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350 TBI won't start when timing bypass is disconnected / unplugged

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Hey guys,

I'm new to this forum and new to owning my first chevy truck (with a small problem - hopefully!) and I wanted to post this here for some help/advice/guidance.

I have done some research on this problem and only found one other post on the internet with this problem so it seems like a rare one! Unfortunately, the post I found died once the OP stopped responding.

So here is the story and the problem:.


  • As of yesterday, I am the new owner of my first Chevy truck! :D It's a:
    • 1989 GMC Sierra 2500 with a TBI 350
    • It's completely stock and runs great! (while the EST Bypass wire is connected)
    • No check engine light
    • Got it for $500 :)


  • There is a problem... 
    • The truck does not pass CA smog testing :( 
      • Smog Visual: Passed
      • Smog Emissions/sniffer: Passed
      • Smog Functional test: Failed
        • The reason it failed (as printed on the smog failure report) is: Ignition timing is not readable. Also vehicle will not run when single wire tan/black connector is disconnected which is procedure for checking timing.


  • This was my first attempt at resolving the problem:
  1. Started the engine and warmed it up
  2. Disconnected the tan/black EST bypass connector WHILE the engine was running and the engine went from idling smoothly to a idling rough.
  3. Using a timing light: I set the timing to 0 degrees. The engine then began to run smoothly!
  4. I reconnected the EST bypass connector. (This was done while the engine was still running - The engine continued to run smoothly.)
  5. I shut the engine off
  6. I started the engine and it ran perfectly fine. (I thought I had resolved the issue!)
  7. Just to double check: I shut the engine off then I disconnected the EST bypass connector again.
  8. I tried to start the engine

    Results: Engine cranks over but does not start
  9. After realizing the engine was not going to start: I reconnected the EST bypass connector.
  10. The engine took quite a bit of cranking before it started. Once started, it ran fine.

Basically I am able to replicate the exact same same problem that the smog shop came across. Based on my research, the correct procedure to check the timing is:

  1. Warm up engine
  2. Shut off engine
  3. Disconnect EST bypass wire
  4. Start engine (Cannot get past this step...)
  5. Set timing to 0
  6. Shut off engine
  7. Reconnect EST bypass wire


I'm guessing this is the procedure the smog shop is trying to follow so seems I need to get this to work for them so the smog test passes. I hope this is enough details for everyone. Please let me know if you have any ideas guys. :)

Thanks for the help in advance!

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Your gonna get prolly 100 different diagnosis options but I have found the best option is to have an ALDL cable and monitor everything that is happening.

On the older stuff it is best to have an OHM meter and check the coolant temp sensor and other sensors. This sensor often times reads wrong and will give the wrong amount of fuel to start at different temps. Also, you can reset your IDLE air control valve or just take it off and clean it, if its out of wake you could have an issue. This is all free stuff to check without throwing parts at it. Also check fuel pressure, fuel pressure is vital on a TBI.


I strongly recommend an ALDL cable and it helps a lot with diagnostics.

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Hey guys! Here’s an update:


I sent a message to BobR (he builds ECMs for our trucks) and brought him up to speed with this problem. 


BobR responded with:


“One thing to keep in mind, is that with the engine not running (under 400 RPM), there is no difference between having the EST/BYPASS open or closed. The ECM opens the EST/BYPASS electrically. This is at least on a properly functioning system.

Best guess is that the pickup coil in the distributor is bad.

However, should do some trouble shooting with the EST/BYPASS connector open. During cranking are the injectors firing? Is there spark”


With this response I checked for spark and fuel and both existed. From here, the pickup coil was the next culprit.


I replaced the entire distributor (since it has a new pickup coil inside) and problem solved!!! :D 


The truck now runs just fine with the EST/Bypass disconnected. I took it to the smog station and it passed! :) 


Hope this helps someone down the road! 

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