Chevy frustrating me

I have good spark so I assumed stuff in distributor like icm is ok the injectors are firing just wont start. If the ecm is bad would I have spark?

Gumby22

It is normal for code 12 to be the first code when diagnostics are initiated. Each code will repeat 3 times before the next code reads out, so if you’re getting code 12 more than 3 times, whatever’s preventing it from starting is not detectable by the ECM.

Having spark and injector pulse indicates the ECM is good, so this may be a timing issue. Have you verified the base timing of the distributor is correct? With the #1 cylinder at TDC compression stroke, the distributor rotor should be pointing at the #1 plug tower on the cap.

Try unplugging the connector circled in your pic - that removes spark timing control from the ECM. If it starts, the ECM may be malfunctioning or it’s unable to properly control spark timing due to wiring or component issues.

Chevy frustrating me

I checked the timing and didn't see anything out of place. I tried to start it without that circled wire plugged in still no luck. I wish I could load the video of the sound the tbi plug made when I unplugged coolant temp sensor.

Chevy frustrating me

The tbi plug sounded like a stun gun sound until I unplugged it then came back once plugged in but it has since gone away and no weird sounds.

Chevy frustrating me

It tried to start somewhat when I unplugged that circled wire instead of rolling over with no kick at all with it plugged in

Gumby22

By “TBI plug” do you mean the IAC valve (pic)?

Chevy frustrating me

Yes that looks like the part.

Gumby22

Ok - the ECM utilizes coolant temp as an input for where the IAC (idle air control) valve should be positioned, so the ECM was probably just reacting to the change in temp reading - unplugging the temp sensor creates a default reading of -40*F.

To test/check the IAC operation, plug the temp sensor back in. Remove the air filter and look down into the IAC bore while a helper cycles the key on (without cranking), then off. At key off, the IAC should extend all the way out, then back off a few steps. If it doesn’t do that, consider it faulty.

I explained the IAC in detail in a post a few months ago if you want more information - https://chevroletforum.com/forum/198...urious-101819/

Chevy frustrating me

Thank you very much for that info. There was no movement on key on or off down the bore as far as I could see so i am going to consider it faulty and replace it. I can't thank you enough for the help I will report back my results

Chevy frustrating me

No luck starting after I replaced iac valve