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Code PO300 random multiple misfire detected

Tiny
PCDJ64
  • MEMBER
  • 1999 CADILLAC SEVILLE
  • 4.6L
  • V8
  • FWD
  • AUTOMATIC
  • 75,000 MILES
Car has new heads, new intake and gaskets, clean throttle body, new plug wires and plugs.
it drives fine, idles rough, there are no vacuum leaks and the check engine light is not flashing.
i have some stuff I got from the portable scanner.

fuel system one and two in closed loop mode
MAP 5.7
RPM goes between 670 and 725
ECT 165
SHRTFT 11.7
LONGFT 37.5
SPARK ADVANCE 11.5 TO 14.5
MAF 0.01
SHRTFTB1S1 17.2 B1S2 99.2 B2S1 10.2

I do not know if this information helps.
i am at a loss, I am ready to give up on this thing and get rid of it.
any help is greatly appreciated.
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Friday, March 10th, 2017 AT 9:54 AM

5 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Check for a vacuum leak with choke cleaner on manifold and all around if rpm changes you found it . check fuel pressure with a gauge may be low auto parts rent it. finally check for air leaks like between engine and air cleaner duct work for holes and grommets on engine leaking air leaks can cause this. see link for fuel pressure.
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Friday, March 10th, 2017 AT 12:14 PM
Tiny
PCDJ64
  • MEMBER
  • 127 POSTS
You read the fuel trim levels and came up with a possible vacuum leak?
All of the gaskets are new, none of the vacuum lines are damaged, I would think that I would have a lack of performance if it was low fuel pressure, there is no lack of performance at all just rough idle.
Is there anyone that can read the numbers I put in the other comment and see if they indicate something is wrong?
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Friday, March 10th, 2017 AT 1:13 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,883 POSTS
Hello,

If I may jump in here, please pull the spark plugs and check the color of each one. I believe you have an intake valve that is leaking causing the rough idle you are speaking of. If you have a leak down tester it would be a good idea to use it, but a compression test may point you to the cylinder that is having a problem.

https://www.2carpros.com/articles/how-to-test-engine-compression

Please run this test and get back to us so we can continue helping you.

Best, Ken

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Friday, March 10th, 2017 AT 3:43 PM
Tiny
PCDJ64
  • MEMBER
  • 127 POSTS
A leaking valve bad enough to cause loss of compression would affect performance, there are zero performance issues, both heads are rebuilt. I have had cars with leaking valves and this car definitely does not run like that, the idle is not that off.
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Friday, March 10th, 2017 AT 4:02 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,883 POSTS
Idling rough is a performance issue. And if the intake valve is leaking a little it will cause that as exhaust enters the intake system. Which would cause the p0300. But if the valve are okay I would start to lean towards the MAF sensor. I would try to clean it first and if that doesn't help replace it. If the MAF is a little off it will cause a lean condition which will create a p0300 code as well.

Please let us know what you find.

Best, Ken
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Friday, March 10th, 2017 AT 4:25 PM

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