Table of contents: Operating principle ↓ Checking the BCDD control valve ↓ Checking the BCDD control solenoid…↓
Note: A hand-held vacuum pump with a vacuum gauge is required to perform the diagnostic procedures described below.
Operating principle
Deep vacuum in the intake manifold when engine speed is reduced (decelerations) leads to the emission of unburned combustible gases and oil into the atmosphere, and an increase in the emission of hydrocarbon compounds. In order to reduce the level of hydrocarbon compound emissions, the carburetor is equipped with a mixture leaning device during deceleration (BCDD) (see accompanying illustration).
When the vacuum in the intake manifold reaches a certain value, the BCDD control valve opens the air bypass channel, which reduces the volume of fuel injected into the carburetor through the cast jet. This increases the completeness of combustion of the mixture, which leads to a reduction in the emission of hydrocarbon compounds.
To prevent engine detonation after final stop, models with manual transmission are equipped with a BCDD solenoid valve, which blocks the operation of the BCDD control valve.
Checking the BCDD control valve
Note: Under normal conditions, the BCDD control valve does not require inspection or adjustment, however, if adjustment is necessary, proceed as described below.
1. On models with manual transmission, disconnect the carburetor electrical connector.
Note: Disconnecting the carburetor connector will de-energize the BCDD and fuel shut-off (shut-off) solenoid valves, preventing them from operating, causing the engine to not idle during this test.
2. Loosen the fast idle damper completely (if such is provided).
3. Remove the top air cleaner cover and connect a vacuum gauge to the intake manifold.
4. Increase the engine speed to 3,000÷4,000 rpm, then release the gas pedal. When you release the gas, the frequency of the sound emitted by the carburetor neck should change. This change in frequency indicates that the BCDD control valve has been activated.
5. Repeat the check of the last paragraph, observing the vacuum gauge readings while changing the sound frequency. On 4.2 L models, the device should register a vacuum depth of 570±50 mm Hg, on 3.0 L models - 580±50 mm Hg.
6. If the sound frequency does not change, or the vacuum depth exceeds the nominal value, remove the rubber plug from the BCDD control valve and turn the adjusting screw clockwise. If the frequency still does not change, remove the BCDD valve from the carburetor and make sure that its plunger operates when vacuum is applied. If necessary, replace the BCDD valve.
Caution! Do not press on the adjusting screw!
7. If the frequency change occurs at a vacuum depth below the nominal value, remove the plug and turn the adjusting screw counterclockwise to achieve the desired result.
8. Adjust the fast idle damper.
Checking the BCDD control solenoid valve
1. Disconnect the carburetor electrical connector.
2. Apply battery voltage to the terminals of the solenoid in the carburetor connector (see accompanying illustration).
1 - 4.2 l
2 - 3.0 l
3. You should hear a clicking sound from the solenoid valve in the carburetor. If not, replace the valve.