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Charcoal canister, and purge valve 200/25/ZR

Whats the actual function of the combined EVAP Canister and Purge valve, and how does it help with controlling the emissions. Where is it fitted and why can it be so damned annoying, to the extent of spoiling the smooth cruising sound of my car with intermittent tapping sound emulating from the purge valve which is attached onto the top of the vehicles charcoal canister.

The canister which is located between the fuel filter and coolant tank is fitted with a triangular rubber isolation mount, and fixed into a metal bracket on the bulkhead as shown below. A poor condition of the isolation rubber, or even a misplaced one, will be a contributing factor how audible the purge valve noise will sound inside of the cabin. 

Basic function of the EVAP system, is creating a vacuum in the fuel tank and fuel lines so fumes can be sucked into the engine to burn, instead of venting to the atmosphere. A damaged fuel filler cap seal wont create much of a vacuum, so forcing the purge valve to activate for periods when maybe its not always required to do so.

How does it work, The purge valve an electrically operated solenoid collects fuel vapors from the fuel tank and stores them in the charcoal canister, fuel vapors are then moved from the charcoal canister to the fuel injector then burned off inside the engine, The purge valve flow is monitored with sensors to determine the precise amount.

EVAP Canister and Purge valve,

Purge valve located top of charcoal canister

The main problem on vehicles with 'really excessive' tapping. could be a leak somewhere in the EVAP system, a damaged fuel filler cap seal, saturated canister pellets, or a worn or missing isolation rubber, any of these faults could make the purge valve work harder and noisier, the purge valve is shown on top of charcoal canister.

The tapping sound given off by the purge valve, can initially be mistaken for sounding like the fuel injectors clicking, high tension leads arcing, or even the sound of a slack tappet giving an obvious cause for concern. The tapping on some purge valves are nothing more than a nuisance.

My own experience concerning the purge valve. On the first cold start of the day, there is no tapping noise. Driving with the engine at operating temperuture, the tapping noise will on occassions  just about be audible only for a short period of time, but not consistent, this will  be when the fuel fumes are being purged from out of the charcoal canister, into the fuel injector rail and then being burnt off in the engine.  

Questions and Answers

Relative to the canister and purge valve's function. 

When I remove the fuel filler cap, there is a hissing sound coming from the fuel tank. Thats normal, no vacuum in the tank most probably will be a leak somewhere in the EVAP system, or just a damaged fuel cap seal. A more pronounced 'whoosh' of air when at the filling station after a good drive....is a quiet, and correctly functioning valve.

On initial start up, we sometimes get a very faint whiff of petrol fumes. The charcoal pellets in the canister will be saturated, these cars are now old, As far as im aware pellets can be removed and dried out by unscrewing bottom of the canister, but not too sure about this.

Tapping sound from my purge valve is really excessive what options do I have to sort this out. An easy way is to enable a by-pass valve but it would affect the environment and upset the ECU, Alternative is to source a used one for a few quid, they are interchangeable, and only clipped on to top of the charcoal canister. I have an older valve on my 2004 MG and all is good.

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