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1999 Suburban 5.7 Runs Rough-need some help.

Discussion in 'GM Powertrain' started by jtriplett, Jun 4, 2009.

  1. jtriplett

    jtriplett New Member

    My 1999 1500 Suburban 5.7 has 25K miles on it. It hesitates under hard acceleration. It runs smooth under light acceleration. Recent work includes new plugs, wires, cap, rotor, coil, both catalytic converters and both 02 sensors behind the converters. The sensors in front of the converters are original.

    Also, the intake had been off twice. First time to fix a coolant leak and the second time to fix a vacumn leak cuased by the first repair. The vacuum leak causes the trans to up-shift very firmly. The up-shifts are no slightly firm on occassion, so I don't think I have a vacumn leak.

    Another symptom is this. The last change was the cap and rotor, after which the engine ran smoothly, and the check engine light went off. The light came back on and the hesitation retunred in less than 500 miles. I removed the NEW distributor cap to discover that the contacts and the rotor were charred, as if they had a lot more miles than 500. I cleaned the contact and it ran better, but has started getting rough again. Planning to inspect the contacts again this weekend, but I suspect I have other issues.

    I read on another forum about valve guides wearing past 100K on these motors, which could foul the plugs.

    However, when I have experiecned similar issues with other vehicles, it was always a problem with the ignition system.

    Need some help please.

    i am a pretty fair backyard mechanic, but I am lost. I am happy to swap knowlege with anyone. I spent 14 years in the car stereo industry, so I can offer some solid help to anyone needing it with their audio/video/security systems.

    Thanks!
    Jeff T.
  2. MrShorty

    MrShorty Moderator Staff Member 1000 Posts

    Do I get the impression that the SES light is coming on? If so, the first step would be to pull codes from the computer. Some places (like Autozone) will pull codes for free in hopes of making money off the result. I might pull codes even if the SES light isn't coming on to check for pending codes.

    I might also check fuel pressure to make sure fuel delivery is keeping up with engine demand.
  3. jtriplett

    jtriplett New Member

    Yes, the SES light is on. Last code checks at Autozone showed bad O2 sensors. I might have replaced the wrong ones, but could these cause it to run rough?

    I forgot to mention that the fuel pump is new also, though I have not done a pressure test. How do I do this?

    Thanks!
  4. werty

    werty New Member

    My 1999 1500 Suburban 5.7 has 25K miles on WHAT thats 25,000 correct?
    is it for sale :great:
    you question the cap.did you put a worthwhile one back on it ?
    did you check a egr valve?
    why was the intake off twice?oh because i cant read but???
    Last edited: Jun 4, 2009
  5. MrShorty

    MrShorty Moderator Staff Member 1000 Posts

    Do you remember what codes were pulled? Have you pulled the codes again after the other repairs? If you pulled codes for the pre-cat O2 sensor(s) and replaced the post-cat O2 sensors, then you probably didn't get at the problem. If you post the actual code numbers you got, that would help us help you. There are several web-sites with OBD-2 code lists (I like www.troublecodes.net) to look up the basic definition of codes. Post-cat O2 sensors don't really effect how the engine runs, they are there to monitor the function of the catalytic converter. The pre-cat O2 sensors, on the other hand, are part of a feedback loop that controls fuel mixture. A problem with the pre-cat O2 sensors can throw the mixture off which results in driveability problems. Of course, anything else that effects mixture can have a similar effect and produce O2 sensors codes, so it's important to have a diagnostic strategy that will identify when the sensor is the problem and when something else is the problem (you can't get that information from the trouble code alone). I would say the strategy at this point is to get the codes again, then post them here and see if we can come up with a good diagnostic strategy.

    If you don't want to wait for us, you might go to the library, locate a good repair manual for you year/model (engine performance, driveability, and troubleshooting volume usually) and copy the diagnostic information for each code.
  6. vncj96

    vncj96 New Member 1000 Posts

    I would start back with the tune-up that was done, checking to see that the distributer was set correctly
  7. jtriplett

    jtriplett New Member

    MrShorty-thanks for the great feedback. I will get the codes and post them here.
  8. cipman5916

    cipman5916 New Member

    I dont know if you already tried this, but i had a similar problem with my suburban, check the fuel filter. mine showed good pressure but under hard acceleration the truck would hit a wall and it would seem as if the tranny was shot. keep us posted on what u find out. :rules:
  9. jtriplett

    jtriplett New Member

    The actual mileage on my 1999 Suburban is 125,000-not 25,000 (K=1000).
    I am going to replace the fuel filter just in case, and thanks for that!

    The Trouble code according to the Actron scanner is PO161, which says O2 sensor in bank two-
    I do not know if this is the sensor before or after the converter. Both of the sensors
    after the converters have been replaced recently.
  10. 2COR517

    2COR517 New Member 1000 Posts

    I would go back to the ignition. You said the cap looked terrible shortly after the tuneup. Use only Genuine AC Delco plugs, cap, rotor. For wire, use AC or NAPA Premium. For plugs, just get the boring old fashioned single electrode platinums. This is what your parts guy will give you. Most importantly, set your gap down to .040. The bigger gap put a tremendous load on your ignition system, and doesn't offer any benefit. One last thing. There should be a vent hole in the bottom of your distributor. If the screen is still there, knock it out. It will allow the distributor to breath much better, and stay dry.

    And there is no timing adjustment on these trucks.

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