car runs bad

2004 CHEVROLET TRAILBLAZER
100 MILES • 6 CYL • 2WD • AUTOMATIC
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HEINRICH101
  • MEMBER
  • 3 POSTS
2004 trailblazer intermittent problems:
1. runs perfect
2. no run, cranks only
3. runs rough
4. only run rough when a/c is on
5. runs rough when cold; rough for the first 5 minutes, after that runs fine all day
6. dtc: p0300
I have replaced:
A. 6 spark plugs
B. 6 coils
C. 1 fuel filter
D. 6 injectors
E. tested fuel pressure 1.(65psi regulated), 2. (90 psi deadhead)
F. have scanned and read all ecm datas, they are within normal paramater
Jan 11, 2011 at 9:46 PM
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ERNEST CLARK
  • ✓Experienced Mechanic
  • 1,730 POSTS
Your fuel pressure should be 55-62 psi. 65 isn't a problem. But 90 means your fuel pressure regulator is bad. But it won't stop the engine from starting. If the diaphragm is leaking fuel into the intake manifold, that will give you a rough idle and the P0300 misfire code. However, if the FPR is also leaking the fuel back to the tank after shut down, then that will give you a hard start.

Pull the rubber hose off the FPR and check/smell/feel for gas. If there's any at all, replace the FPR. Also, when you do a pressure check, keep pressure gauge connected and turn key off and watch gauge for at least 15 minutes. If pressure falls, replace FPR.

One last piece of advice: Don't replace components without testing them first.
Feb 16, 2011 at 4:04 AM
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