Got a 91 base coupe, stock.
Just replaced lots related to intake/vacuum under the hood. Intake pulled. New gaskets. New sensors. New injectors. New vacuum lines. Car runs good when warm, but, a little hard starting when cold. So, as most of us, I just can’t leave it alone.
I hook up the timing light to see what I am running.

With EST unplugged, I was running “after” TDC about 5 degrees. I had marked the distributor when I pulled it for the intake job. Maybe I was off a little on the re-stab?? I don’t think I was, so it is possible it has been like this for a while.

The under hood sticker says 6 degrees BTDC.

After I dicked with it a few times, I have it set to 9 degrees BTDC. Decided to test run it to see how it does. Reconnected EST wire and took it for a about a ten mile drive.

The engine seems to be running much better than before. It idles perfect. Runs very smooth and is very spunky from the stop sign.

I have read somewhere that with the L98, the ECM is sort of “dumb” on timing in that it can’t really identify the current base, so it adds or subtracts given the inputs from whatever base happens to be. Is that true?

FSM is a little vague, or I just haven’t found that chapter.

Is my engine really running better?
Should I keep nudging until I get it exactly on 6 BTDC? Is 9 good or something even better?

Thank you for advice.

 

So yes and no. My understanding is like yours that it will advance and retard irregardless of base timing because it has to trust you have it set right. On the flip side it will begin to pull timing from the knock sensor if you have it too far still. The advance side more or less works the same as a regular old mechanical advance type distributor just with the knock sensor able to adjust for too much.

As far as if 9 is okay... plug a scanner in and log retard events. If you aren't getting any its fine and I'd just leave it. My car liked anything from 8 to 12 on the base and wouldn't pull any timing according to logs. To play it safe, the book set point is going to be 100% safe. But it may also take a little more. My advice is to test it. But 3 degrees isn't outside of the ECMs ability to compensate for either.

 

Thanks 84.

I just got the Tuner Pro cable. I’m still trying to download the software.$@$&?!/ damn technology.

As soon as I get it figured out, I will follow your suggestion. Until then, the Timing Light is it.

Will the Base Timing being so far off cause poor running in Open Loop?

 

Reset the ecm first disconnect / reconnect the ecm connector near the battery, the ecm will adjust timing retard etc if any knocks have been sensed.

Then set it to 6 deg btdc "unless the car has been modified"
do not rely on the harmonic balancer to be accurate, the outer ring can slip. Best to confirm no 1 tdc with a screwdriver first.

Depending on what grade fuel you use, and or any build up of carbon deposits on piston and combustion chamber. You should hear any pinging and feel the ignition retard to compensate.

Hard starting ... check fuel pressure that it holds when ignition is turned off.
TPI runner gaskets, any air leaks can affect smooth running. Only big leaks would affect start.
If the fuel pressure leak down quickly, don't assume injectors first. Remember the pulsator between fuel pump and sender can crack, make sure you have no injector O ring leaks if using bosch 3 injectors. They need 3 o rings one top and two at bottom.

Good luck
Gerard

 

It would have to be a whole lot. There is always going to be a coating but whether there is enough to cause hot spots, I haven't seen in a modern engine unless something else is FUBAR.

IDK. How would you hear pinging when the ECM retards timing? IIRC, it decreases timing 2 degrees and then advances it half a degree at a time till it hears pinging. Not sure about you but my ears and butt aren't that sensitive.

 

All the responses are interesting and helpful. Thank you.
Still don’t have the following question answered:

Will the Base Timing being so far off cause poor running in Open Loop?

So, if the timing is off ten degrees, and the ECM is not controlling yet (Open Loop), can this condition cause poor engine performance?

It seems to me it will because there is nothing I am aware of adjusting for the spark timing problem. Is there something during Open Loop that can adjust timing?

 

so long as it is close... like within a couple of degrees. It should run fine. On cold start I believe it hits base timing then ramps into the ecm set advance. Watching mine it used to take ~15 seconds before EST advanced it fully.

As far as poor running... I don't believe so. I believe SA retard is always active even in open loop. I can't see being set at a couple more degrees advance causing a massive running issue. Being retarded a couple of degrees can and will however. My car I knocked a bit and ended up around 7 degrees ATDC and it ran... how do I say... bad. Very rough and not well... it sounded like it was cammed but was still stock at the time. So it really depends.