View Full Version : My 2000 Deville has the p0300 code, and possible p0420 code. Need help

04-28-10, 12:18 AM
well I think this is going to be long-winded.

I've tried to diagnose/fix the problem for about a week with the help of you all just by lurking here. I wouldnt know where to start if not for this site. Well my 2000 Deville began to start rough, shake and run rough like it was losing power. I couldnt really accelerate, but after some time on the road it would kind of hiccup and then smooth out and run regular. I also detected a smell, like a rotten egg smell(??) that I've seen people type about on here. I pulled the code and got the p0300 which is the random misfire. I changed the spark plugs(sidebar) and the spark plug boots. Didnt fix the problem the same thing was happening. I dont remember when this happened but it got to a point where my Deville would not start rough and shake. It will run smooth for maybe 30 minutes and then start misfiring, the SES light would come on and start blinking is how I would know and it would be riding rough. I used the carb cleaner to spray all the hoses and around the engine to see if it was a vacuum leak. The idle didnt change not one bit. I used the Ethos scanner to diagnose where the misfire was coming from. They were coming from 2,4,6 and 8, the front coil pack. I did what I read on here and flip flopped the coil packs, and sure enough the misfires were comin from 1,3,5 and 7 now. So I figured I found the problem. Strapped for cash I bought a USED coil pack from a junkyard, but when I put it on, the misfiring started instantly when I started up the car instead of after I rode for 30 minutes or so. But now the misfire was on both sides but only in 2 places. In 3, and 4. In the mist of all this I got the p0420 code a couple times which is the catalyst fuel efficiency, but that code dont show every time after I would clear it. Mechanic said the misfire is causing that code/messing up the catalyic converter. And only 1 time I got the p0174 code which means its running lean. I also checked the fuel pressure, and I was operating low at 35 Ksi when it suppose to operate at 41-47. So I took the USED coil pack I got off cuz it was misfiring as soon as I started the car and put the original coil pack back on and now its back to the same thing of misfiring in cylinders 2,4,6 and 8 after it let me ride a little while.

(sidebar)the spark plugs that were in there when I changed them were NGK platinums. I read here about my car needing the AC Delcos, but being strapped for cash which is the reason Im doing the work mostly myself I got NGK again. Now Im thinking you all will say thats the problem, but will they only mess up on one side and not the other? And when I changed to the USED coil pack, will only 2 mess up and not the other 6?? why are the different coil packs causing different misfires? maybe both were bad, and thats the problem? Something I read on here is about the plenum may cause this. I havent looked at that nor do I even know where to look right now.

Sorry to be SOOOOOOOO long-winded but I tried to go step by step as best I could remember. Any help would be greatly appreciated as you all have already helped me so much without knowing it. Thank you

04-28-10, 01:38 AM
NGK, Bosch, woo-woo, been there, done that. They are like known to not fix your misfire. Only the Ac-Delco #41-950 .050" pregapped plugs will work. Northstars are picky on the plugs and are going to cry when you serve them a wrong kind of dish.

I think that's why you're still getting a misfire after changing your coil packs. Your original coil pack seems like it's gone forever.

Also the misfire on 4 cylinders is very bad situation for your engine and possibly messing up with your catalytic converter. I always believed that rotten egg smell was an indication of bad catalytic converter, but one of our member once said it's because of a rich mixture. And think about it, if your engine misses, guess where the unburned fuel goes?

04-28-10, 10:20 AM
For his engine it's AC Delco #41-987.

Those coil cassettes also have removable ICM's - those little black boxes clipped into the L leg of the cassette. There's another sealed electrical connector in there - carefully remove the ICM's and check those inner connectors for cleanliness/moisture.

Do all the injectors sound alike while idling ?? Use a stethoscope (Walgreen) with a length of vacuum hose in place of the chest diaphragm.

The "plenum" - that rubber connector between the throttlebody and intake manifold - it sometimes cracks down at the bottom - just next to the clamp that you can see down in the valley at the throttlebody. #2 in the drawing.

04-28-10, 11:45 AM
Thanks for the responses.

I guess my first order of business should be to get the AC Delcos #41-987 which I should of gotten in the first place instead of trying to save $3 a plug. But wouldnt ALL the plugs react bad with the Northstar instead of just a few?? So wouldnt that mean at minimum I have ANOTHER problem at least?

Also, Im not THAT mechanically inclined, so "with a length of vacuum hose in place of the chest diaphragm" is throwing me right now.

As far as the plenum and the ICMs on the coil cassettes, I guess I should check them first??

Mark C
04-28-10, 11:49 AM
Is the check engine light flashing? If so you are hurting the catalytic converter by running a rich fuel mixture thru it. If the P0420 code doesn't clear after you resolve the misfire your going to be looking at a $1000 plus catalytic converter replacement job.

04-28-10, 11:58 AM
Is the check engine light flashing? If so you are hurting the catalytic converter by running a rich fuel mixture thru it. If the P0420 code doesn't clear after you resolve the misfire your going to be looking at a $1000 plus catalytic converter replacement job.

yes, its flashing...thats what I thought which is the reason I stopped driving it altogether....Im hoping its not destroyed when I fix the misfire

Mark C
04-28-10, 02:21 PM
Only took driving my Seville (with 140K on it) about 4 or 5 miles to waste mine. guess it depends on your mileage and how much of the cat is left at this point.

04-28-10, 02:38 PM
The "chest diaphragm" on a stethoscope is the flat thingy that the doc listens to your heart and lungs with - you pull it out of the tube and replace it with an 18" length of vacuum hose and use that apparatus to listen to engine parts/places for whirrs, hisses, clicks, buzzes, and other nasty noises.

Or, you can spend a bit more and look for a "mechanics stethoscope" in an auto parts store.