How To Test The EGR Valve (1994-1995 2.2L Chevrolet S10, GMC Sonoma)

TEST 4: Making Sure The EGR Valve Pintle Is Not Stuck Open

The exhaust gas the EGR valve channels into the intake manifold carries quite a few carbon particles. Eventually, these carbon particles will build up on the pintle and its seat.

The result (of this carbon buildup) is a pintle that will not fully close on its seat, allowing exhaust gas to enter the intake manifold at all times.

To check for this condition, we'll remove the valve and blow compressed air into the valve's exhaust port to see if it comes out of the valve's intake air port.

If the EGR valve is OK (not stuck open), air will not pass from one port to the other port (see image above).

These are the test steps:

1. 1

   Remove the EGR valve from its location on the side of the cylinder head once the engine has cool down.

2. 2

   Turn the EGR valve over.

3. 3

   Visually check to see if the valve's pintle is stuck open.

4. 4

   Blow compressed air into the bore (that contains the pintle) and see if this air comes out the outlet.

5. 5

   Air should no pass thru to the other side.

Let's take a look at what your test results mean:

CASE 1: Air did not pass thru. This is the correct test result.

You can conclude that the EGR valve is bad (damaged diaphragm) and needs replacement if you have:

• Confirmed that the EGR valve IS NOT receiving vacuum when the engine is idling (TEST 1).
• Confirmed that the EGR valve's pintle DOES NOT rise and the engine's idle is unaffected when applying vacuum to the EGR valve (TEST 2).
• Confirmed that air DOES NOT pass thru the valve (this test section).

NOTE: You can re-use the same EGR valve gasket if it isn't damaged. Whether using the same gasket or a new one, you must install the gasket dry. In other words: DO NOT use any sealer (like RTV Silicone) on it.

CASE 2: Air passed from one port to the other. This test result tells you that the EGR valve's pintle is stuck open. Replace the EGR valve.

TEST 5: Checking For Blocked EGR Passages

Up to this point, the previous tests have confirmed:

• The EGR valve IS NOT receiving vacuum when the engine is idling (TEST 1).
• The EGR valve's pintle and diaphragm moved up/down when you applied/released vacuum to the valve, but the engine's idle DOES NOT worsen (TEST 2).

There's a good possibility that the exhaust gas passages in the EGR valve's adapter block or the intake manifold are blocked.

In this test section, we'll perform a very simple test that'll help us confirm whether or not the EGR passages are blocked.

IMPORTANT: If the engine has been running for any length of time and the EGR valve is hot, wait till the engine cools down completely before removing the EGR valve.

Let's get started:

1. 1

   Remove the EGR valve from its place on the engine.
   
   NOTE: Remove any and all parts, nuts, tools, or anything that may fall into the open ports on the EGR valve adapter.

2. 2

   Start the engine and let it idle for no more than 10 seconds.

3. 3

   After no more than 10 seconds, turn the engine off.

4. 4

   You'll see one of two results:
   
   1.) The engine started and ran with a very high idle and/or stalled.
   
   2.) The engine started and ran normally (no high or rough idle).

Let's interpret your test result:

CASE 1: The engine started and idled normally (no high or rough idle). This test result confirms that the exhaust gas recirculation passages are blocked, preventing exhaust gas from entering the intake manifold.

Your next steps are to remove the EGR adapter block and check it for carbon blockage (obstruction) and possibly even remove the intake manifold to check it for EGR passage blockage.

CASE 2: The engine started and idled high and/or the engine stalled. This is the correct and expected test result and tells you that the EGR passages ARE NOT blocked.

You can conclude that the EGR valve is bad if you have confirmed:

• The EGR valve's vacuum hose IS NOT supplying vacuum to the valve when the engine is idling (TEST 1).
• The EGR valve's pintle and diaphragm rose when (you applied vacuum to the valve), but the engine's idle DID NOT worsen (TEST 2).

How Can I Tell If I Have A Negative Or Positive Back Pressure EGR Valve?

The 1994-1995 2.2L Chevrolet S10 (GMC Sonoma) comes with a negative back pressure EGR valve.

But it's not unusual to find that the EGR valve has been replaced by an aftermarket positive back pressure EGR valve.

If the EGR valve on your vehicle has application numbers stamped on its diaphragm housing (see the image above), you can determine its type.

The numbers stamped on the EGR valve should have a trailing letter N or a letter P. The letter N indicates a negative back pressure EGR valve.

The image above shows an EGR valve stamped with the letter N indicating it's a negative back pressure EGR valve.

An EGR valve stamped with a trailing letter P indicates a positive back pressure EGR valve.

Why is this important? Because only the negative back pressure EGR valves can be bench-tested off of the vehicle with a vacuum pump.