EGR Function Testing GMC Vandura I

Vacuum Modulated & Negative Back Pressure Types

1. Check for proper hose routing, according to appropriate diagrams. See VACUUM DIAGRAMS article. Check EGR signal tube orifice for obstructions.
2. Hook vacuum gauge between EGR valve and carburetor and check vacuum with engine running at normal operating temperature. With engine at 3,000 RPM, there should be at least 5 in. Hg.
3. Manually depress valve diaphragm. While depressed, hold finger over source tube and release diaphragm.
4. Check for diaphragm and seat movement. Valve is okay if it takes over 20 seconds for diaphragm to move to seated position. If less, replace EGR valve. Check operation of Thermal Vacuum Switch.

Positive Back Pressure Type

1. Check for proper hose routing, according to appropriate diagrams. See General Motors VACUUM DIAGRAMS in this section. Check EGR signal tube orifice for obstructions. Check operation of Thermal Vacuum Switch. Remove valve from engine. Apply external vacuum of 10 in. Hg or more to EGR valve signal tube. NOTE: A constant vacuum supply must be used.
2. Valve should not open. If it does, transducer control valve is stuck closed and EGR valve must be replaced.
3. With vacuum still applied, apply a stream of air from a low pressure source 15 psi (1.05 kg/cm 2 ) into EGR valve exhaust gas intake passage. Valve should open completely. If it does not open at all, transducer control valve is stuck open or exhaust passages are plugged. Replace EGR valve.
4. If EGR valve and control valve are both functioning properly, clean mounting surfaces. Using a new gasket, install valve on engine. Reconnect vacuum hose.

EGR-TVS TEST (HOT)

1. Remove EGR valve vacuum hose at EGR valve and connect hose to a vacuum gauge. Start engine. With transmission in Park or Neutral, open throttle partially. As throttle is opened, vacuum gauge should respond with an increase in vacuum reading. If operation is satisfactory, remove gauge and reconnect hose to EGR valve. If gauge does not respond to throttle opening, proceed to step 2).
2. Remove carburetor-to-TVS hose from switch and connect hose to vacuum gauge. Start engine. With transmission in Park or Neutral, open throttle partially. If vacuum gauge responds to throttle opening, switch is defective. Remove switch and replace with new part. If gauge does not respond to throttle opening, check for plugged hose or defective carburetor.

EGR-TVS TEST (COLD)

1. Engine coolant must be below 85°F (29°C). Drain coolant to below level of switch. Disconnect vacuum lines and remove switch. Inspect switch to make sure it is in good condition.
2. Connect a vacuum hose to lower nipple of switch, marked "C" or "CARB". Connect a vacuum gauge to upper nipple, marked "E" or "EGR". Place switch in water at 75°F (24°C) and submerge completely for 2 minutes while agitating water thoroughly. Apply 12 in. Hg to hose on lower nipple of switch. Under this condition, switch should be closed. NOTE: Leakage of up to 2 in. Hg in 2 minutes is allowable and does not mean a defective switch.
3. If operation is satisfactory, reinstall switch. If switch is defective, replace with a new part. Replace coolant and check level.