Volvo Coupes/Sedans/Wagons 1970-1989 Repair Guide

Evaporative Control System (Charcoal Canister)



See Figures 1 and 2

All 1970 and later Volvos are equipped with an evaporative control system to prevent fuel vapors in the fuel tank and (in carbureted models) the float chambers from escaping into the atmosphere. An expansion tank above the fuel tank provides for thermal expansion of fuel vapors in warm weather. Those vapors which do not condense and return to the fuel tank are displaced and drawn into an activated charcoal canister in the engine compartment.

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Fig. Fig. 1: Details of the fuel evaporative system-fuel injected engines

When the engine is at idle or is shut off, the charcoal canister absorbs and stores the vapors from both the tank and the carburetor. Throttling the engine causes the vapors to be drawn out of the canister into the air/fuel mixture bound for the engine. The vapors enter the engine through the intake manifold and are burned in the combustion chambers.

On carbureted models, the float chamber vapors are diverted from the canister to the air cleaner upon acceleration. As a result of these fumes being vented to the air cleaner, an overly rich (proportionally too much fuel for the amount of air present) fuel mixture may develop, leading to starting difficulties-especially in warm weather. A hot-start valve is located between the float chamber and the air cleaner on carbureted models which returns the vapors to the charcoal canister until the engine can handle the extra-rich mixture.

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Fig. Fig. 2: Typical evaporative control canister mounting

On 1974 and later models, the expansion tank was relocated inside the fuel tank and an equalizing or balance valve is used to regulate pressure buildup in the fuel tank. When the expanding fuel vapors create too much pressure in the tank, the valve opens and allows the vapors to be fed into the charcoal canister. When too much vacuum is present in the fuel tank (due to sudden cooling or low fuel level) a second valve opens and allows air from the venting filter back into the fuel tank to prevent the tank from collapsing. The equalizing valve also prevents fuel from entering the vapor line during hard cornering.


See Figures 3 and 4

The charcoal canister is located in the engine compartment on all models except the 1800E and 1800ES, on which the canister is located under the left front fender.

On 1970-73 models the canister has a removable foam plastic filter in the bottom which should be replaced every 24,000 miles (38,647 km). The filter is exposed and can be removed from the bottom of the canister when the canister is removed from the car. On 1974-78 models, the entire canister should be replaced every 45,000-48,000 miles (72,464-77,295 km), depending on the model and year of the vehicle. On 1979 and later models, the canister is nonserviceable and requires no maintenance, other than checking lines for kinks or obstructions. If the car has experienced a severe fuel-related problem (continued flooding, polluted or incorrect fuel, etc.) the canister may need to be replaced. It cannot be cleaned or reconditioned.

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Fig. Fig. 3: After you have disconnected and labeled the lines, unbolt and remove the bracket. Hold the canister with your other hand so that it will not fall

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Fig. Fig. 4: Lower the canister and bracket, then disconnect and label the rest of the lines

Before attempting to remove or service the charcoal canister, label each hose and its matching port. Hose placement is critical to the proper operation of the engine. Make sure hoses are reinstalled in their proper place and held firmly with clamps.

To remove the canister on 1970-73 models, disconnect the hoses from the top and side of the canister, loosen the canister bracket bolt and slide the canister out. On 1974 and later models, pull the valve with its hose off the top of the canister, remove the other hoses (if equipped), loosen the clamp screw and slide the canister out of its bracket.

The hoses should be routinely checked for cracks, kinks and restrictions. The ends of the vacuum hoses can become brittle and leak at their connections. The lines to the fuel tank should also be checked periodically for any crushing or kinking under the car, and always after any impact to the underside of the car from solid objects or ice.