When I go to time my 91 305 tbi firebird, i do all the steps of what i think is correct. (unplug EST, set base timing to zero, shut car off, plug EST back in and unplug battery) After all of that i go to check my timing again (with EST plugged in of course) and its showing my timing is almost 10-11 degrees advance any ideas?? Also i know i possibly need new valve seals but it smokes like a freight train while setting the timing.. why is that? normally it only has a good cloud when i start it... Anyways this didnt seem healthy to me so i retarded the timing with the EST plugged in to get it to run at about 4 degrees advance. (I tried that with it unlpugged and it threw the timing very far advanced still) this is giving it a rough idle and a dead spot in acceleration.. PLEASE HELP!

i wish i could help. I had the same problem with my 91 RS.. Did you just get this car?

Are you sure the timing is zero?acording to the book the timing is 6,cant verify since i dont have a tbi 91 but i do know on my 90 tpi is 6 trans in drive

well heres something intresting. Have you tryed your ALDL connector? It says here you could connect something on your diagnostic link to prevent advance timing.

1) Base timing on a TBI car is zero. You can set it a tad higher (some have done 4), but specs call for zero.

2) You can't check timing with the EST plugged in - when the EST wire is plugged up, the computer is controlling the timing, and it could be anywhere, advanced (BTDC) or retarded (ATDC), and it will be erratic and change constantly (since the computer is changing it umpteen million times a minute).

So, you unplug the EST, start the car, set the timing where you want it, turn off car, plug up the EST again, and done. In the future, if you want to check base timing again, you have to unplug EST, start car, check timing, turn off car, plug up EST.

make sure after you set the timing to turn off car, disconnect battery, THEN plug in est, wait about a min. and then hook battery back up again. That way the ecm can really register the change. I have found that is how my car likes it.

I've had this car for a little over a year now, and I did all of those steps that you just described, I just got a little curious and checked the timing again with EST plugged in and was wondering if the computer should be advancing the timing to 10+ degrees at idle? What exactly is an ALDL connector?

The computer can advance the timing as much or as little as it deems necessary - it will be erratic and all over the place - I'm surprised it 'sat still' at 10 long enough for you to even see it. Checking timing on a running computer-controlled car while the computer is controlling it is just not really possible.

The ALDL connector is the 12 prong connector under the dash, where you can read the check engine codes from. I've never heard of shorting anything on the ALDL to disable computer control of timing, but it may be possible. But, with the EST wire (and given that unplugging it is the GM method of checking/setting base timing), why bother?

If the computer is advancing/retarding timing on it's own, then all is fine ... as long as you have your base timing set to spec (or any recommended setting that's proven effective).

CamaroNewb is correct. Set the base to zero with the EST unplugged and the ECM spark tables will take over from there. The PROM tables are set to run with a zero degree base timing. Whatever you set as base, the ECM adds from there. Zero is recomended for stock LO3's. The stock 169 swirl ports are fast burn heads and by design no not need any advance (like TPI which has a factory pre set of 6 or so).

Ok thanks! I just didn't want the computer to damage anything because of a bad sensor causing it to advance the timing. Lol I guess I knew what an ALDL was, I have a snap-on scanner of my own... Well I guess today I'll set the base timing back to zero and just check everything with my scanner to make sure there's no knocks. Thanks again! I'll keep ya'll updated!

Ok! timing is set and my car is running a lot smoother! Still has a slight miss but i think some plugs and wires should fix that... is there an easy way to get to the back spark plugs? Lolz Also I hooked my scanner up to my car its showing no codes or knocks, but it is showing it running lean?..

slight miss is more than likely a bad spark plug wire IMO. I could be wrong tho. How do you know it is running lean? What are your BLM's looking like?

Block Learn Multiplier? I know my scanner shows Block learn, but I dont know much about BLM much less what my car is at or what it should be running at. 128? I just read an article on BLM, if I understand correctly its basically adjusting air/fuel ratio?
the scanner also shows MAF but im not sure what it should be at either..