This tutorial addresses a very common problem I've seen on the 1987-1995 GM 4.3L, 5.0L, and 5.7L engines with throttle body fuel injection system: a stuck open EGR valve.
And when the EGR valve gets stuck open, it'll cause a rough idle condition that may hard to diagnose.
If you suspect that the EGR valve is behind your engine's rough idle condition, this tutorial will help you to troubleshoot it with 2 simple tests.
NOTE: This tutorial covers the EGR valve of the 1987-1995 throttle body fuel injection (TBI) system of the 4.3L, 5.0L, and 5.7L GM engines.
Contents of this tutorial:
Symptoms Of A Stuck Open EGR Valve
As you're probably already aware, the EGR valve's job is to let a metered amount of exhaust gas (that is oxygen free) into the intake manifold to reduce the amount of oxygen entering the engine. This reduction in oxygen from the air/fuel mixture reduces the amount it creates when it combusts (inside the cylinders) and thus reduces the amount of oxides of nitrogen (NOx) created.
This process of letting exhaust gas re-enter the engine's cylinders is known as re-circulation in tech terms. This re-circulation should happen only after the engine has warmed up and when the engine is under load (like when you're accelerating the vehicle from a stand-still).
At idle, the EGR valve should NOT be letting any amount of exhaust gas back into the cylinders. So, when the EGR valve gets stuck open... it isn't able to stop the flow of exhaust gas from entering the intake manifold and thus the combustion chamber and your pickup or SUV will idle rough.
Here's a few more specific symptoms you'll see:
- Rough idle.
- Code 32: EGR System Problem.
- Code 33: MAP Sensor Signal High.
- Bad gas mileage.
- Won't pass the state mandated emission test.
TEST 1: Continuous Vacuum to EGR Valve
The fuel injection computer is the one that regulates the opening and closing of the EGR valve thru' the application and release of vacuum (through a vacuum solenoid).
One side of the EGR vacuum solenoid (the one that has only one vacuum inlet port) connects to the vacuum port labeled with the letter J on the throttle body.
In case you're wondering, the letter J is embossed above the vacuum port on the throttle body.
The other side of the EGR vacuum solenoid has 2 ports. One is a vent port (that vents vacuum to the atmosphere) when the engine is idling. The other port connects to the EGR valve and should only have vacuum when the PCM commands the solenoid to activate.
So, the first thing we'll do is to make sure that vacuum is not being applied to the EGR valve when the engine is just idling.
IMPORTANT: Be careful and take all necessary safety precautions while working around a running engine. Think safety all of the time.
These are the test steps:
Disconnect the vacuum hose that connects to the EGR valve vacuum port. Leave the other end that connects to the EGR vacuum solenoid connected to it.
Connect a vacuum gauge to the vacuum hose you just disconnected from the EGR valve.
NOTE: If you don't have a vacuum gauge... don't panic. You can still check to see if vacuum is present in the vacuum hose.
Start the engine and let it idle and check to see if the vacuum hose has vacuum.
The vacuum hose that connects to the EGR valve should not have vacuum.
Let's take a look at what your test results mean:
CASE 1: Vacuum WAS NOT present. This is the correct and expected test result since manifold vacuum should NOT be present when the engine is idling.
The next step is to remove the EGR valve from its place on the intake manifold and make sure it's valve pintle is not stuck open. For this test, go to: TEST 2: EGR Valve Pintle Stuck Open.
CASE 2: Vacuum was present. This is a problem, since vacuum should not be present when the engine is idling.
This could be caused by one of two possibilities... either the EGR solenoid is defective or the vacuum hose is attached to the wrong port on the EGR solenoid.
Resolving this ‘continuous vacuum to the EGR valve at idle’ problem will solve the EGR valve and rough idle issue.