: 98 STS code P0404 (and P0300)



enderle_nathan
04-02-06, 07:56 PM
I have a 1998 Seville STS with a slight problem. I am a bit stumped, and wanted to see if anybody here has had a similar problem, and knew how to fix it. The car has 100,500 miles on it, I have had it for the last 25,000 miles, with no repairs besides a fuel pressure regulator. When the car is cold it has a rough idle at stop lights. Anything above idle, and when the car is warm, it runs great. The problem is worse on cold days (<40 F). This problem has existed a while, but there were never engine codes till recently. I now regularily (every other trip or so) get a P0404 code, and once got a P0300 code. P0404 is EGR Open Position Performance and P0300 is misfire detected. The misfire code makes sense to me, because of the rough idle. I checked the EGR valve function as shown on this page

http://www.cadillacforums.com/forums/cadillac-tech-tips/59077-how-clean-egr-valve.html

and the spring loaded thing works. Also, the symptoms were wrong for this to be a sticking EGR valve, according to that page. My thoughts were that the spark plugs and wires need to be replaced, and given the miles on the car I will do this soon. Could bad plugs cause the EGR code, though? My experience with fouled plugs on other cars is that they run bad all the time, not just cold at idle on cold days. Any help is appreciated, and thanks in advance.

Nathan

Ranger
04-02-06, 08:17 PM
No, I don't think plugs or wires would cause an EGR code. I think the EGR operates in closed loop so the PCM is getting info back from it. Maybe it is intermittently sticking.

1993 eldorado green1
04-03-06, 01:26 PM
I have a 1998 Seville STS with a slight problem. I am a bit stumped, and wanted to see if anybody here has had a similar problem, and knew how to fix it. The car has 100,500 miles on it, I have had it for the last 25,000 miles, with no repairs besides a fuel pressure regulator. When the car is cold it has a rough idle at stop lights. Anything above idle, and when the car is warm, it runs great. The problem is worse on cold days (<40 F). This problem has existed a while, but there were never engine codes till recently. I now regularily (every other trip or so) get a P0404 code, and once got a P0300 code. P0404 is EGR Open Position Performance and P0300 is misfire detected. The misfire code makes sense to me, because of the rough idle. I checked the EGR valve function as shown on this page

http://www.cadillacforums.com/forums/cadillac-tech-tips/59077-how-clean-egr-valve.html

and the spring loaded thing works. Also, the symptoms were wrong for this to be a sticking EGR valve, according to that page. My thoughts were that the spark plugs and wires need to be replaced, and given the miles on the car I will do this soon. Could bad plugs cause the EGR code, though? My experience with fouled plugs on other cars is that they run bad all the time, not just cold at idle on cold days. Any help is appreciated, and thanks in advance.

Nathan



your plugs may not be able to burn all the extra fuel when its cold. that would be my only guess. also some of the gm platinum plugs were prone to loose the platinun tips and it caused crazy missfire problems. i had a camaro ss that this happened too and couldnt figure it out either, sure enough a major tune up and it was back to normal again..

the plugs and wires are fairly cheap and only takes like a half hour to do, i would also replace the fuel filter while your at it.

another thought after the tune up is to find a shop who has "BG" fluids and the equipment to properly do the flushes. the fuel system has been proven by all the mechanics that i know to really work well and has actually stopped random missfire codes because of dirty injectors and such..

danbuc
04-03-06, 07:15 PM
P0404 is the computer reading an error in EGR system performace. The valve itself has a pintle that moves up and down depending on the amoutn the valve is opened (variable stepper motor). The PCM reads the position of this valve in relation to the commanded position of the EGR valve itself. When the PCM reads a great engouh difference between the two values it throws a code. It's usually cause by a sticking EGR valve. The PCM command it to (let's say 50% at 1/4 throttle cruising down the highway). If the PCM reads recieves a value from the pintle position sensor that says the EGR valve is only opened up say 38%, it determines that the EGR valve is not performing properly for whatever reason, and it throws a code. Being an emmisions code and all, it will turn the check engine light on.

Bad plug wires will nto cause an EGR System Performance code to be thrown. The EGR valve doesn't care what gases you pump threw it. All it cares about it whether or not it's actually flowing the commanded amount through the valve. Here's the info right out of the factory service manual.

Conditions for Setting the DTC's
-Ignition voltage 12 volts or less
The linear EGR pintle position error is greater than 75 counts

-Ignition voltage more than 12 volts
The linear EGR pintle position error is greater than 25 counts.

On a side note, it as mentioned above by ranger, it is posible for the valve to stick intermittently. Mine sticks every so often and will throw the P0404 code. It only happens every other week or so, having started just a month or so earlier. I haven't bothered with it, since I just clear the code and go about my business. If you went ahead and cleaned it out, then you should have any problems until it get caked up with carbon again. If it's still throwing a code even while moving freely, then it's time to invest in a new EGR valve.

piers
04-12-06, 05:25 PM
i,ve recently been working on a 97 deville which has thrown both these codes at me aswell as others such as fuel trim codes .replacing the o2 sensors has cleared the fuel trim codes . i replaced the egr valve and have since not seen the P0404code again but when cold idle is still very lumpy and occasionally feels like its surging , trying to pull away by it self . i plugged a scan diagnostic tool into it which detects misfiring on almost every cylinder yet no codes have appeared for a while . all leads , plugs have been changed , luckily i have a "donor" northstar (which performed fine) from which i have swapped all the coil packs and ignition module but this has still not rectified the problem . this only happens when cold, 2 minutes of running the engine warms up and behaves perfect . with no codes to point me in any direction i'm pulling my hair out . next step i think i'm gonna remove and refit the inlet manifold with new gaskets and make sure there is no air/vacuum leaks . i've read on this forum that after replacing parts u can reset the pcm by disconnecting the battery for at least an hour and a half ... worth a try .
Have u had any further developments ?