIM systems
These EVAP systems still function the same way as their predecessors. They still use a charcoal canister to contain or trap HCs, and a purge valve to allow those HCs into the intake manifold when ready. The difference is in the leak detection methods … that is the whole purpose of these ESIM assemblies ... to detect EVAP system leaks. First, let’s look at what is inside these ESIM assemblies. Then, how Chrysler uses them to determine leaks. Finally, we can review some common faults you may see and how to test for them.
Chrysler ESIM Parts and Operation
So what’s inside the ESIM? Take a look at these pictures, you might be surprised to see there are really only 5 parts that make up this unit.
The case or housing (See Figure 3)
A vacuum-operated contact switch (See Figure 4)
The diaphragm used to seal and control the contact switch (See Figure 5)
Small weighted vacuum valve (See Figure 6)
Large weighted pressure valve (See Figure 6)
The ESIM EVAP systems are normally sealed. Both the large and the small valves are weighted and held in the closed position by gravity. Because of this, it is very important that these units be installed in the upright position. The ESIM is installed on the charcoal canister along the path to the fresh air vent. It can be thought of somewhat like a vent valve. When the vehicle is being fueled or when the fuel temperature rises, pressure in the fuel tank builds and vapor needs to be vented. The vapor is allowed through the charcoal canister trapping HC’s, then on to the ESIM. The large valve in the ESIM becomes unseated at about .5 inches of water allowing the pressure to vent through a filter to the atmosphere. Negative pressure is present in the fuel tank and EVAP system when fuel is being drawn into the engine when fuel temperature drops or during purge operation. If this negative pressure reaches 2.2 inches of water, the small valve is lifted from its seat allowing air in through the fresh air filter. (See Figure 7)
ESIM Leak Detection
Pretty simple so far right? The whole purpose of the ESIM is to determine when an EVAP system leak is present. As we have seen in the images of the ESIM, there is a vacuum diaphragm and a simple contact switch. When the vacuum in the EVAP system reaches roughly 1 inch of water, the diaphragm is pulled causing the contacts on the switch to make a connection. Chrysler determines leaks using two different strategies:
First: Non-Intrusive Leak Testing
This takes place when the engine is shut OFF. Yes, that is correct, after the engine is shut off and the fuel tank has been naturally pulled into a vacuum. This is caused by cooling of the fuel or diurnal (normal daily) ambient temperature changes. This can only happen if the fuel tank and EVAP system are sealed explaining why the ESIM is normally closed. When this natural vacuum reaches roughly 1 inch of water, the switch inside the ESIM closes and a signal is sent to the PCM. If the switch closes in a predetermined amount of time and this occurs during a predetermined number of key off cycles, it is determined that the EVAP system is sealed and the non-intrusive test is passed. If the switch does not close as described, then the test is considered inconclusive and the intrusive test will be run on the next key cycle that certain criteria are met (outlined next). This does not mean a DTC indicating an EVAP leak will set, this is the start of more testing to determine a fault.
Second: Intrusive Leak Testing
Before this test is performed certain criteria must be met:
The engine coolant temperature must be within 50 degrees F (10 degrees C) of ambient temperature to indicate a cold start.
The fuel level must be between 12% and 88%.
The engine must be in closed-loop status.
Manifold vacuum must be greater than a minimum specified value.
Ambient temperature must be between 39 degrees F and 98 degrees F (4 degrees C and 37 degrees C).
The elevation level must be below 8,500 feet (2,591 meters).
Once these conditions are met, the PCM will activate the purge valve to create a vacuum in the EVAP system. The PCM then measures the time it takes for the vacuum to dissipate. This is referred to as the decay method. For example, if the switch opens quickly after the purge valve is closed, a large leak is recorded. If the switch closes after a predetermined amount of time, testing of a small leak will continue. If the small leak test continues to fail a predetermined number of times, a small leak code will be set.
Other EVAP system fault tests
These intrusive and non-intrusive tests are performed to determine leaks in the EVAP system. The PCM still needs to test or to verify there are no faults in the system operation. To do this, Chrysler uses two additional tests, the purge monitor test and the ESIM switch stuck closed monitor. Here’s how those two tests operate:
Purge Monitor Test
This test is performed to verify the purge valve is functioning and will only be run if the small leak test has passed. Since the small leak test has passed, we know the purge valve is capable of sealing the EVAP system from the intake manifold. We don’t know if the purge valve is opening or if a good manifold vacuum is present. You may have noticed some of these vehicles using an ESIM have a fuel tank pressure sensor (FTPS) and others do not. The FTPS was added in 2012 on most models and is used ONLY for the purge monitor test, it is NOT used for any leak testing.
No Fuel tank pressure sensor 2007- 2011
The Purge monitor test is performed in two stages. The first stage monitors the vapor ratio during normal EVAP purge operation. The vapor ratio is a calculation made by the PCM by monitoring the short-term fuel trims during the purge operation. If short-term fuel trims do not react as expected, the stage two purge monitor test will take place. For the stage two test, the PCM operates the purge valve more aggressively. If the vapor ratio does not respond, a code P0441 is set indicating a purge flow fault.
Fuel tank pressure sensor around 2012 and newer
Vehicles with an FTPS perform the purge monitor test in two stages, as well. The first stage monitors the fuel tank pressure during normal EVAP purge conditions. If pressure changes in the EVAP system and are not as expected during purge operation, the second stage test will be performed. For this test, the PCM increases and decreases the purge valve in a more controlled or deliberate manner, while monitoring the FTPS for the expected changes of pressure in the system. If the pressure in the EVAP system does not change as expected, a code P0441 is set.
ESIM Switch Stuck Closed Monitor
The final part of this system that the PCM needs to test is the vacuum switch in the ESIM. This is done at key off, engine off. The PCM energizes the purge valve for up to 30 seconds, this will vent any vacuum that may be present in the EVAP system. If the switch is already open or opens when this test is performed, the test passes. If the switch was closed and stays closed, a fault is recorded and a code P0452 set.
Faults and Trouble Codes
Now that you have a better understanding of how these systems work, we can look at what faults you may encounter and how to diagnose the cause. As you know, there are many trouble codes that can be set on today’s complex vehicles. It would not be practical to list each of them here. Also, I do not believe it is good practice to memorize things that can easily be referenced in-service information. Most of the trouble codes relating to this system fall under one of only a few categories. Here is a list of those common categories along with a short testing plan and the likely fault code:
Note: Tests are listed in this order since later tests will include performing tests listed earlier. They are not listed in order of most common.
ESIM switch fault (P0452)
The ESIM switch has two wires, a 5v reference from the PCM (usually about 4.5v actual) and a ground supply. Here’s how to test its operation:
With no vacuum in the EVAP system, a voltmeter should show 4.5-5v when connected by back probing the two pins. (Figure 8)
If not, you have a circuit fault. Determine if you are missing the PCM supplied 5v or the ground path.
2. Now apply a vacuum to the EVAP system by using the purge valve controlled with a scan tool. You should see the voltage drop to near 0, if not, replace the ESIM. (Figure 9)
If the switch does not close you will want to verify the purge valve did open and there are no leaks in the EVAP system. After all, if the system was not pulled into a vacuum of more than 1 inch of water, the switch should not close.
3. If the switch does operate as it should, disconnect the two wire connectors and connect an ohmmeter directly to the ESIM switch terminals.
Now test the resistance with the switch closed. It should have low resistance (around .5 — 2 ohms). If you find high resistance (over about 2 ohms), replace the ESIM. (Figure 10)
In my experience, most scan tools will not allow you to monitor the ESIM switch PID while controlling the purge valve (real convenient). Also, I have found most scan tools to display only a value of open or closed, not a voltage reading. Meaning you cannot monitor bias voltage in place of using a voltmeter.
EVAP system leak
(P0456 small leak, P0455 large leak)
Before any testing, make sure there are no fault codes stored that could cause the leak test monitor to be inaccurate. Such as the P0452 code indicating an ESIM switch fault. Then follow this test plan:
I always start any EVAP system leak fault testing with a visual inspection of the system. Many times you will find something obvious like a damaged part, torn hose or missing gas cap.
2. You will now need to perform a system leak test. Here’s the method I prefer:
Disconnect the fresh air vent hose from the ESIM and connect the smoke machine in its place. You may need to make an adapter using various sizes of hose as seen in the image. Since you have installed the smoke machine on the vent path the system should now be sealed, and you did not need to disrupt any fittings or valves. (Figures 11 and 12)
You can now turn on the smoke machine and monitor for leaks being sure not to use excessive pressure.
3. If you did not find any leaks, you will now want to test the ESIM switch operation as described for the ESIM switch fault code (P0452).
4. Verify the ESIM vent filter is not restricted. It should allow air to flow freely.
5. You have now tested all possible causes excluding the valves in the ESIM. You can remove the ESIM and connect a vacuum/pressure pump along with a gauge to test the valves for sealing OR I
prefer to replace the ESIM at this point. You can call it the process of elimination diagnostics. Since we have tested all other causes, this has to be our failure.
Note: a small leak code (P0456) will only be set after a large/medium leak test is performed.
Gas cap loose/gross leak (P0457)
This fault is monitored and tested the same as a large system leak; however, the PCM also noted a large change in fuel level. Perform testing the same as other EVAP system leak faults. You may want to start by testing the gas cap if you have the equipment (many emissions testing stations). During smoke testing, pay close attention to the gas cap.
Purge system performance (P0441)
Perform testing for any leak faults or ESIM switch fault codes first if also stored.
On vehicles with a fuel tank pressure sensor monitor the sensor value on a scan tool while controlling the purge valve on. On those without a fuel tank pressure sensor (2007 - ~2012), connect a vacuum gauge in place of the fresh air vent on the ESIM. You should see pressure drop steadily to a manifold vacuum. If not:
Verify the purge valve opened and the circuit was energized.
Verify there is a good manifold vacuum present to the purge valve.
Test the ESIM fresh air vent for restriction.
It should flow air freely.
Test the hoses from the purge valve to the fuel tank for restriction.
Verify the ESIM vent filter is not restricted.
It should allow air to flow freely.
General EVAP system fault (P0440)
This fault will set if the ESIM switch does not close during the intrusive leak test. Since there are many possible causes, the PCM cannot determine what the failure is and sets a general fault code. Perform the following tests as they were described previously.
Start by performing a system leak test.
Test the operation of the ESIM switch.
Test the purge system performance.
If all the above tests pass, replace ESIM.
It’s been said many times that a good technician can fix anything if he understands how it works. Hopefully, you now understand how these Chrysler ESIM systems work. Armed with this understanding and a logical testing approach, you can quickly find the problem the next time a Chrysler with an EVAP system fault comes into your bay.