Understanding how to diagnose EVAP issues will save shop time and lead to a quicker and more accurate assessment, improving your shop’s productivity and increasing customer satisfaction.
Of all the OBD II Diagnostic Trouble Codes, the EVAP codes can cause you the most trouble. A faulty evaporative emission control system almost never affects engine performance or gas mileage, so aside from maybe a whiff of fuel odor, the only symptom is the malfunction indicator light (MIL).
Some EVAP codes indicate specific malfunctions, like a short circuit in the purge valve (P0445) or a faulty fuel tank pressure sensor (P0451). While codes alone aren’t enough for a complete diagnosis, at least these can point you in the right direction.
The real challenge comes from codes that don’t point to a particular malfunction, like P0440 that just means the powertrain control module (PCM) thinks there’s something wrong with the EVAP system. Chasing down vague codes like this requires a bi-directional scan tool that lets you force the PCM to run the EVAP monitors. This gives you a chance to recreate the conditions that set the code.
However, as always, your most valuable tool is knowledge and understanding of the system. So first we’ll briefly review the basics of the evaporative emission control system. There are almost as many different EVAP systems as there are car manufacturers, so we’re going to focus on just one: Chrysler.
How it works
The EVAP system captures fuel tank vapors so they don’t evaporate into the air. The system must function under two different conditions; while the vehicle is being driven and while the vehicle is parked with the engine not running; so there are two different operating strategies. When the car is parked, ambient temperature and pressure can change inside the fuel tank. If the temperature increases, pressure increases too, and a pressurized fuel tank can leak fuel vapors. Rather than just vent the pressure into the atmosphere, it’s vented through hoses to a canister filled with charcoal. With the canister open to outside air, the pressure can escape but fuel molecules in the vapor stick to the charcoal, much like steel sticking to a magnet. This is a completely passive operating strategy that will always work correctly if there are no leaks in the system (and the canister hasn’t been flooded with liquid fuel).
When the engine is running, manifold vacuum draws fresh air through the canister, stripping the fuel vapor off the charcoal and sending it to the intake manifold. This is called “purging” the canister. For the canister purge strategy to work correctly, the fuel system must be in closed-loop so the PCM can reduce injector pulse width as needed to avoid running too rich. The canister purge is actually a controlled vacuum leak, and it’s managed by the PCM with a pulse-width modulated valve so it can prevent the engine from running too lean.
Emissions regulations require the on-board diagnostic system to detect leaks in the EVAP system. Beginning with 2004 models, it had to detect leaks that are as small as a 0.020 inch (0.5 mm) hole. Beginning with 2007 models, the diagnostic system must monitor EVAP purge flow to make sure the fuel vapors are being drawn from the canister into the engine.
Chrysler’s EVAP system has changed significantly since it was first introduced in 1996, but the centerpiece of their on-board monitor has always been a switch. Most other systems have a fuel tank pressure sensor, and Chrysler finally added one to their system in 2013, but the switch is still there and is still the primary monitoring device.
An on-board EVAP leak test is basically the same for all vehicles. The PCM closes the EVAP system at both ends, applies pressure or vacuum to the system, and then calculates the size of a leak by measuring the time required for the vacuum or pressure to decay or leak off. The maximum pressure or vacuum is extremely small, just 7.5 inches of water (0.25 psi), and a pressure sensor designed to operate in that range is very sensitive to ambient temperature. So instead of monitoring a sensor with compensating circuits and complex algorithms, Chrysler’s PCM simply monitors a switch to see if it’s been pushed open or closed by vacuum or pressure in the EVAP system. Over the years, they’ve used three different methods to operate that switch.
Earlier models use a leak detection pump (LDP), which consists of two solenoid valves, two check valves, a diaphragm, a calibrated spring and a reed switch. When the engine is running, the PCM closes the normally-open canister vent valve so air cannot enter the system. Then it opens the LDP solenoid valve to supply manifold vacuum to the chamber above the diaphragm. The vacuum pulls the diaphragm up against the spring, and filtered air is drawn into the pump below the diaphragm.
The rising diaphragm pushes the reed switch open, turning off the LDP solenoid valve. With the vacuum turned off, the spring pushes the diaphragm down again, pumping the air out of the lower chamber and into the EVAP system (check valves in the lower chamber manage the air flow in and out of the chamber).
This pumping cycle continues until pressure in the EVAP system equals the spring pressure above the diaphragm. When that happens, the air pressure holds the diaphragm up against the spring, keeping the reed switch open. Eventually the pressure in the EVAP system leaks down, and the spring pushes the diaphragm down, closing the reed switch to start the pump again. The PCM measures the time the switch stays open to determine if there’s a leak in the EVAP system.
Emissions regulations tightened in 2004, and Chrysler took the opportunity to upgrade several systems. In 2003 they introduced their next generation controller (NGC) that combined the engine and transmission controls into a single powertrain control module (PCM). The fuel pressure regulator was moved to the fuel pump module, eliminating the fuel injection return line and reducing evaporative emissions. They also eliminated the LDP and put the reed switch and vent valve into a smaller housing, creating the natural vacuum leak detector (NVLD). The inlet of the NVLD is the canister vent valve. The outlet is connected to the canister with a hose or the NVLD may be mounted directly on the canister. The canister is connected to the fuel tank above the check valve in the filler tube, so the NVLD can detect a loose gas cap.
The leak test runs when the engine is turned off and the vent valve closes to seal the EVAP system. A diaphragm above the vent valve is exposed to atmospheric pressure on one side and to the EVAP system on the other side through a tiny passage in the NVLD housing. The diaphragm operates the switch and a push rod that connects to the vent valve.
Unless ambient temperature is really high, the temperature in the fuel tank will gradually decrease when the vehicle is parked. Fuel pumps operate at about 100 degrees Fahrenheit (38 degrees Celsius). Since the vent valve is closed, pressure in the fuel tank will decrease along with temperature, naturally creating a very slight vacuum in the tank and the EVAP system. When that vacuum reaches just 1 in. of water, atmospheric pressure pushes the diaphragm and closes the NVLD switch.
One-inch-of-water vacuum is almost nothing, but the EVAP system won’t hold it if there’s a leak equivalent to a hole that’s only 20 thousandths of an inch (0.020 in.).
By the way, if you read the description and operation of this system in Alldata, be careful. It says the switch will close when vacuum reaches 19 in. of water, but that’s a misprint. It should say 1 in. of water.
The spring in the vent valve is very light. If vacuum in the fuel tank reaches 3 to 6 in. of water (about what it takes to suck a drink through a straw), it unseats the valve and air is allowed in to relieve the vacuum in the tank.
However, if fuel tank pressure increases a mere half inch of water above atmospheric, it pushes the diaphragm the other way, and the push rod opens the vent valve to release the pressure.
Remember, this is on the outlet of the charcoal canister, so the fuel vapors have already been trapped.
When the engine is started, the vent valve solenoid is turned on to open the valve. However, the PCM periodically closes the vent valve and uses the purge valve to pull a slight vacuum in the tank. If the switch closes, the EVAP system has passed the large-leak and loose gas cap tests.
The NVLD connector has pins for three circuits: power to the switch, power to the vent valve solenoid, and a shared ground. The PCM supplies 12 volts to the NVLD switch when the ignition is on and 5 volts when the ignition is off. Since the other side of the switch circuit is connected to ground, the PCM only needs to monitor current flow in that circuit to know if the switch is open or closed. There is a 130-ohm resistor in the NVLD switch, so current draw is 92 milliamps (0.092 amps) with the ignition on and only 38 milliamps (0.038 amps) with the ignition off. Current draw for the vent valve spikes at about 1.5 amps when it first opens, but then the valve is held open on a duty cycle averaging only 150 mA.
In 2007, Chrysler eliminated the NVLD and replaced it with the evaporative system integrity monitor (ESIM). Like the earlier system, the ESIM combines the canister vent valve and monitor switch into one assembly, but instead of a solenoid vent valve, it uses a pair of weighted check valves to control fuel tank venting. The ESIM is mounted directly on the charcoal canister to keep the weighted valves in the proper position.
The larger check valve opens at 0.5 in. of water to vent fuel tank pressure. The smaller valve opens at 2.2 in. of water to relieve fuel tank vacuum. The switch is operated by a diaphragm that’s exposed to fuel tank pressure/vacuum on one side and atmospheric pressure on the other side. There’s an air filter on the atmospheric side, usually remotely mounted.
There are only two wires connected to the ESIM, power and ground for the switch. The PCM supplies 4.5 volts to the switch at all times, and when fuel tank vacuum reaches 1 in. of water, the switch closes to complete the circuit to ground. There’s no resistor in the switch, but the PCM limits current draw on that circuit to 5 milliamps (0.005 amps).
On-board leak tests
On NVLD and ESIM systems, the small-leak test starts after the engine is turned off. The PCM monitors the switch until it closes, verifying that the system can hold vacuum. Depending on ambient conditions, this can take several hours. If the small-leak test is passed, there’s no need for the large-leak test. If the test is failed, the PCM stores a pending code and the large-leak test will run on the next drive cycle. If it passes the large-leak test, then two consecutive small-leak failures will set a hard code and turn on the MIL.
The following conditions must be met for the small-leak test to run:
The PCM uses manifold vacuum to test for large leaks and a loose gas cap. On the NVLD system, the vent valve is closed (turned off) for this test. The purge valve is operated to pull a vacuum on the system, then the purge valve is closed and the PCM measures the time required for the vacuum to decay enough for the NVLD or ESIM switch to open. If the switch never closes during this test, it could mean there’s a leak or it could mean mechanical problems with the purge valve. This is a two-trip code, meaning the test must fail on two consecutive trips if the right test conditions are met.
The test conditions are:
On-board purge monitor
The PCM must regulate the amount of purge air flowing through the canister, and the correct amount varies with engine operating conditions. Purge airflow is controlled with a pulse-width regulated solenoid valve. Since there is no airflow meter in the EVAP system, the PCM looks at engine operating conditions and calculates a purge vapor ratio: the purge airflow as a percentage of the total amount of air flowing into the engine. The PCM measures purge airflow by operating the purge valve and looking for expected changes in short-term fuel trim.
The PCM must also verify that air is actually flowing through the canister. It does this by gradually increasing the purge valve duty cycle beyond the correct purge vapor ratio and looking for expected changes in the purge adaptive calculation, and it also looks for NVLD or ESIM switch closure (remember, that switch closes at just 1 in. of vacuum). If the purge vapor ratio calculation does not reach a specific level and/or if the switch does not close, the PCM will assume there is no air flowing into the canister. This could mean a leaking purge hose, a stuck purge valve or a clogged air filter on the vent tube.
Purge flow is actually a controlled manifold vacuum leak, and the affect on short-term fuel trim shows clearly on a scan tool (long-term fuel trim is suspended during this test). The purge flow monitor conditions are:
Service bay testing
The first rule of troubleshooting any EVAP system is, don’t touch anything before you check the trouble codes. If you tighten the gas cap or wiggle a connection, you might temporarily fix or mask the problem.
When chasing EVAP leak codes, remember that the PCM tests the NVLD and ESIM systems with vacuum, while a smoke machine uses pressure. In fact, the smoke machine uses a lot more pressure than is required to open the vent. That’s one reason the EVAP system test port was eliminated. To use a smoke machine for leak testing, connect it to the end of the vent tube, “downstream” of the vent valve.
If there are no EVAP leaks, the most common reason for P0440 (EVAP system), P0450, P0451 or P0452 (sensor/switch circuits) is a problem in the NVLD or ESIM switch circuit. You can quickly test this with a scan tool or DVOM and a gas cap adaptor that lets you connect a hand-operated vacuum pump.
Remember, that switch should close with just 0.5 to 2.2 inches of water vacuum, and the gauge on the hand pump might not even read that low before the tank vent opens. If the switch doesn’t close, look for obstruction in the vent tube or vent air filter (spider webs are common).
Also look for corrosion at the connector (more common on the NVLD).
Code P0441 indicates the purge vapor ratio is below specified rate. As noted earlier, it could mean a mechanical problem with the purge valve or a clogged vent tube, but it’s often caused by an intermittent fault in the NVLD or ESIM switch. This is easy to check with a bi-directional scan tool: simply command purge flow and watch short-term fuel trim and the NVLD or ESIM switch. If fuel trim goes negative but the switch doesn’t close, the system is working but the switch is not.
Troubleshooting EVAP performance codes is more art than science. If you know how the system is supposed to work and your scan tool will command the purge valve, you can clear the codes and manipulate the system to recreate the failure.
It’s always easier to find the problem when you can recognize known-good, so next time there’s a Chrysler product in your bay, connect a scan tool and manipulate the purge valve to see what it will show you. ■
Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He currently holds ASE Master Technician and L1 certifications.