My 94 Deville has a off idle knock at light throttle, it's very easily heard. There are no codes coming up, and I only use 93 octane. Any ideas? I'm thinking the EGR system is getting plugged?

My 94 Deville has a off idle knock at light throttle, it's very easily heard. There are no codes coming up, and I only use 93 octane. Any ideas? I'm thinking the EGR system is getting plugged?

Classic sign of the EGR tubes plugging. Hold the throttle blades wide open and look down the throttle bores. See the two tubes sticking up at you from the floor of the intake below the throttle blades...? Those are the EGR tubes and they are not supposed to be full of carbon. Rod them out thoroughly with a piece of stiff wire until they are clean and free of carbon. Clean the throttle bores and backside of the blades while you are in there. After cleaning the throttle bores and blades disconnect the battery negative terminal for 60 seconds to reset the idle learn offsets to the default "clean throttle body" values.

I did that a few weeks ago because it was "bogging" off-idle. I didn't notice the knock then, but now the bog is gone after I cleaned out the EGR tubes in the intake and I cleaned the throttle body. I'm not sure if maybe the EGR valve itself is getting carboned up, or what else is now causing this!

Thanks!

The following was cribbed from another post in the Deville section but it applies....


First thing to do is to reach under the EGR valve "hat" with your finger tips and feel the diaphragm that operates the actual EGR valve pintle. With the engine idling, reach under the valve with your finger tips and pull upward gently to manually open the valve and the engine should stumble or die if the EGR is flowing. That will at least tell you that the EGR passages are clean and capable of flowing exhaust gas.

Use a helper and have them GENTLY crowd the throttle while the car is idling in gear. In drive, with the engine warmed up thoroughly, have them hold the brakes firmly and while in drive just open the throttle slightly to gently load the engine against the torque converter. While they do this, reach under the EGR valve hat and feel the diaphragm with your finger tips. It should quiver and open on it's own when the throttle is opened. If not, then the EGR is not operating correctly. If it is not opening, then check the vacuum supply to the EGR solenoid and the vacuum signal to the valve when the throttle is opened IN DRIVE with the engine warmed up. The PCM controls the EGR solenoid and will open it when the throttle is opened in drive witht the engine warmed up completely. If it does not have an adequate vacuum supply then it cannot operate. If there is vacuum to the solenoid but it does not open and supply vaccum to the valve when the throttle is opened in drive then it may not be operating correctly.

When you cleaned the EGR tubes did you go all the way thru them to the passage below...?? The tubes are 1 to 1.5 inches long and open into the EGR passage from the EGR valve below. You need to clean all the way thru them to make sure the EGR is flowing.

The EGR valve is pretty robust on that package so I would not jump to the conclusion that it is failed without doing some more diagnosis.