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350 starting rough and backfiring when cold

Discussion in 'GM Powertrain' started by Crawdaddy, Dec 1, 2013.

  1. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    I recently rebuilt the TBI on the '91 Suburban and around that time, the engine started dieing and backfiring when starting the engine from cold. I'm not certain if it was doing this before I rebuilt the TBI, but I don't recall it. If I don't keep my foot on the accelerator, the engine dies almost immediately. When I keep my foot on the pedal, it will stay running, but backfires for about a minute until it starts running evenly. After that, the engine runs great with no misfires or loss of power. All the vacuum hoses appear to be intact. I'm not sure what issue would exist only when starting and for the first minute and then resolve itself. Any ideas? I feel like I should know where to go with this, but it's not making itself apparent to me.
  2. zippstripp

    zippstripp New Member

    Maybe the coolant temp. sensor thinks it's colder than it really is. After a minute or so of running it may wake it up and it starts reading correctly. Just a thought.
  3. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    That was actually a brief thought I had, especially since it's been cooler recently. I'm not getting any codes about it though. I'm pretty sure I might have a new spare of that sensor hiding somewhere that I can try.
  4. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    Just an update; I think I got this resolved. Yesterday I changed the coolant temperature sensor, and it didn't seem to fix it. Due to the nature of the problem, I pretty much only get once chance per day to test fixes. I read that on the TBI engines, there is no feedback from the actuators and since I disturbed the IAC, it didn't know what position it was in.

    There's a relearn procedure where you disconnect the battery for a couple minutes, then jump the 2 diagnostic pins in the OBD-I connector and turn the ignition on for a minute without starting it. This resets the IAC to the full closed position and allows the computer to figure out where it was. After the IAC reset, it cranked without issue, but there could be an issue that was also fixed by the ignition being on for a while. With the ignition on, the fuel pump would have had more time to build pressure. Tomorrow I'll give it another shot and if it cranks quickly, I think I've solved the issue.
  5. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts

    The issue isn't solved. It seems like the issue only occurs when it's cold outside. Today was in the 50s. Any temperature-related ideas that could explain the issue?
  6. zippstripp

    zippstripp New Member

    here's a hunch, try a different ignition coil.

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