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TA 98 rough idle problem

This is a discussion on TA 98 rough idle problem within the General Help forums, part of the LSx Technical Help Section category; Hi all. My 98 TA 5.7L is having some issues with idle. The car has 55k miles on and I ...

  1. #1
    Senior Member JayTA98's Avatar
    Join Date
    Dec 2007
    Orlando, FL

    Mean Black
    1998 Trans Am

    TA 98 rough idle problem

    Hi all. My 98 TA 5.7L is having some issues with idle. The car has 55k miles on and I own it for 6 months now. When I got the car the idle was almost normal to me. It does have a rough idle and sometimes it skips (like it is going to shut off but it doesn't) the problem is intermitent but it happens frecuent. I have replace spark plugs twice, the 1st time I put the OEM from AC delco and replaced the spark plug wires (aftermaket ones)since one of them showed open. It got better to a point but the idle was still rough. I have cleaned the TB and idle control unit with TB cleaner. I also cleaned the MAF sensor and replaced the air filter. I used Seafoam thru the brake booster line, engine oil(300miles before oil change) and I pour it in the fuel tank. I noticed a change in TB response and some idle but not that much. I replace my oil every 3k miles. I flushed the tranny fluid at 50k miles. I always use 93 fuel on it. I don't hear any vaccum leaks. I don't have any codes stored. The car as far as I know is stock. It has the slp type lid, KN filter and Borla exhaust.

    Today I replaced the spark plugs with NGK's TR55 (gap them at .055) and I found 2 spark plug wires open again. The old spark plugs looked in good shape. I replaced the 2 bad wires and I still got the rough idle but it got a little better when is in gear but kinda worse when it is in P gear. No idle skip so far.

    I know these cars idle is not perfect but it has changed since I got it. A friend told me to replace the idle control unit but I'm not getting a SES light. I just don't want to waste the money and then I can't return it. I'm going to order some good MSD wires because I'm not liking when the wires are going bad. I think that's part of my problem. Could the Idle control unit be having issues and not light the SES light? Do you suggest replacing it? it around 100 bucks. The other thing I'm thinking is if I have a bad coil. I can't find a way to test those out. I have being told that if a coil is bad it will light up the SES.
    I want to get the MSD coil kit but I can't afford it right now ($700) unless I have to really replace a coil.

    I don't know what else to try. I read another post on a similar problem to this but no solution at the end. Any more suggestions? Also how do I reset the timing in the PCM? I heard that you can do that and the PCM will learn to adjust to the current setting. Thanks in advance and by the way this is one of the best LS1 sites I seen so far keep up the good work guys and girls.
    Last edited by JayTA98; 01-06-2008 at 02:15 PM. Reason: question just came to my head

  2. #2
    Life in the Fast lane sunsetorangess's Avatar
    Join Date
    Aug 2005
    Bragg NC

    Met. Sunset Orange
    01 SS

    You replaced the wires and said you found one open, then you found two open, less than a few months later, were they the same wires? I wouldnt replace the Iac valve, You said you cleaned it, that should be good. What exactly is the car idling at? Rpm? Does it jump around or stay steady at that rpm? Whats your tranny fluid level at?
    I had rough idle issues and was throwing codes with my pos K&n air filter so i ditched it for a paper one. K&N filters are just a headache anyhow.

    If it is eating the same wires, take a look around for an exhaust leak in the area if you cant find one, then take your car for a sprinted run at night, just to get it good an hot, then pop the hood and focus on the area where its melting plug wires and see if your manifold is glowing, or if you have a nifty lil "no touch" thermometer check the temp on each manifold.
    2001 SS, Its not the car its the Driver that matters....

  3. #3
    Veteran 0rion's Avatar
    Join Date
    Aug 2005

    98 Formula
    06 duramax

    I tell you what I recommend. Hunt around locally for someone that has autotap, hptuners,efilive, or some other type of datalogging software. It will tell you pretty quick what's going on. I'd try to isolate the miss and at least then you know what cylinders to start looking at. Have you tested fuel pressure at the rail yet? Should be ~58psi at idle. I wouldn't spend the money on MSD coils, the stock coils are plenty hot enough so if you do end up replacing a coil just find a stock one somewhere cheap.
    Someone in another thread was having a miss problem similar to this and it ended up being a clogged injector so that's something else you can look at. Make a poor mans stethoscope out of a real long screwdriver and put your ear to the handle and the other end on each injector and see if you hear a difference in the injectors as they're firing.
    Just a couple of things to check.

  4. #4
    Senior Member kool-aide's Avatar
    Join Date
    Dec 2006
    Deltona, FL

    Sebring Silver Metalic
    2002 Camaro Z28

    hahahaha....i like the stethoscope idea. I saw that on shade tree mechanic once. I used it while deciding which f-body to buy, 98 T/A or a 99 z28, obviously the 99 z28 made less noises so i bought it.

    good luck, hope you find the problem. I would do what Orion suggests, get a tool that can log data and maybe try listening to each injector.

    I have a 99 z28 with 97k+ miles. I have replaced a lot of old worn out parts, and still going. I wouldn't jump into expensive parts like coils or fancy wires until you have a real good idea of whats wrong.

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