The EGR system is used to control oxides of nitrogen (NO x ) by recycling some exhaust gas through the combustion process. A small amount of exhaust gas is admitted into the intake manifold by the EGR valve.
Exhaust gas flow is a function of the pressure differential between the exhaust and intake systems. It is controlled by the vacuum operated EGR valve located on the intake manifold. The valve is open at closed throttle (idle) and closed at wide open throttle.
The EGR control unit monitors coolant temperature and engine speed to determine if the EGR control unit output signal should be sent to open the EGR valve. The control unit de-energizes the EGR control solenoid, which in turn shuts off the vacuum to the EGR valve under cold start and warming-up conditions. During these periods, exhaust gas recirculation is not desirable. As the coolant warms up to normal, vacuum is allowed to operate the valve proportional to throttle position. If the throttle opens beyond a preset level, a vacuum reducer is engaged, temporarily limiting vacuum to the EGR valve and effectively turning it off during the wide open throttle period. The vacuum signal to the valve is also reduced at high altitude, reducing tailpipe smoke.
A vacuum pump capable of producing more than 10 in. Hg (33.8 kPa) of vacuum will be needed to perform this test.
- Start the engine and warm it to full operating temperature. This can be achieved quicker by driving the vehicle than by allowing it to idle.
- Disconnect the vacuum hose from the EGR valve and install a vacuum pump and short piece of hose. Alternately, disconnect the EGR hose at the control solenoid and connect the hose directly to the vacuum pump.
- With the engine running at warm idle, draw a vacuum with the hand pump; the EGR valve should open. The engine sound should change, possibly even stalling, depending on how far the valve is opened.
- Draw and release vacuum several times and at different speeds. The response of the valve (and therefore the idle speed/quality) should be proportional the the vacuum signal from the pump.
- Disconnect the test equipment and reconnect the vacuum hose.
REMOVAL & INSTALLATION
- Label and disconnect the hoses from the valve. Carefully loosen and remove the retaining bolts, remembering that they are probably heat-seized and rusty. Use penetrating oil freely.
- Remove the valve and clean the gasket remains from both mating surfaces.
- Inspect the valve for any sign of carbon deposits or other cause of binding or sticking. The valve must close and seal properly; the pintle area may be cleaned with solvent to remove soot and carbon.
- After cleaning and inspection, recheck the motion of the valve with a hand vacuum pump. Watch the shaft area closely. A stuck or binding valve will not open properly; a weakened valve will open too soon. If either condition is encountered, replace the valve.
- Install the valve with a new gasket. Tighten the bolts evenly to 5 ft. lbs. (7 Nm).
- Connect the hoses and lines to their proper ports.