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help please

Discussion in 'Chevy Trucks' started by plowin207, Nov 30, 2011.

  1. plowin207

    plowin207 Member
    Messages: 89

    i replaced the knock sensors in my 01 sierra with the 4.8L after i replaced them it idled rough but got better as it warmed up. my first thought was vacuume leak but i didn't get why it went away. know when you start it it seems to idle fine but when going down the road when you go to hold speed. i notice it most within 5 to 10 mph either side of 40 the rpms surge and level out and over and over again. any ideaws? thanks so much in advanse
  2. 2COR517

    2COR517 PlowSite Fanatic
    Messages: 7,115

    Why did you replace the knock sensors? Did you do a fuel pressure/volume test?
  3. plowin207

    plowin207 Member
    Messages: 89

    i relaced the knock sensors cause they were both throwing codes. how would i do that test?
  4. theholycow

    theholycow PlowSite.com Addict
    Messages: 1,180

    Knock sensors are a common failure. Did you replace the harness too?

    Fuel pressure test is done with a fuel pressure tester. Harbor Freight sells one for $20 that does the job just fine for these trucks. I don't know how a proper fuel volume test is done, but if you watch the fuel pressure gauge while you load the engine (high RPM + WOT) you will know. Anyway, if there was insufficient fuel volume I imagine it would throw a code for running lean - and the symptom would be worse while accelerating than while cruising.

    A vacuum leak could go away after it warms up because of thermal expansion.
  5. plowin207

    plowin207 Member
    Messages: 89

    yes i replaced the sensor too. i checked the tranny fluid and it was way low i put some more in but haven't tried driving it again the check engine light came on so i am going to go to auto zone and see what the code reads
  6. plowin207

    plowin207 Member
    Messages: 89

    oops i mean the harness. also the idle seems kinda low know and the oil pressure seems alittle high not sure if this information helps