Engine misfire when warmed up, code P0300

Tiny
RUSSELL CROSS
  • MEMBER
  • 2003 CADILLAC SLS
  • 4.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 116,000 MILES
Car runs fine when first start it up in the morning. Once the engine temperature gets to normal it starts to miss. Throwing a p0300 code. It is missing on 5 and 7 thought head gasket but it runs so nice when cold I am not sure. It never heats up. I have changed coils. Fuel filter boot on back of throttle body and PCV vacuum elbow under intake. I am just stumped on this one. I have also changed the crank position sensors too.
Tuesday, August 27th, 2019 AT 9:58 AM

3 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Hi,

Do you have the ability to look at live data? We need to see what the o2 sensors are saying.

Multi cylinder misfires are set because there is not a consistent misfire on a given cylinder.

If we can look at the o2 sensor data we should be able to tell what is happening. It runs well when cold because it is in what is called open loop. This means it is looking at just the air flow sensor and temp data. It doesn't start looking at o2 sensor data until it is warmed up. If this is when it is misfiring then we need to see what the o2 sensors are telling the PCM.
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Tuesday, August 27th, 2019 AT 4:56 PM
Tiny
RUSSELL CROSS
  • MEMBER
  • 2 POSTS
I have a small scanner I have looked at live data don't remember numbers, but o2b2s1 and o2bis2 is jumping around. O2b1s1 is steady which makes sense cause missing on bank 2. I am going to put another set of plugs in it this weekend and get numbers from live data then and see how it runs. Thanks for your help so far.
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Wednesday, August 28th, 2019 AT 7:38 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. Thanks. Take a look at the short term fuel trims for both sides as well. It may be easiest to just get a video of the scan tool with those numbers that way I can watch them if they are moving around. Thanks
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Wednesday, August 28th, 2019 AT 6:06 PM

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