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THE ABCs of EGR, Part II

Clearing the air

In part one of this feature on the EGR system, we covered both the development and the basic theory behind its operation. Now we'll get into some important information you'll need to know in order to identify and service your car's vacuum-operated EGR system. (As noted previously, electronic systems are another matter better left to a service pro.)

Variations

There are a number of variations to the vacuum-operated theme. They all try to accomplish accurate metering of exhaust gas, regardless of their configuration. Here's the breakdown:

Non-Backpressure Controlled: Early systems used this method where the amount the valve opened was controlled solely on the strength of the vacuum signal applied to it. A few manufacturers improved on it by hyper-accurately controlling the vacuum signal to the valve, or adding an additional throttle-actuated mechanism in the intake passage.

Backpressure Controlled/Externally Modulated: In this system, a modulator controls the strength of the vacuum signal to the valve. The modulator has a port connected to the exhaust system and, depending on the amount of backpressure present in the system (more during acceleration), the appropriate amount of vacuum is applied to the valve.

Backpressure Controlled/Internally Modulated: In this system, instead of exhaust backpressure being used to control the vacuum signal to the valve, it is used to provide additional lift to the valve. This is accomplished via an internal exhaust passage and special valve design.

Testing

Armed with the basics on the different types of vacuum-controlled EGR systems, you are now ready to test the system. We recommend consulting a service manual for your specific vehicle, paying special attention to safety precautions.

Keep in mind that a lot of configurations exist. For instance, certain models have a vehicle speed sensor that is used in connection with vacuum delivery to the valve. Unless the vehicle is traveling above a certain speed (25 to 30 mph), vacuum will not be routed to the valve. You can imagine that testing such a system would require specific techniques and safety precautions. The manual will inform you of such information.

Rules of Repair

Here are a few basic rules of thumb for diagnosis and repair. All tests should be performed on a fully warm engine (although the "passage test" could be done before the engine is fully warm, as noted below). If the "Check Engine" malfunction indicator lamp is illuminated, a diagnosis should be performed to determine the cause. It is likely that a failure in the EGR system will cause the lamp to light up. If that is the case, proceed with the testing.

In addition, the exhaust and intake passages to the valve should be clear. An easy way to test this is to push up on the underside of the valve while the engine is idling. (Again, be aware of how hot the valve is to the touch! Let the engine cool down a bit, if needed, before proceeding with this test.) The engine should idle roughly, or stall if the passages are clear.

On the non-internally backpressure-controlled valves, it's possible to apply vacuum to the valve with a handheld vacuum pump. If the valve holds vacuum and lifts, and the engine idles roughly, you then know that the valve is good and the passages are clear.

When the idle quality does not change while the valve is lifted, then the problem is a carbon-blocked valve, passage, or passages. You'll need to remove the valve and inspect it and the passages.

Cleaning the valve assembly is pretty straightforward, but it is not recommended to completely immerse the valve in any cleaning agent. It is sometimes possible to clear a blocked passage with a drill-driven rotary flex tool, or "snake," similar to the technique used to quickly unblock drain plumbing in a home. It takes skill and practice to effectively use one of these, so you might want to visit a repair professional instead.

In addition, the valve should open at the appropriate time. You can safely test it by feeling the bottom of the valve with your finger and revving the engine simultaneously. If the valve is too hot to touch, you should be able to view its operation, possibly using a mirror tool, if necessary. (On the backpressure controlled/internally modulated style, this is the only practical method of testing, although more complicated methods exist.)

If the valve doesn't lift at the appropriate time, you'll have to check for presence of vacuum at the valve and back to the source, if necessary. You'll have to check for presence of exhaust at the external vacuum modulator backpressure port, if so equipped, and back to its source. If it's carbon-blocked, it is sometimes possible to use a "snake" to clear it as well. If vacuum is reaching the valve, and it's not opening when tested, the valve is likely bad.

So, take a little time to do your homework on your EGR system. This extra effort can save you money—and clear the air at the same time.