Ignition timing question

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

  1. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    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?

  2. unplugged

    unplugged Epic Member 5+ Years 1000 Posts

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

    ct9a Epic Member 5+ Years 1000 Posts

    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?
  4. zippy

    zippy Rockstar 100 Posts

    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.
  5. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    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?
  6. zippy

    zippy Rockstar 100 Posts

    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.
  7. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    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.
  8. zippy

    zippy Rockstar 100 Posts

    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.
  9. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    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.
  10. bart6180

    bart6180 Member

    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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