Mercedes Benz Vapor Canister Purge Solenoid Troubleshooting Guide

To troubleshoot the vapor canister purge solenoid on a Mercedes-Benz, scan for EVAP codes (P0440 to P0446), then bench test the valve: it should be closed with no power, hold vacuum from a hand pump, and click open the moment you apply battery voltage. If it leaks vacuum or does not click, replace it.

The purge valve is part of the Evaporative Emission Control (EVAP) system. Fuel vapour from the tank is caught and stored in a charcoal canister, and the purge valve decides when, and how much of, that vapour goes into the engine to be burned like ordinary fuel. On modern Mercedes-Benz cars the purge valve is an electrically operated solenoid controlled by the engine computer. It is closed when the engine is off and opened gradually once the engine is running.

All these parts are connected by hoses. Dust and debris can block the canister, and the purge valve can crack or fail with age and heat. When it fails, the EVAP system stops working properly and emissions go up.

TL;DR

• Cause: a purge solenoid stuck open, stuck closed, clogged or with a circuit fault.
• Symptoms: check engine light, rough idle, weak acceleration, hard starting, poor fuel economy.
• Codes: P0440, P0441, P0443, P0444, P0445, and P0446 for the related vent circuit.
• Test: apply battery voltage to the terminals (a 9 V battery is enough) and listen for a click.
• Leak test: a good valve holds hand-pump vacuum for at least 1 minute with no power.
• Fix: replace the valve if it leaks or does not click; it is a US federally warranted emission part for 2 years or 24,000 miles.

If this is not your case, read How to fix EVAP Canister Problems.

Table of Contents

1. What Does the Canister Purge Valve Do?
2. What Happens When the Canister Purge Solenoid Goes Bad?
3. What Are the Symptoms of a Faulty Purge Valve?
4. How Is the Solenoid (Purge Valve) Tested for Mercedes Benz?
5. Is the Purge Valve Covered by Warranty?

1. What Does the Canister Purge Valve Do?

The charcoal canister can only hold so much vapour. It stores the vapour from the fuel tank until the engine is running under the right conditions, then the purge valve lets it into the engine to be burned. The valve also controls exactly how much vapour goes in.

The purge valve opens and closes the passage between the fuel tank vapour lines and the intake manifold. It works like a switch: open, vapour flows into the intake; closed, it stops.

On modern Mercedes-Benz cars the valve is an electrically actuated solenoid controlled by the engine computer. With the engine off it is closed. With the engine running, the computer opens it progressively so a set amount of vapour is burned.

With no power to the solenoid, the valve stays closed. When voltage is applied, it opens. If the solenoid fails, the valve can no longer open or close on command, and that causes a chain of problems.

Many modern Mercedes-Benz engine control modules only purge vapour while the car is moving at a steady speed, usually in higher gears. Purging at low speed or under load can upset the air-fuel ratio and cause misfires.

2. What Happens When the Canister Purge Solenoid Goes Bad?

A weakened purge valve can cause a vacuum leak, which directly affects idle speed and idle quality. The valve can also stick open, stick closed, or clog so the vapour cannot be purged.

A faulty valve can also release vapour at the wrong time, which upsets the air-fuel ratio. To work properly, the purge valve has to open and close at exactly the right moments.

When the purge control solenoid fails, it can set several trouble codes, which can make the fault harder to pin down:

3. What Are the Symptoms of a Faulty Purge Valve?

A failing purge solenoid affects the EVAP system and the car's emissions. It will usually show one or more of these five symptoms.

Mercedes Benz Experiences Rough Idling

A rough idle is one of the first signs. The engine runs unevenly when stopped or at low speed. If the purge valve sticks open, it creates a vacuum leak that affects idle speed and quality.

A broken purge solenoid, or a damaged hose connected to it, can also create a vacuum leak. Fix it quickly, because it can make the engine stall.

Engine Performance Problems With Mercedes Benz

Besides a rough idle, a failed purge valve can make the engine feel weak, without enough power to accelerate.

You press the pedal and the car feels like it is holding back. That slow acceleration comes from disturbed combustion, and it needs repairing.

Difficulty in Getting Started With Mercedes Benz

Hard starting is another common sign. If the purge solenoid causes a vacuum leak, the car may not start reliably.

The leak lets unmetered air into the engine, which upsets the air-fuel ratio and disrupts combustion. Eventually the engine may not start at all.

The Check Engine Light in Mercedes Benz Is Turned On

If the engine computer sees a problem with the purge solenoid circuit or signal, it turns on the check engine light.

Many other faults turn on the same light, and fueleconomy.gov notes it can be as minor as a loose gas cap. Read the codes with an OBD2 scanner before buying parts.

Low Fuel Efficiency With Mercedes Benz

Poor fuel economy is another sign. Instead of being burned, the fuel vapour is lost through the EVAP system, so the car wastes fuel instead of using it.

The purge solenoid is an emissions component, so it is key to the car meeting emission rules. It keeps fuel hydrocarbons from escaping into the air.

If you suspect the purge solenoid, have the car diagnosed to see whether the solenoid or a vacuum hose needs replacing.

4. How Is the Solenoid (Purge Valve) Tested for Mercedes Benz?

The exact test procedure is in the Mercedes-Benz service manual and can vary by model.

The purge valve on most cars is normally closed: closed with no voltage, open with voltage.

The Mercedes-Benz service procedure is to apply battery voltage to the purge valve terminals and listen for a click. For a bench test, a 9 volt battery is enough to make the valve click.

The next step is to check that the valve closes properly and does not leak. Connect a hand vacuum pump to the purge valve and apply vacuum.

If the vacuum drops quickly, the purge valve is leaking and needs replacing. A car with this fault will usually have the check engine light on, and a scan may return P0446 (Evaporative Emission Control System Vent Control Circuit Malfunction).

A leaking valve like this shows up as a slow leak when the EVAP system is checked, so you have found the root cause.

For comparison, a good purge valve holds a strong vacuum for a long time, still holding after 1 minute.

Now connect battery voltage. The vacuum should drop as soon as the valve clicks open. That shows the purge valve is working.

EVAP check engine codes are often caused by a faulty vent valve or gas cap as well, so check those if the purge valve passes. The EVAP canister guide covers testing and cleaning the canister itself.

5. Is the Purge Valve Covered by Warranty?

In the US, often yes for newer cars. The EPA's emission warranty fact sheet lists the purge valve, purge solenoid, fuel filler cap and vapour storage canister as emission parts covered for the first 2 years or 24,000 miles on 1995 and newer light-duty cars and trucks under 8,500 pounds. Check your Mercedes warranty booklet for any longer cover.

Working with the rest of the EVAP system, the purge valve keeps fuel vapour out of the air. If you see any of the signs above, have it checked and fixed early, both for the car and for the environment. On diesel Mercedes models, the emissions faults are different; see Mercedes Benz NOx Sensor Issues and how to fix it?.