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i was driving my 91 coupe deville and the engine soon came on i pulled the code. it was the EGR. every now and then it comes back on and when i check the code it is still the EGR how can i get rid of it. or how much would it cost to change the EGR? will it affect my engine if not changed?
 

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The '91 has the older vacuum controlled EGR if memory serves me correctly. With the engine idling, get your finger under the EGR (the mushroom head so to speak). You should be able to feel the diaphram through the cut outs. Run the RPM up and hold it. Now you should feel the diaphram move (engine should be at operating tempurature). At idle, if you push up on the diaphram, the engine should stumble and/or stall. You might try to pull it off and clean any carbon you find around the pintle. Also check the diaphram to be sure it holds vacuum. Pull the vacuum hose off of the solenoid and leave it connected to the EGR. Now suck on it and then place your tongue on the end of the hose to seal it. Does it hold vacuum? It should. If the pintle is not sticking and it holds vacuum, the EGR valve itself is good. Now check the vacuum line leading to it from the solenoid that controls it. If that is good, maybe the solenoid is bad, disconnected or has dirty or corroded connections. A factory service manual would be a big help in diagnosing it.
 

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EGR related problems (spark knock at slight throttle when engine is hot, bad emission, SES light) are common for 4.9. Cleaning passages and replacing the components helps. In addition to the procedures outlined by Ranger I would recommend cleaning with a stiff wire the two EGR tubes inside TB under the plates. Usually it's enough to get rid og SES light.
 

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Forget the valve and such for the moment.Take the air cleaner off, hold the throttle blades wide open and look down the throttle bores with a strong flashlight. See the two tubes sticking up from the floor of the intake ... down below the throttle blades...?? Those are the EGR tubes and they are not supposed to be full of carbon and crud. Simply take a piece of stiff (coat hanger) wire and rod them out to clean out the carbon so the EGR can flow. If it is really bad and baked too hard you might have to pull the throttle body off and use a drill bit to clean out the tubes. Likely that will cure the problem. Don't worry about the loose carbon you dislodge. It won't hurt anything.
 
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