How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

TEST 2: Applying Vacuum To The EGR Valve

Now that you've verified that the EGR valve is not getting a continuous vacuum, the next step is to start the engine and use a vacuum pump to apply and release vacuum to it.

The purpose of this test is to see how the EGR valve diaphragm moves up and down as vacuum is applied and released.

You should also notice that the engine's idle should become rough as the diaphragm moves up and the valve's pintle opens.

If applying vacuum to the EGR valve does not affect the engine's idle RPM, you can generally conclude that the EGR valve is bad or that carbon build up is blocking the exhaust gas flow into the intake manifold.

NOTE: You'll need a vacuum pump and a telescoping mirror to perform this test. If you don't have one, you can run to your local auto parts store (AutoZone, O'Reilly Auto Parts) and borrow one from them. If you'd like to buy a vacuum pump, I recommend the Actron CP7835 Vacuum Pump (at: amazon.com).

CAUTION: This test is done with the engine running. Be careful and take all necessary safety precautions.

Let's get started:

1. 1

   Disconnect the vacuum line from the EGR valve.

2. 2

   Connect your vacuum pump to the EGR valve.

3. 3

   Have a helper start the engine and accelerate it to 2000 RPM.

4. 4

   Apply vacuum to the EGR valve.

5. 5

   You should see/hear a noticeable drop in engine RPM and the EGR valve diaphragm should move up as the EGR valve opens.
   
   You can observe this movement with a telescoping mirror.

6. 6

   Release the vacuum you applied to the EGR valve.

7. 7

   Engine RPM should increase and the EGR valve diaphragm should move down as the EGR valve shuts off exhaust gas recirculation.

Let's interpret your test result:

CASE 1: Engine speed dropped and the diaphragm moved up when you applied vacuum to the EGR valve. This is the correct test result and confirms several important things:

• The EGR valve is functioning correctly.
• Exhaust gas is able to freely enter the intake manifold (no carbon buildup blocking its flow into the intake manifold).

If an EGR valve diagnostic trouble code is illuminating the check engine light, the most likely cause is:

• The EGR system vacuum hoses are misrouted or broken.
• The EGR valve vacuum solenoid is bad.

You can find the EGR vacuum solenoid test here: How To Test The EVRV Solenoid (1988-1993 2.8L Chevrolet S10, GMC S15 Pickup, GMC Sonoma).

CASE 2: Engine speed DID NOT drop and the diaphragm DID NOT move up when you applied vacuum to the EGR valve. This test result indicates the EGR valve is bad (usually the result of a ruptured diaphragm).

Replace the EGR valve if you have:

• Confirmed that the EGR valve is not receiving vacuum with the engine running in idle RPM (TEST 1).
• Confirmed in this test section that engine idle speed did not change and the diaphragm did not move up/down when you applied vacuum to the EGR valve.

CASE 3: Engine speed DID NOT drop but the diaphragm did move up when you applied vacuum to the EGR valve. This test result usually indicates the EGR valve is stuck in its closed position or carbon buildup is blocking exhaust gas flow into the intake manifold.

Your next test step is to go to: TEST 3: Manually Pushing The EGR Valve's Diaphragm.

TEST 3: Manually Pushing The EGR Valve's Diaphragm

In this test section, we'll manually push the EGR valve's diaphragm with the engine at idle RPM.

With the EGR valve diaphragm depressed, you should see two things happen:

• The EGR valve opens and allows exhaust gases to enter the intake manifold.
• The engine's idle RPM should drop.

CAUTION: The EGR valve and engine can get very hot. If the engine is hot, let it cool down before starting this test.

CAUTION: This test is done with the engine running. Be careful and take all necessary safety precautions.

Let's get started:

1. 1

   Disconnect the vacuum line from the EGR valve.

2. 2

   Have a helper start the engine and let it idle.

3. 3

   Manually push the EGR valve's diaphragm upward to open the EGR valve.

4. 4

   You should see/hear a noticeable drop in engine RPM as the EGR valve opens.
   
   NOTE: As the engine RPMs drop, engine idle will become rough and the engine may stall.

5. 5

   Release the diaphragm.

6. 6

   Engine RPM should increase as the EGR valve shuts off exhaust gas recirculation.

Let's interpret your test result:

CASE 1: Engine speed dropped when you pushed the EGR valve's diaphragm open. This is the correct test result and confirms that:

• Exhaust gas is able to freely enter the intake manifold (no carbon buildup blocking its flow into the intake manifold).

If in TEST 1, your test result confirmed that engine speed DID NOT drop when you applied vacuum to the EGR valve, you can conclude the EGR valve is bad and needs replacement.

CASE 2: Engine speed DID NOT drop when you pushed the EGR valve's diaphragm open. This test result usually indicates the EGR valve or the intake manifold is blocked with carbon buildup and preventing exhaust gas flow into the intake manifold.

To further investigate this, go to: TEST 4: Checking For Blocked EGR Passages.