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Thread: Rough, miss-like idle

  1. #1
    Join Date
    Feb 2016
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    Cincinnati, OH
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    Rough, miss-like idle

    After weeks of trying to diagnose a running issue, nothing I have done so far has helped. Actually I feel I've made the issue worse. The car ran "ok" before I messed with it, but not 100%. It would pop on the engine light for a minute or two then go out. Codes said something about the DIS and it going lean. Swapped a DIS, checked IC piping, checked visible vacuum lines for leaks, swapped cam sensor, swapped crank sensor, and swapped coil pack. Still no dice. Replaced the IAC since the center wall was cracked.
    Next thing I will try is replacing the spark plugs and wires. They were an issue before... Other than that, I'm not sure what else I can do other than sell the car.
    Thanks.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  2. #2
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    How's the outer ring on your harmonic balancer?

  3. #3
    Join Date
    Feb 2016
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    Cincinnati, OH
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    163

    Harmonic ring

    The outer ring is perfectly ok. No damage to it or play.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  4. #4
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    Dec 2007
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    Salem OR
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    First thing I'd do is stop firing the parts cannon at it.

    Can you re-check and list current codes?

    Have you had a look at the plugs yet?

    Do you have a reliable means of checking fuel pressure?

    When you checked vacuum lines, did you see any raw fuel in the line at the fuel pressure regulator?

    What major work has been done recently?

    How many miles on the fuel injectors?

    Is the engine all stock or are you running mods?

  5. #5
    Join Date
    Apr 2002
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    Norcross, GA
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    You might want to check your O2 sensors, also. A bad O2 sensor can make it run really rough.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  6. #6
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    Feb 2016
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    Cincinnati, OH
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    Yeah, not trying to just hang parts on it, just suck at diagnosing.

    -I have not re-checked for codes(have not bought a scanner)

    -was going to check plugs when replacing wires

    -I have an in-car fuel gauge that is quite reliable

    -no fuel in the vacuum line off the regulator

    -there hasn't been any major work to recently. Mid last year was when work was done.

    -I have no idea how many miles are on the injectors. They are 38lb's and the car has about 104,000miles on it with 12,000 on the engine.

    -some mods to the engine. Stage 1 cam, long headers, underdrive pulleys, a/c delete, EGR delete, ported 89 SC w/ 10% pulley and raised top, and a double IC.

    -O2's were replaced mid last year with the other work done.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  7. #7
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    Salem OR
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    Can you get it to a local auto parts store and ask them to read the codes for you?

    I'd like to know more about why you mentioned something about being lean...

    How much vacuum are you seeing at idle?

  8. #8
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    Feb 2016
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    When I checked codes on it a few months back, it gave me a code 19, 41, and 91. As for vacuum, it was about 9-10 on the gauge.
    Last edited by Matt_aka_Dale; 02-12-2019 at 11:54 AM.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  9. #9
    Join Date
    Feb 2016
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    Cincinnati, OH
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    New spark plug wires didn't seem to help, but now I only have one code. Code 19(Distributorless Ignition Cylinder identification circuit failure). Since i'm terrible at diagnostics, it wouldn't surprise me if I keep looking past something. PLEASE tell me i'm missing something here.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  10. #10
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    Disconnect the cam sensor and try to fire it up...might take more than one try. See if it runs differently/better.

    Double check plug wire layout:

  11. #11
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    Sounds like DIS or coil pack problem or any connection between them. Once had an idle stall problem which turned out to be the sc belt rubbing on the wire bundle to the cam and crank sensors, so you never know what you might find.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  12. #12
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    Feb 2016
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    Spark plug wires match firing order. Unplugged the cam sensor and it wasn't as hard as I was imagining it would be to start. It seemed to run a little better, but I can't see having to replace a cam sensor for a 3rd time.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  13. #13
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    Quote Originally Posted by Matt_aka_Dale View Post
    It seemed to run a little better, but I can't see having to replace a cam sensor for a 3rd time.
    At least it's something to go on Like I said...don't keep firing the parts cannon at it.

    Might be wiring/harness. Inspect the harness for the DIS/cam and crank sensor closely for oil contamination, breaks in the wrap, loose/worn connector pins that might not be making good connection and/or show green corrosion, etc. Make sure the cam sensor connector has only one red rubber gasket. Have any engine parts been painted, such as the accy. brackets or heads? Do you know if all engine grounds are in place, clean/healthy?

    Sorry if I missed it - what year is your SC? I've got some good/re-furb'd harnesses. I think Victor/SpinningWheels has some too on occasion.

    I've seen some good testing procedures in the factory emissions guide, but some steps require a break-put box and others run the risk of masking issues just be moving the harness around if you're not careful and/or have good probes to help when backprobing connectors. I try to avoid piercing wires if at all possible when testing circuits.

    I had issues that pointed to front sensors at one time and fresh ones didn't help...ended up swapping to a better condition harness before things behaved. Be sure when juggling sensors that you disconnect the battery/EEC keep alive memory fuse so that the computer relearns against the new hardware.

  14. #14
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    The ol' girl is an '89. The harnesses are, in my opinion, not the best looking. I know one plug has one wire that is bypassing the plug itself(the plug that connects the back harness to the front harness). That was how it was when I got it. As for grounds, they all seem okay, but I can go over them again.
    Matthew "Dale" Johnson
    ~"If it looks dumb and works, it's not dumb"

  15. #15
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    Quote Originally Posted by Matt_aka_Dale View Post
    The ol' girl is an '89. The harnesses are, in my opinion, not the best looking. I know one plug has one wire that is bypassing the plug itself(the plug that connects the back harness to the front harness). That was how it was when I got it. As for grounds, they all seem okay, but I can go over them again.
    SCs live/die by their electrics/electronics. A compromised harness can include visual damage as well as hidden. Some wires are shielded to fight off EMI..electrical interference in signals that can confuse the EEC. When the shielding is damaged/missing, funny things can happen and be difficult to identify/isolate. When pins are loose/corroded, signals can drop out entirely.

    Grounds are part of the same environment, making it hard for reliable signals to come from and go to the EEC and all those sensors. Inspect the cable on the pass. side motor mount that goes over to the body. Corrosion can hide inside the cable sheath and where each end is bolted up. Best to install a new one there, just don't use one longer than needed. Be sure the metal and bolt hole on the body are clean shiny. Replace any sad looking battery cables as well.

    Confirm the ground wire at the back of the head where the fuel-pressure regulator bolts up is also clean/tight.

    The mounting surface where the DIS sits needs to be bare metal, no paint (yes, you still use the paste that a new DIS comes with in the box)...same where that entire accy. bracket bolts to the head/engine. Painting those components can hamper the ground path back to the battery and cause signal issues.

    Be sure the voltage at the battery is 11.5 or higher, both engine off and running, where it could be as high as 14+ volts. With the key off and the headlights on, check to see what volts are present across the battery terminals...keep checking for a minute or so to confirm it's ok, or if it drops like a rock.

    All this can be tedious, i know, but an older/neglected SC is a prime example of hard to find niggling electrical issues....pretty sure that's why I was able to buy mine - I think the previous owner got tired of not being able to figure out why it would run ok one day and act up the next
    Last edited by KMT; Today at 12:51 AM.

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