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Thread: Warm-ish idle issue

  1. #1
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    Warm-ish idle issue

    When I start the car cold, it idles up, and then back down just like it is supposed to do, and idles between 600-750. About 1-2 minutes after that, the idle starts to vary just a little according to the gauge, but you can't feel it. If you sit at idle for 2-3 minutes, the car will suddenly drop to barely running, cough and sputter, and occasionally stall. It will start back up immediately, but will still idle like crap. If you just rest your foot on the gas peddle, it stops doing this, and will idle just fine, and if you do this when stopped, it doesn't try to die. Also, if you drive the car after it has done this, it will eventually go back to working (2-3 minutes of driving, and it's back). This started after I pulled the supercharger off to replace valve cover gaskets, and wasn't a problem before. While I was in there, I replaced a bunch of vacuum hoses, and I've gone back over them, and none seem to be leaking. I've also redone the thermal paste between the DIS module and it's mount. One thing I did change while I was in there was; there was a catch can plumbed into the PCV lines incorrectly, so I just pulled it out, and plumbed it straight into the vacuum lines it was already hooked to. I'm leaning toward the IAC starting to fail, but I'm not sure that's the problem, so I was hoping somebody might've experienced something similar.

  2. #2
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    Something similar yes, sure, BTDT.

    Have you run the codes yet?

    What year is it?

    ...started after I pulled the supercharger off to replace valve cover gaskets, and wasn't a problem before.
    I'd go back over the intercooler tube seals. What did you use to seal them when you put it back together?

  3. #3
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    It's an 89, and I haven't run the codes, as the check engine light hasn't come on. Didn't think I'd get anything useful from it if it wasn't on. I used copper gasket sealer with the IC tubes, read somewhere on here while figuring out the valve cover gaskets that it should be fine. Just odd to me that it pulls vacuum better then it did before the work, it's now at ~16 inches of vacuum at idle, normally, and ~21ish on decel. It was at 14 and 20 before I did vacuum lines. I'll double check them this weekend though.

  4. #4
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    Quote Originally Posted by nomind View Post
    It's an 89, and I haven't run the codes, as the check engine light hasn't come on. Didn't think I'd get anything useful from it if it wasn't on.
    Might be stored codes....not everything throws a light. Good habit to get into, I think. Both when chasing issues and after repairs such as you've just done. Helps to know what you're -not- chasing, and to avoid just firing the parts cannon at it.

    I used copper gasket sealer with the IC tubes
    Not sure I'd use copper sealant on the iC tubes. I use the re-usable teflon gaskets on the tubes and black RTV on the top and plenum-to-intake.

    Looked inside the throttle body to see how clean it is at and behind the plate?

    IAC, when old/failing, typically honks like a goose and isn't something you'd always need to remove/fool with just to pull the supercharger. Might want to confirm the connector is ok.

    In your case, remember you're looking for a common problem that could occur after a specific repair, so if you've been checking posts here for common issues based on symptoms alone, you risk greasing a rabbit hole.

    Why was the supercharger pulled, and what other parts, such as plugs/wires were removed during that process?

    Vacuum leaks tend to clear up when the engine gets warm, unless, of course, you can drive a truck thru them...like when a hose is off, and the IC tubes leak and aren't tight, etc.
    Last edited by KMT; 01-03-2019 at 02:02 AM.

  5. #5
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    I'll pull codes tonight. Supercharger was pulled off to do valve cover gaskets. It developed a leak that was causing oil to get onto the exhaust manifolds (ended up being the power steering pump o-rings), but since I had them, I went ahead with replacing them anyway (TB gasket was replaced as well). When I pulled it apart, I did pull the IAC from the TB, and cleaned the TB (didn't do the IAC).

  6. #6
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    Good on you for cleaning the TB. The IAC might not tolerate it too much tho - can be fragile, so... I do a visual, and if it looks pretty foul and I know it's age within a decade or so, consider replacement instead of cleanup. Cleaning might do more harm than good (but if you do, be gentle and keep chems off the bellows), a new one gets you a new gasket too.

    If the IAC is fouled, I'd want to know why, other than old age, of course. I don't think that's the problem at this time, at least not root cause.

    But again, I'd focus on the IC tube sealing status. A smoke test would be ideal at this point, I think.

  7. #7
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    Didn't get to pulling codes on it, but discovered another issue while looking it over, and after some research, may have found another problem with my work (might be the issue, or part of the issue). I said I removed a poorly installed catch can, well, when I did that, I tied the PCV on the passenger side to (what I thought was) the PCV on the drivers' side. After doing some looking/digging, etc. it appears that the drivers' side isn't PCV. They were both plumbed together and into the catch can when I pulled it apart, but I don't know where the DS valve cover should vent to. Does it run to the rubber intake hose on the intake? I found there was a hole just sitting open on the intake. Put a vacuum cap in it this go round as well. Really wish I wasn't traveling for work this week, but will try disconnecting the two this weekend, if I can figure out where to route the DS vent.

    Edit: I did check my IC lines, and found a leak at the screw on collar. Sealed that before I left for the week as well. Gonna smoke check it again this weekend and see if anything else pops up.
    Last edited by nomind; 01-09-2019 at 04:39 PM.

  8. #8
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    Yes, the driver's valve cover vent is plumbed off the air intake tube, between the MAF and throttle body. I wouldn't just cap it off, or plumb it to the PCV.

  9. #9
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    I'll redo all that plumbing again when I get home. Also have a cat (I think) rattling really badly. Driver's side sounds like someone is gargling marbles. I'm really starting to hate it when I buy someone else's project car, though this one is worlds better then the last one was.

  10. #10
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    Quote Originally Posted by nomind View Post
    Also have a cat (I think) rattling really badly.
    Last time I had a rattle-cat, it was also way plugged...boost would go sky high. Running 'test pipes' now.

    Feel your pain...stick with it, you'll know when to bail

  11. #11
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    Well, it won't be until after the Power Tour if I do, the wife wants to take it this year.

  12. #12
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    It seems to be better now. Took it out and drove it around for ~30 minutes or so, idle never dropped or acted funny at all. I resealed the IC piping and fixed the vent line into the intake hose, and hooked up the PCV correctly. While I had it apart again, I gasket matched the throttle body and the intake. Don't know if it'll make any difference in the performance, but it made me feel better. Now to get the cats replaced. I'll probably just get some high flow cats, and have an exhaust shop make me some new down pipes. Probably be cheaper then ordering the down tubes without any cats from SC Performance (or at least close, and I'll still have cats).

  13. #13
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    Thanks for the followup. I'd go back over the IC tube fasteners after a couple hundred miles, but it sounds like you've got it sorted.

    Quote Originally Posted by nomind View Post
    I'll probably just get some high flow cats, and have an exhaust shop make me some new down pipes. Probably be cheaper then ordering the down tubes without any cats from SC Performance (or at least close, and I'll still have cats).
    I bought new down pipes w/hi-flow cats from SCP, good price, 8 yrs. ago, then found a local shop to put them in. The shop seemed sure they could have set me up w/their materials and labor for less, but oh well.

    I try to buy from vendors that support our cars first, but you might want to get a quote from a local shop just so you can make an informed deci$ion. Pretty sure labor rates have gone up since when I had mine done

    Quick note on pipes and hi-flow cats. Expect an indicated boost drop. Doesn't mean a performance loss, but as boost is a measure of backpressure, you may think you've lost something somewhere. I used it as an opportunity to swap to a different pulley and gain back original indicated.

    Don't forget to run the codes - make sure that the O2 sensors are happy, at least.
    Last edited by KMT; 01-13-2019 at 03:37 PM.

  14. #14
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    Will do on the O2's. And I'm prepared for the boost number drop (at least mentally). Plan is to upgrade to a late model blower, new TB and intake later on this year. I'll also drop the exhaust and clean up what I can on the exhaust manifolds, for a little peace of mind.

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