Random Misfire P0300 code
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Northstar Engines and System Technical Discussion Discussion, Random Misfire P0300 code in Cadillac Engine Technical Discussion; Well after doing head gasket job, I time serted the block and the task went extremely well. I got the ...
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    RICOMRD is offline Cadillac Owners Member
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    Random Misfire P0300 code

    Well after doing head gasket job, I time serted the block and the task went extremely well. I got the engine back together after initially having trouble understanding how to time the cams. Duhhh age is catching up with me!! Thanks to the support members of this board and AJ I could gasp the concept and that part I believe went well.

    Now I got a P0300 code random misfire. The engine idle quality is poor. I took it for a drive and on highway speeds, the engine responds well. Drove the car for about 17 miles with the goal of perhaps more codes would appear in order support a better assessment. No such luck just the same P0300 random misfire code. If I erase the code, no improvement of idle quality is achieved and the failure code re-appears and naturally accompanied with MIL lamp glowing like a Christmas tree.

    I did a vacuum test, and vacuum reading is below 20, and the needle is steady at idle. This leads me to believe that I have a vacuum leak. But I am dreading the worst-case scenario. Plug wires are new and I tested them, Plugs are new, plus the plugs appear to be firing normal. I tested the ignition coils, and changed the ICM with a known good running unit. I additionally did a compression test. Could some of the experts on this board lead me in the right direction?

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    Ranger's Avatar
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    Re: Random Misfire P0300 code

    Have you checked all the possible vacuum leak points? What year is it?

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    RICOMRD is offline Cadillac Owners Member
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    Re: Random Misfire P0300 code

    I was also thinking it could be a vacuum leak. I was going to do a check with propane. It is 97 Seville STS.

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    Re: Random Misfire P0300 code

    RANGER - This evening my 96DeVille just popped the same code. Noticed rough idle at start-up, code appeared 8 miles later and with 25 miles of driving it didn't smooth out. No time to look into things yet, however, could you point out the "... possible vacuum leak points."? I've always thought my life would be a lot smoother if those damn lines were color coded. Only ones on the DeVille I've found that are, are under the dash where you need to stand on your head to see "em!!!

    Is the P0300 indicative of a vacuum leak, what else might trip it? K_C

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    Re: Random Misfire P0300 code

    K_C, Personally, I would suspect plugs or wires to cause a rough idle before a vacuum leak. I would expect a vacuum leak to cause a high idle. That said, check the PCV valve seal in the rear cam cover, vacuum lines to the EVAP solenoid on the rear cam cover. Probably a few more on the TB. All vacuum lines will originate from the manifold or TB.

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    Re: Random Misfire P0300 code

    Thanks RANGER. Further investigation has pointed to the O2 sensor issue I'm having. I was getting an intermittent response code on Bank2 Sensor1 [0153, currently history].

    Procrastination has now produced a current 1153 code [same sensor] on a cold start. Looks like it's time to buy that special socket, an O2 sensor and skin my knuckles, whatdaya think? K_C

    edit: BTW the 0300 code went history before a full in park warm up.

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    Re: Random Misfire P0300 code

    Autozone: $60 for Bosch O2 and got the loaner tool. Interesting that all codes went history on the trip over. Changing sensor any way in that it's been acting up.

    As far as 0300 code, I saw it after a sudden rise in temps [outside - not engine]. Environmental changes can bring out the worst in a NorthStar. Just as going into winter raises hell with things, so does coming out of it. Just an observation on my part. Sometimes it's advisable to hang in there, stop trying to heard the cats and avoid jumping to worst case conclusions. K_C

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    Submariner409 is offline If it won't run, chrome it
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    Re: Random Misfire P0300 code

    Don't forget that an O2 sensor reacts to mixture changes, as in rich: plugs, wires, injector, etc. and in lean, as in vacuum leak, fuel pressure, some timing hiccups, etc. O2 sensors are normally very reliable. Try cleaning the connector first.

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    Re: Random Misfire P0300 code

    Yo sub, I always listen to those who support my procrastination, thanks.

    I'm thinking, however, that with a prior 1132 problem then a recent 1153 - both relating to the same sensor it could be failing, however, your insight is valued so here I go to return the sensor and save a knuckle bust for now. So these bad boys aren't prone to fail, but when bad just crap out? I'm learning that the NorthStar hiccups can be cured by procrasti.... no no, patience and a new day. K_C

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    Re: Random Misfire P0300 code

    K_C, the Guru has always said to stay away from Bosch O2 sensors and stick with A/C Delco. Just something to keep in mind should the problem return.

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    Re: Random Misfire P0300 code

    Now that I'm on to it, going to newgmparts and getting the real deal is worth $10 more, thanks for the pointer. I can alway "borrow" that socket deallie from Autozone should the need exist, no need to own one, right Ranger? K_C

    edit: BTW, I'm seeing conflicting info on this site as to the location of the Bank2 Sensor1 O2 on my vehicle. Haven't dug out the FSM to confirm, but where is it firewall or radiator side. Should locate by the #2 cylinder... not sure what side that's on either.

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    Re: Random Misfire P0300 code

    Bank 2 Sensor 1 is on the firewall side exhaust manifold.

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    Re: Random Misfire P0300 code

    Roger Ranger K_C

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    Re: Random Misfire P0300 code

    RICOMRD - didn't mean to cut in line man! How's your Seville, last I heard you were about to test for vacuum leak with propane. Things running smooth??? K_C

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    RICOMRD is offline Cadillac Owners Member
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    Re: Random Misfire P0300 code

    Yeap! I took the manifold out in order to inspect the gasket placement in manifold to head, I additionally replaced a rubber gasket behind the throttle body and replaced all injector o rings. Checked the intake manifold for cracks. Cleaned everything and put her back together. The idle quality is better but not the Cadillac smooth and silky idle. The code P0300 is still there. So I will do the inevitable and recheck for cam timing. I will do this next weekend; I got to do my homework now.

    I just dread the thought of cracking open the damper bolt again and half way dropping the cradle to check timing.

    I was thinking; is it feasible to check cam timing by piston Number 1 TDC method?

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