350 TBI won't start when timing bypass is disconnected / unplugged
03-22-2019, 12:24 AM
#1
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Car: 1991 Pontiac Firebird
Engine: 355 TBI
Transmission: 7004R
Axle/Gears: 3.27/42 ?
350 TBI won't start when timing bypass is disconnected / unplugged
Hey guys,
You are the most knowledgeable TBI experts I know so I wanted to post this here for some help. I did some google searching and only found one other post 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:
.
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:
Thanks for the help in advance!
You are the most knowledgeable TBI experts I know so I wanted to post this here for some help. I did some google searching and only found one other post 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:
.
- Yesterday, I bought a truck:
- 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
- The problem...
- The truck does not pass smog
- 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.
- The truck does not pass smog
- 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
Thanks for the help in advance!
Last edited by Napster134; 03-22-2019 at 12:32 AM.
03-23-2019, 09:38 PM
#2
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Join Date: Jun 2009
Location: California
Posts: 396
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Received 2 Likes
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Car: 1991 Pontiac Firebird
Engine: 355 TBI
Transmission: 7004R
Axle/Gears: 3.27/42 ?
Re: 350 TBI won't start when timing bypass is disconnected / unplugged
Hey guys! Here’s an update:
I sent a message to BobR (most of ya know who he is but if ya don’t: He builds/sells custom tunable ECMs for our cars. The best one too as far as I’m aware) 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!
I sent a message to BobR (most of ya know who he is but if ya don’t: He builds/sells custom tunable ECMs for our cars. The best one too as far as I’m aware) 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!
03-24-2019, 06:16 PM
#3
Re: 350 TBI won't start when timing bypass is disconnected / unplugged
I'd have guessed "ignition module" rather than "pickup coil".
If you had spark and fuel spray, the pickup coil is working. I think some--but not all--of the internal module circuitry is defective.
Easy enough to find out. Swap ignition modules, see if the problem comes back.
If you had spark and fuel spray, the pickup coil is working. I think some--but not all--of the internal module circuitry is defective.
Easy enough to find out. Swap ignition modules, see if the problem comes back.
03-25-2019, 07:37 PM
#4
Re: 350 TBI won't start when timing bypass is disconnected / unplugged
i've seen pickup coils that will allow engine to run for 20-40 minutes then vehicle dies and will restart after a "cool down" period.
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