Engine misfire at certain times of the day - 2003 STS
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Northstar Engines and System Technical Discussion Discussion, Engine misfire at certain times of the day - 2003 STS in Cadillac Engine Technical Discussion; Okay this thing has me stumped. It has only been happening for 3 days. I drive 14 miles to work ...
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    crezzy2k1 is offline Cadillac Owners Member
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    Engine misfire at certain times of the day - 2003 STS

    Okay this thing has me stumped. It has only been happening for 3 days.
    I drive 14 miles to work each day, there and back. Car starts fine first thing no problems.
    I then drive to lunch which is about 2 miles each way. Car starts fine no issues.
    Now after lunch i come back to the car start it up and all is well for about 2-5 minutes, then in light traffic it will develop a nasty misfire (it won't even rev higher than 2000rpm), the SES light will begin to flash, i get on to an open stretch of road and it calms a little but still light mis. By the time i get back to work the mis-fire is over and the engine is running normally again. The flashing SES has stopped. I shut the car off and go back to work.
    I come out of work about 4 hours later, start her up, have a solid SES light and P0300 (mis-fire) code Current (the only code)

    I start the car the next day in the morning and all is well and the Current P0300 goes into history, i then restart the car after lunch and back to the mis-firing.

    This is driving me nuts!!!

    What i have done so far is pull all the plugs, investigate them all of them where fine apart from one which looked fine apart from from the base of the thread which was wet, the electrode was to be expected though. I have since swapped that plug and its boot to the cylinder next to it to see if i can rule out the plug and ignition cassette.
    Last time the plugs where changed was 57,000 miles ago and they all look good.
    Something i should mention is that i have had P0420 (catalyst low efficency) come up twice in the past 40 days the last time it came up was last week. The car seems to run fine when not mis-firing.

    Do you guys think that a clogged or partially clogged cat could be causing the engine to strain and cause a mis-fire?

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    Re: Engine misfire at certain times of the day - 2003 STS

    To me, the misfire and the low cat efficiency point to a raw fuel source in your exhaust. Either the plugs are bad, which is debatable, or you have a fuel delivery issue. The clogged cat MAY be a possibility, but I'm not sure. Keep in mind, I'm still learning myself, just my 2 cents, but that's what it sounds like.

    Do whatever's cheapest. Check the cat, listen to your injectors, and check the fuel pressure regulator. Then go from there.

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    crezzy2k1 is offline Cadillac Owners Member
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    Re: Engine misfire at certain times of the day - 2003 STS

    Thanks for the input CadillacLuke24. I have checked the injectors with the screwdriver to ear test, all of them are clicking along nicely. I have checked the regulator and see no problem with fuel in there or in the vacuum line. What i haven't checked yet in the fuel filter, i do have a new one lying around that i have been putting off installing it so I'll put that in and see if anything improves.

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    Re: Engine misfire at certain times of the day - 2003 STS

    Just had this problem about a month ago Drove me nuts car wouldnt throw a code until it was real bad. Then it would go away for a while them come back again . My guess is the ICM . Mine had a intermintint problem I replaced it with one I had from a old coil pack I got luckly and guessed the right coil pack to attack from looking at the plugs. Look under (WOT PROBLEM and PO300) a lot of good ideas there and help from the forum.

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    Re: Engine misfire at certain times of the day - 2003 STS

    Thanks ELDO GUY i read your thread yesterday, (i always do a bit of homework on these here before i post, so much info on here!) i was thinking about getting myself over to the nearest junkyard and picking up a coil cassette, but i'm not gonna be able to do that until the weekend. Did your plugs show any signs of problems while the ICM was bad?

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    Re: Engine misfire at certain times of the day - 2003 STS

    Just got home from work. Car started and immediately started mis-firing at idle, i gave here a little gas and she sounds and smells terrible. I let the car warm up a little before i took the highway home, once on the highway the SES flashed on the onramp then went to a steady SES for the rest of the trip, once the car was upto speed there was again no issues. When slowing down for a stop light i no longer had the mis-fire.

    I pulled into the garage and let the car idle a bit, no mis-firing. I pulled the codes and the only one is P0300 History, i then shut her off then restarted a minute later, i noticed this time the idle was about 650 as opposed to about 750 when i pulled into the garage, not sure what the computer is doing properly compensating for the mis-fire. I'll let her cool down for a bit then i'll go out there and start her again and see if she mis-fires, if she does i'll pull the plugs again and see whats going on.

    While i was in the garage i gave the Cat a little tap with a broom handle (sophisticated test, i know ;-) i can definitely here rattling in there so I'm pretty sure its had it.

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    I would go to the junk yard and try and find one that looks good. The date of mfg is stamped on the back so id go for the newest i could find. Some of my plugs looked good and some not so much I think. I posted pictures of the plugs in that thread. If you swap out icm's make sure it all comes out some times a. Small connector stays in there if you miss it youll really have a problem.

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    Re: Engine misfire at certain times of the day - 2003 STS

    then in light traffic it will develop a nasty misfire (it won't even rev higher than 2000rpm)
    While i was in the garage i gave the Cat a little tap with a broom handle (sophisticated test, i know ;-) i can definitely here rattling in there so I'm pretty sure its had it.
    I was leaning towards the ICM or ignition cassette, but that CAT does sound highly suspicious.

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    Re: Engine misfire at certain times of the day - 2003 STS

    I read a sign in the Midas here that if the CAT went bad, it's an indicator that something else is amiss. For the knowledge files, can a CAT just get old and go out?

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    Re: Engine misfire at certain times of the day - 2003 STS

    Get a spraybottle of water. When the engine is running rough and misfiring as badly as it will, spray the exhaust manifolds with water and you'll be able to tell pretty quickly which cylinder/s are misfiring.
    Don't mess with Binky Bear!


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    Re: Engine misfire at certain times of the day - 2003 STS

    Quick update:

    Last night i went back to the car after it had been shut down for about 2 hours, started her up no mis-fire. The SES was on with P0300 in History, i shut her off.

    I had a new fuel filter so i decided to go about putting that in just incase, i relieved the fuel pressure from the schrader valve in the engine bay (lots of pressure!) then went about trying to get the old fuel filter out, what a PITA it is to get that nut off!, anyway i got it out it looked fine. Put the new one, stated her up and she was running fine. Decided to take her onto the back roads for a few WOT's, again no issues at all. Filled her up with BP 87, then bought her home. All this time the SES was on.

    Got out there this morning and stated her up for the trip to work, again no issues. SES went out about 1/2 mile down the road.
    Got to work and she was running fine.

    Would the ignition cassette/ICM really be that erratic?

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    crezzy2k1 is offline Cadillac Owners Member
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    Re: Engine misfire at certain times of the day - 2003 STS

    Quote Originally Posted by dkozloski View Post
    Get a spraybottle of water. When the engine is running rough and misfiring as badly as it will, spray the exhaust manifolds with water and you'll be able to tell pretty quickly which cylinder/s are misfiring.
    Bit tight in there to even spray the manifolds. Plus it hasn't ever mis-fired when i have been at home so even checking the plugs doesn't do much good.

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    crezzy2k1 is offline Cadillac Owners Member
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    Re: Engine misfire at certain times of the day - 2003 STS

    Quote Originally Posted by CadillacLuke24 View Post
    I read a sign in the Midas here that if the CAT went bad, it's an indicator that something else is amiss. For the knowledge files, can a CAT just get old and go out?
    You're right on that one. But i think the CAT has just decided to quit. 9 years/120000 miles, car originally came from New Jersey so perhaps lots of traffic has killed it. That being said this is the first car i have ever owned with a CAT that is on its way out.

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    Re: Engine misfire at certain times of the day - 2003 STS

    Quote Originally Posted by crezzy2k1 View Post

    Would the ignition cassette/ICM really be that erratic?
    It's possible that as it heats up the heat affects something. I've had a starter fail only when it was hot. Contacts expand and break contact.

    Everything you ever wanted to know about CAT's.
    http://www.aa1car.com/library/converter.htm

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    Re: Engine misfire at certain times of the day - 2003 STS

    Thanks Ranger! More info for the knowledge bank

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