Misfire 1998 STS
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Northstar Engines and System Technical Discussion Discussion, Misfire 1998 STS in Cadillac Engine Technical Discussion; Well I picked up my first caddy today. Its a 1998 STS with the 4.6 Northstar. I am getting a ...
  1. #1
    nhradiotech is offline Cadillac Owners Member
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    Misfire 1998 STS

    Well I picked up my first caddy today. Its a 1998 STS with the 4.6 Northstar. I am getting a P0302 Cylinder #2 misfire. I pulled the #2 plug and it was covered in oil. I replaced it and the new plug fouled with oil immediately. I am very unfamiliar with these motors so any help is appreciated. What I have done so far:
    Compression test, 130PSI and held until I released it
    Spark test, good
    Replaced Plug
    Swapped coil with known good

    The previous owner had some overheating issues when he got it. He replaced the thermostat and life was good. I am open to any and all suggestions. I have done a few searches but I am not finding any threads with this particular problem. Thanks in advance!

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  3. #2
    nhradiotech is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    No ideas? I am starting to suspect the HG's but I would like to be sure. Has noone else had this problem?

  4. #3
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: Misfire 1998 STS

    An oil fouled spark plug is usually NOT an indicator of Northstar head gasket problems. (Assuming the plug electrodes in the cylinder are fouling, NOT oil on the shell or insulator, which indicates a weeping cam cover plug well O-ring.)

    If the plug tip is oil fouled, then you have either grossly worn valve stem seals or a bad set of piston rings.

  5. #4
    nhradiotech is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    Thanks for the reply! I will check the cam cover o ring. The electrode is actually soaked with oil. Is there anything special about pulling the valve covers?

  6. #5
    Ranger's Avatar
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    Re: Misfire 1998 STS

    Yeah they are a PITA. Front one has the water pump drive pulley driven off of the intake cam, so the shaft extends through the cam cover. The rear is too close to the firewall. You'll have to lower the cradle or rock the engine forward. Go to the Technical Archives at the top left of the forum page and read up on oil consumption.

  7. #6
    Submariner409's Avatar
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    Re: Misfire 1998 STS

    Valve seal replacement on a Northstar, particularly if the engine is in the car, is NOT for the faint of heart.................Incredible amount of very tricky disassembly involving the entire valve train. You'll absolutely need a GM/Helm shop manual or subscribe your car to www.alldatadiy.com.

    The cam cover O-rings have nothing to do with oil on the plug electrodes - only on the insulator or shell hex flats.......unless there is so much oil in the plug well(s) that it runs all the way down the plug as a liquid when you pull it out of the well.

  8. #7
    nhradiotech is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    There is zero oil in the plug well. Only on the electrode. Do vale stem seals in these fail this catastrophically? I have had other motors with bad valve stem seals that blow blue smoke but are not soaked with oil when pulled.

  9. #8
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: Misfire 1998 STS

    Jake has better data on Northstar valve seals, but the subject has come up twice in here in 4 years that I know of - statistically insignificant. The compression looks a tad low. Again, Jake would know the warm pressures to expect from a late 90's Northstar. I'll guess 150 - 160 psi. But no leakdown??? Now you're in the realm of a head gasket failure that has also compromised the valve train oil feed passages.

    If you got this car from a dealer or used car lot, you're on borrowed warranty time......and I do NOT like the reference to previous overheating issues. A simple thermostat change does not usually solve Northstar overheating problems, particularly in a 1998, the center of the most failure-prone 3 years. Go up to the sticky thread on head gasket failure polling - click on the lower right corner - view results - to see where this unscientific tally places your car............

  10. #9
    nhradiotech is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    I understand why you would be wary of the overheating I am too. But this thing does not over heat now. Is there a definitive process for testing for a bad HG? I don't want to fix the wrong problem. I have been searching this site and I dont seem to be experiencing a HG failure in the same manner. I have no water in the oil and the coolant doesnt smell like exhaust as of yet. My only symptom is #2 cylinder with a oil covered plug. If you have or know of a how to on diagnosing the motor I would love to try it to know one way or the other. Thanks!!

  11. #10
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    Re: Misfire 1998 STS

    I have no water in the oil
    The Northstar doesn't do that.
    Is there a definitive process for testing for a bad HG?
    Yes, either buy or rent a block test kit and test the air above the coolant for exhaust gases or do a cylinder leak down test on each cylinder (much more involved).

  12. #11
    buggin123 is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    You should contact Jake at northstar performance, He's done a lot of these engines and can give you an idea whats going on, it sounds to me like you have a cylinder that has oil rings that are baked with carbon from the engine being babied its whole life. The recommendation to do WOT weekly helps keep this from developing. I doubt you have that much oil coming down from the valve train.

  13. #12
    nhradiotech is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    Well afetr talking to Jake I tried some more tests and I found that the injector was completely clogged. After a little cleaning it runs perfect. But now I found I am indeed in the failed head gasket club. While troubleshooting I found some sparkly gold flakes in the coolant reservoir. I wonder if it will hold till the weather warms back up. I am just going to find another long block and put jakes studs in that one then swap out the long block.

  14. #13
    Submariner409's Avatar
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    Re: Misfire 1998 STS

    If its not overheating, blowing coolant out of the reservoir cap and losing coolant at a great rate, you're not in the head gasket club. A failed head gasket won't put sparkly gold flakes in the reservoir. It's entirely possible that the previous owner dumped some sort of magic sealant in the reservoir - some of that stuff has copper and aluminum dust in it.

    You may have cured your wet, fouled plug problem with the injector cleaning.

    Drive it for a while and see what happens.............and have your local radiator shop do a test for gasses in the airspace over the coolant in the reservoir after you have driven it and not messed with it for a week or so.

  15. #14
    nhradiotech is offline Cadillac Owners Member
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    Re: Misfire 1998 STS

    Will do. I am hopefully getting plates for it this week so I will put some miles on her and see how quick the coolant goes. I am certain the previous owner put stop leak in the coolant. That is what I was implying. Thank you very much for all the help so far!

  16. #15
    Ranger's Avatar
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    Re: Misfire 1998 STS

    It's possible he did that to stop an external leak.

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