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

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!

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:

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

Results: Engine cranks over but does not start

After realizing the engine was not going to start: I reconnected the EST bypass connector.
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:
Warm up engine
Shut off engine
Disconnect EST bypass wire
Start engine (Cannot get past this step...)
Set timing to 0
Shut off engine
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. Please let me know if you have any ideas

Thanks for the help in advance!

I cannot access bbb industries but my thoughts on this is the input for icm is the crank sensor I would verify input although it being a bypass system I forget wether input for crank goes to ecm or icm but I would check crank sensor input not sure if that's the right direction

Hey guys! Here’s an update:

I sent a message to BobR at DynamicEFI (he builds ECMs for these cars/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!!!

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!