I just picked up a 1991 Firebird 305 ci. It has been running really rough and sometimes stalls at idle I did replace Spark plugs. I pulled the code tonight after reading on this board how to do so. I got 12. My research says "Diagnostic mode / ECM functioning No reference pulses to ECM."
Gotta be honest, no clue what that means. This is my first Firebird. Is this as simple as just replacing the ECM? Thanks in advance for the help.

 

Thats the first code you will get when you read for codes, It basically means you have connected to the ecm, in other words, say you had a code 33 and 42, you would read a 12 first, 33, 42. Here's a link, http://repairpal.com/gm-obd-code-12

 

Ok so if it just repeats this "12" code over and over it means I have no codes?

 

Yes. Does your engine start and run okay?

 

It start after about 5-10 seconds turning over, runs rough, but will idle, just rough

 

Check all the vacuum lines for cracks or breaks. Are you sure it's not flashing any codes after the 12 flashes?

 

I will recheck in AM. I will check vacuum lines too. Any reference for what lines go where or what ones are essential? I have a feeling last owner messed with stuff, as air cleaner was off car.

 

I don't know what 305 you have TBI or TPI. You can do a google search for an emission hose diagram images.

 

I actually found one on this website. This place rocks. It is a TBI, w/auto trans. Looking forward to checking all this out tomorrow. Thanks for help, will post results.

 

Update: Red Rock, you were right. I didnt watch the codes long enough. It flashed 12 3 times then went to a new code.....................23. Look like TPS???
I did check all the vacuum lines and they seem ok, even put factory air cleaner back on it.
Just go out and grab a TPS and cross fingers?
Thanks in advance

I should also add specific issue. It starts ok, even idles ok for a bit (kinda sporatic) but eventually the idle fluctuates and stalls.

 

https://www.thirdgen.org/tpimod2

 

I will try the TPS adjustment. When I read through the IAC adjustment it seems like they wanted you to start it while jumper was still in the Diagnostic port? That ok?

 

Ok I tested the TPS
Test one was to check voltage in 4.5. 5 volt range it read 4.2 on GRAY WIRE
Test two was to check voltage in11 to 12 volt range - it read 11.2 BLACK WIRE
Test three was to check voltage in the .5 to .9 range (at closed throttle) - it read .3 BLUE WIRE
I also kept BLUE WIRE circuit open while i manually opened throttle, Was suppose to read in the 4.5 to 4.9 range - it read 4.2

Im guessing its still probably the TPS just intermittently failing. Cheap enough, I think Iwll pick one up tomorrow and replace.

 

On the IAC, no jumper in the aldl port when adjusting. On the TPS, with a digital ohm meter, read your blue wire with the key on but not started, it should read between .54 and .75, when you turn the throttle blade, it should roll steady volts. I set mine to around .60-.65

 

It was reading around .30
Like I said trying a new TPS cant hurt. Will eventually end up replacing everything anyway, just want to make it run good as is right now. Thanks for help, will update when I replace TPS

 

Ok, Replaced TPS. Didnt solve issue. However it is no longer throwing the 23 code.
When I run the diagnostic test I am getting 12 over and over again, stopped counting after like 10 repeats.

I noticed that the cowl on the hood must have leaked as the air cleaner is very corroded. Injectors seem to be ok though.
All Vacuum lines appear to be in place.
New Spark plugs as well.
What next?

 

Hows the fuel filter, and the fuel pressure? Did you clean the IAC when you were checking it? Is the IAC getting voltage?