I have been getting the P0404 EGR cicuit code lately and a misfire when the engine is cold. The misfire seems to slowly go away as the engine reaches normal operating temperature, but there is still a MISS when under load. I replaced the EGR valve and the P0404 code still appears. I thought the miss may have been a plug wire or a plug itself, but I changed all of them too. The P0404 code also seems to be sensitive to the outside temperature. On very cold days ( < 30 degrees ) I get the misfire worse, but no code. On days above 60 degrees, I don't notice the misfire and no code either. It seems to pop up a lot when the outside temp is between 30 and 60 degrees.

Please help me out on this one. My gas mileage is dropping to about 18 MPG where I usually get 21.

Thanks

Donnie
1999 Deville Concours

I've been having the same problem as well. I recently changed all of my plugs to Bosch Platinum 2 Plugs and so far I've only had a few slight minor sputters/ misfires during the cold early mornings with the 04040 code but once the engine gets warm she runs like a champ. Over the last two or three days there was been to codes but I still can feel that somethings not right so I just recently brought some AC Delco 41-950 Platinum Plugs and hopefully the minor sputters/misfires will go away.

Well.....I have followed this thread with interest and suprised there is no input. Is it just the three of us? Out of place because I have a 97 STS but follow the DeVille threads because of the one I just lost.

Here is my P0404 story. Code set three times in a month. Clear codes and fine, till the next time my special circumstances came up.

#1. Temp, 25 degrees, leave home, engine warmed to 1/4 of 1/2 gauge normal. Easy on the throttle but a good head wind.
#2. Same.
#3. Same temp, warm up time but 6 inches of snow on the ground.

Hey, I am puzzled. New EGR because the code popped? No. Plugs at 87,000 solved many questions. Tryin to keep that Cadillac Style goin but these forums can spawn nightmares.

I remember this post but did not respond because I real am not sure what to say. Was hoping someone more knowledgable would chime in.
P0404 - Exhaust Gas Recirculation (EGR) Open Position Performance
That would lead me to say, clean the EGR but you replaced it. I dunno (shrugg).

Is the EGR vacuum controlled? What shape is the vacuum line in? Any leaks?

EGR is vacuum operated and electrically controled via an EGR control valve on the 4.9. On the Northstar it is electronically controlled and operated.

Hmmmm, to add to the confusion the (P0404) set again today. Very flat in Mid-Michigan so lugging the engine in overdrive is rare. Headwind, deep snow and today a 90 degree curve taken aggressivly. No down shift, just maintained my plan and kept the throttle even. Hope this won't put me in the code of the month club but odd. Runs fine but wonder why????