Rough idle, surging while driving, hesitation on takeoff but not always.

Discussion in 'Vortec 4.2L I-6' started by Firelancer, Mar 4, 2015.

  1. Firelancer

    Firelancer New Member

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    Just bought this 2004 Envoy with the 4.2 in it with 129k miles on it. Been trying to track down a problem with how its running with not much success so far. Have replaced cam position actuator solenoid because it was leaking oil replaced plugs with ac delco's, took off the throttle body and cleaned <did not replace gasket, should have I?> with b-12 chemtool and alot of elbow grease as it was pretty bad which made a huge improvement to how it ran. Will be replacing the fuel filter today as i'm sure it just needs to be replaced, peaked at the one on it and it looks like its the original <or its been there a long time> though I don't think it is the problem. I have no engine light on. I got lucky and my mother has a gmc canyon that uses the same coil packs and she just replaced all hers new so I borrowed her truck to test with as i had a caddy at one point that had a bad coil but didn't show a CEL. Replacing them seem to make a difference, but not a huge one. I'm very in tune with things, noises, vibrations etc, but this one is throwing me for a loop. I do believe i have a bad motor mount and thats whats causing my vibrations at 600 rpms but there is something causing it to randomly hesitate when i give it gas, happens more often in park. While in park and idling the rpm's fluctuate about 50-100 and seems to be a pattern to it, not completely random by any means. Sometime's feels like its bogging down if at a stop sign, turning and letting off brake at the same time its kinda like it wants to die but picks right back up in a second or less especially if giving it gas, though sometimes it even seems like its going to stall out when giving it gas. It has yet to die on me, seems to have decent power and pickup but i just got it and never drove one before so i might be wrong there. Also while going down the road, rpms will surge up and down slightly even while accelerating. This seems to stop at highway speeds 55+ I have been reading and reading for about 2 weeks now without trying to just throw parts at this but i'm running out of options. I think I have it narrowed down to a few things based of things i have read. I have no scan tools of any sort but any input in a direction I should go would be greatly appreciated. Also when idling it seems to be running "rough" even if the rpm's are not dropping at the time, sometimes the rpm's drop sometimes they don't. When vehicle first starts and is still cold its like its less noticeable, the roughness of the engine running i mean. the vibration from the motor mount is more evident when its cold but yea i'm stumped please help! lol

    Thermostat and temp sensor should be changed <my temp needle sits left of the middle mark read a few places i should change them both if this occurs> if i remember right this can cause a vehicle to run poorly cause its uses temp readings to determine fuel mixture?

    Possibly a slow responding pre-cat o2 sensor?
    Possible Cam and crank position senors?

    Also forgot to mention, 98% of the time its like it runs fine, the hesitation doesn't happen often, most of the time the engine seems to be running great but it seems like over the last few weeks i have had it its gotten slightly worse becase it didn't start to seem to almost stall till the last few days or so. Alot of the time when idling the rpm's arent up and down like its running fine. Seems like though that it always has a very small surge to it when driving but not always, somehow only thing i noticed before i bought it was that it needed rear sway bar links lol
  2. NJTB

    NJTB Well-Known Member

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    Hook up a scan gauge and check the sensor readings. Mine acted similar when the upstream O2 sensor was going out. It also took quite a while for the CEL to illuminate.
    FWIW, when it does finally go south, there's no mistaking it.
  3. Alec Venable

    Alec Venable Member

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    I'm currently having the same issue with my Envoy. Were you ever able to fix the issue?
  4. northmick

    northmick Member

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    I had problems just like yours did about the same fixes, plugs, coils, cleaned throttle body, also found that my TB did not seem to throw codes right away, one thing that made a big improvement was a new MAP senser. If your a/c compressure (clutch) is going out it will cause erratic ide
    Remember that the a/c system works with the heat system and defrost so I could cause problems at differt times. A pressure test should let you know if its the a/c

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