Symptoms of bad purge valve
Here are the symptoms of a bad purge valve. A purge valve is a part of the Evaporative Emissions System. The EVAP system captures fuel vapor as the fill the tank and stores the vapors in a charcoal canister during fill up. When you start the vehicle, a solenoid purge valve opens and allows engine vacuum to suck the gas vapors out of the charcoal canister. At the same time, a vent valve opens, allowing outside air into the canister to replace the fuel vapors.
Purge valve causes rough idle
Since the engine is sucking additional fuel vapors during this
purge cycle, the ECM reduces fuel from the injectors. Based on how much fuel you added during fill up (The ECM knows this because it recorded the fuel gauge level before and after the fill up), the ECM calculates how much fuel vapor it expects to see coming into the engine from the charcoal canister.
If the flow of vapor is lower than expected because the purge valve doesn’t open or is partially clogged, the engine will run rough. This is because the engine is running slightly lean because the ECM has already reduced fuel injector feed because it’s expecting fuel vapor from the canister.
On the flip side, if the purge valve is stuck open, and the system has an “always open” vent valve, the engine will idle rough because the stuck open purge valve will act like a vacuum leak, causing a lean idle condition.
Purge valve causes hard starting
A stuck open purge valve can cause hard starting for the same reason as above; it’s a vacuum leak that’s allowing unmetered air into the engine.
Diagnose a purge valve
A purge valve is nothing more than an electronically controlled solenoid valve. You’ll need a handheld vacuum pump and jumper leads to test it.
A purge valve has two vacuum line ports; one vacuum line supplies fuel vapor form the charcoal canister. The other port is connected to the intake manifold or throttle body. Remove the vacuum line running to the intake or throttle body. Apply vacuum to this port and see if the purge valve holds vacuum. Then apply power and ground to the electrical terminals to see if the purge valve opens and closes properly. Also, check for charcoal granules in the valve. That’s an indication of a failed charcoal canister. If you find charcoal, you must replace the canister and flush the lines. If you replace just the clogged purge valve, it will just plug up again.
©, 2020 Rick MuscoplatPosted on by Rick Muscoplat