A vapour canister purge valve is a part of the EVAP system on any gas-engine vehicle. Gasoline evaporates and causes harmful emissions, so a system was developed in the 1970s as part of the Clean Air Act to control that. Any modern car has an activated carbon filled canister that collects the gasoline fumes that escape from the tank. This canister must be purged periodically, or it will become saturated and ineffective. This is done by connecting a hose from the canister to the engine’s air intake manifold and, under specific running conditions, “purging” gasoline vapours into the engine and burning the fumes there. There must be a valve on this hose so that it can either open to let fumes into the running engine when appropriate or close to seal the system under other circumstances. A purge valve is that.
The canister is often located near the fuel tank, the purge line runs forward alongside the fuel lines, and the purge valve is mounted somewhere convenient on top of the engine. The purge valve has an electrical connector, a line-in hose, and either a line-out hose or a direct mount to the engine’s air intake manifold.
How Purge Valves Fail
Being a simple device, there are only two main ways it can fail. First, the seal may fail and allow air to enter. That’s by far the most common failure.
Second, the electrical canister purge valve solenoid (EVAP purge solenoid) that opens and closes the canister purge valve can fail. The PCM, which is the engine computer, controls this solenoid, which is often integrated into the purge valve. Like most solenoids, it’s pretty much just a coil of wire that creates a magnetic field, opening the valve when current runs through it. The purge valve won’t work if there is something that stops electricity from flowing, like a broken wire inside.
Signs that a Purge Valve Might Need Replaced
The main thing that can go wrong with a canister purge valve is that the internal seal can fail. In that case, several things happen. More gasoline fumes get into the engine than are intended, and once those are done, too much air gets in—basically a vacuum leak. When there is a vacuum leak, the PCM, which is the brain of your car, has a hard time keeping the desired idle speed and fuel mixture ratio. This can cause performance issues such as reduced fuel efficiency, stalling, etc.
In practice, PCMs are usually pretty good at that, so the problem is often only noted by an engine light. There are a variety of engine light trouble codes that can be set, ranging from “idle speed control faults” to “lean running conditions” and often “purge flow errors”. It can vary with how badly the purge valve leaks and the vehicle’s specific self-diagnostic system. Checking the purge valve is one of the first steps in many different diagnostic procedures.
If the failure is electrical, the purge valve wouldn’t open and the evaporative canister could become saturated. Then it may be difficult to fill the gas tank, and the pump may keep shutting off (though other evap problems can also cause that). The PCM usually (but not always) recognizes that current isn’t flowing when it should and triggers an engine light with a “purge valve circuit error” code.
How to Diagnose a Faulty Purge Valve
For whatever reason, most vehicles have a purge valve that is uncharacteristically easy to access and remove in comparison with most other parts. It’s usually right on top of the engine, with one electrical connector and two hose connections. It usually only takes a few minutes to remove purge valves, though some are more difficult to remove than others.
Once removed, the ends can be wiped clean, and then you try to blow through it. If you can at all, it’s no good. If you can’t, it’s probably fine, at least as far as the internal valve. Checking the electrical part of the purge valve is more difficult but also not necessary most of the time, depending on the codes or symptoms.
Costs of Purge Valve Replacement
For some rough estimates of purge valve replacement costs, assuming a shop rate of £100 an hour:
For a 2004 Ram 1500 with a 4.7 litre engine, the labor time to replaced the purge valve is .3 of an hour. An OE part costs about £85, and various aftermarket valves cost about £25. That would make the job about £115 using factory parts, or about £55 using aftermarket parts.
For a 2017 Ford Escape with a 2.0 litre engine, the labour time to replace the purge valve .6 of an hour. It’s a little harder than most as it has permanently attached lines that run to harder-to-access connectors than usual. An OE part costs about £100, and an aftermarket Dorman part, which splices into the lines rather than replacing them as the OE part would, costs about £25. That would make the job about £160 using factory parts, or about £95 using aftermarket parts.
For a 2013 Honda Accord with a 3.5 litre engine, the labour time to replace the purge valve is .6 of an hour. An OE part costs about £25 and a Standard Products replacement costs about £20. That makes the job about £85 using factory parts or about £80 using aftermarket parts.
It’s also important to remember that most shops will charge an hour of diagnosis in addition to repair costs when replacing code-based parts. That would typically be added to the cost of replacing the canister purge valve.
What Else Can Seem Like a Bad Purge Valve?
Anything that causes a vacuum leak can cause similar symptoms, and there are many things that can cause vacuum leaks. A smoke check would be the usual way to figure that out, though that’s usually done after ruling out the purge valve with a quick check.
The evap system also has a vent valve that can fail and cause the canister to become saturated, which can lead to some of the same symptoms as a purge valve. Usually, but not always, code-based diagnosis can tell the difference between the two pretty early.
Can I Drive With a Bad Purge Valve?
In most cases, yes, a vehicle with a purge valve problem can still be driven. It can depend on how bad the part is, but in most situations, the vehicle’s PCM will keep you informed of that through the “Check Engine” light on the dash. If there is no engine light, then the PCM doesn’t see enough of a problem yet, and it should be fine to drive. If there is a solid engine light, that means the PCM does see a problem but is still able to keep the engine running well enough to drive, if necessary.
But if the engine light is flashing, that means the PCM is not able to keep the engine running well enough to drive, and it shouldn’t be driven. Usually, a flashing engine light means there are cylinder misfires, which can damage the engine, and/or the catalytic converters. That wouldn’t usually happen with a purge valve problem, but it is possible.