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Missfire

Tiny
TACKDADDY
  • MEMBER
  • 1999 GMC SIERRA
  • V8
  • 4WD
  • AUTOMATIC
Severe missfire at or about 3000 rpm, runs fine below those rpm
Thursday, January 6th, 2011 AT 8:11 PM

5 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Check your fuel pressure as it should be55 psi minimum. Also if you have small bolts for the intake like 8 or 10mm heads see if they are loose. Don't tighten them like a gorilla though.
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Thursday, January 6th, 2011 AT 9:18 PM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Is the check engne light on? The following below can contribute to the problem:

Defective spark plug
Inadequate spark/coil, defective spark plug wire.
Faulty fuel injectors.
Insufficient fuel pressure.
Contaminated fuel.
Oxygen sensors.
Throttle position sensor.
Manifold absolute pressure sensor
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Saturday, January 8th, 2011 AT 7:07 AM
Tiny
TACKDADDY
  • MEMBER
  • 2 POSTS
No check engine light, no codes, runs fine up to 3000 rpm, does not have to driven to misfire, leave it in park and run up to 3000 and it starts going stupid, all rpm related, all elec parts are new, fuel press is good, ecm checked out, cps new, cam sensor new.
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Saturday, January 8th, 2011 AT 2:45 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Have your crank sensor checked for ohms. It may be to h igh and causing this. Your crank sensor can get high ohmsand not set a code.
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Saturday, January 8th, 2011 AT 2:51 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Also check for a vacuum leak
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Saturday, January 8th, 2011 AT 2:52 PM

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