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Ignition timing question

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by ct9a, Jul 2, 2008.

  1. ct9a

    ct9a New Member

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    Ok, here's the deal. My '94 1500 'burb won't pass emissions due to high hydro carbons. The CO content was also high, but within the limits (barely). I was told that this typically means that the engine is running too rich.

    One of the things that I've taken a look at is the ignition timing. The sticker
    under the hood says 0 degrees BDC, is this right? I've come across other threads on here that have quoted anywhere from 4 degrees to 8 degrees.

    At any rate, when I went to check my timing, the mark is so far advanced that it's off the indicator markings on the timing plate. Does ignition advance cause an engine to run rich? Could this be the source of my headache?

    TIA.
    #1
  2. unplugged

    unplugged New Member

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    I doubt that timing is the culprit. It rarely is. How many miles on the odo? Has the cat ever been replaced/.
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  3. ct9a

    ct9a New Member

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    189,000 on the ticker and I have no idea if the cat has ever been replaced. Anyone want to recommend a good one that won't kill my wallet?
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  4. zippy

    zippy New Member

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    Did you disconnect the "computer advance" wire before you used your timing light? I can't remember if it's tan and white, or tan and black. You should see an isolated connector to unplug in the engine's harness at the firewall. The zero advance is intended without the computer's advance. It's like in the old days when you disconnected the vacuum advance before checking the timing at idol.
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  5. ct9a

    ct9a New Member

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    Assuming that I had the right wire, yes I disconnected it. It was under the fuse relay box under the hood, was tan and black leading into a connector and a different colored wire on the other side of the connector. It also had a bus amp that came out when I disconnected the line. Right line? Maybe this is why my timing looks so off?
    #5
  6. zippy

    zippy New Member

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    Sounds like the correct wire. Set your timing to zero with the wire disconnected. Then reconnect it, and you should see some advance at idol. Perhaps, in the past, someone dialed some more initial advance looking for more power.
    Back in the day, the first step to more power was frequently adding to the initial and total advance. In addition to setting the timing and dwell, we were endlessly changing the advance weights and springs in search of the elusive perfect advance curve. Look in some old fart's tool box and you'll find a tube of distributor cam lube and a few old advance weights and springs. And, there's that little file to recondition breaker points.
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  7. ct9a

    ct9a New Member

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    Ok, I f'd up. I had the wrong wire disconnected. The wire was actually under the glove box, which my manual says is only on the '95 models (mine is a '94). Timing is actually dead on. Now I have to figure out WTF is wrong and making my truck run rich.
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  8. zippy

    zippy New Member

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    Okay, it doesn't appear to be a timing problem. Now you know how to check the timing for next time.
    Does it run rich under load, or just at idol??? If it's just at idol, perhaps a cleaning and re-calibration of your Idol Air Control Valve will get you squared away. It's a cheap thing to check first.
    Good luck.
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  9. ct9a

    ct9a New Member

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    According to the readings that the emissions place gave me, the problem seems to be mostly under load between 10-20 mph. I'll check out that IACV though, thanks.
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  10. bart6180

    bart6180 New Member

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    high hdrocarbons on an emissions check normally points to the 02 sensor--had the same problem on one of my other trucks--high hydrocarbos--changed 02 sensors and passed emissions with flying colors
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  11. ct9a

    ct9a New Member

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    Well, I went to remove the O2 sensor, and the corners had been rounded off pretty bad by the previous owner, anyone have some advice as to how to get the f*cker out?
    #11
  12. ct9a

    ct9a New Member

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    Well, I finally got the O2 sensor out, and put a new one in. Failed emissions again, this time the results were worse.

    I think I'm going to end up swapping the cat and the injectors out. Anyone know of a decent upgrade? I might as well get a little performance out of this while I'm just shotgunning parts.
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  13. zippy

    zippy New Member

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    I went with a Magnaflow hi-flo to replace the OE cat a few years ago. Two months, I failed smog (for the first time) with a fresh new crate motor. You guessed it, that low bucks, just-out-of-warranty, cat was the the culprit.

    New injectors are far from inexpensive. IMHO (I'm a cheap old fart), I'd try having them cleaned and tested before shelling out for new. Have you considered a local or mail-order injector service shop? A quick google turned up: www.witchhunter.com (no endorsement implied).
    #13
  14. ct9a

    ct9a New Member

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    Did you get the universal cat and have it welded or is there a direct fit unit? All the direct fit units I've seen are ~$250.

    I didn't think about having the injectors cleaned and tested. I'll have to look into that. Thanks.
    #14

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