Start up issue

[08z51vette]

Having issue with start up. Book said to raise base idle.rpm, base idle airflow and dynamic airflow. Did that and played with it. Got it better but far from right. Also added fuel and nothing seems to get it right. It cranks and slowly builds speed and starts or if adjust too much wheel it starts it revs to 2000 and comes down. Do I need to play with timing possibly. Car has headers, cam, ported ls6 heads. I can get cam.specs in a little bit. Idles fine now thru temp range it is just getting it to start with no throttle input. Thanks in advance for your help.

[08z51vette]

66705-1-1.jpg
Here is cam specs if it helps. Ported 243 heads

[08z51vette]

Anyone?

[dermotw]

Usually you have to change the IDLE>AIRFLOW>STARTUP friction airflow and startup airflow, and the delays, in my case it was the friction airflow that made a difference, although the books say it should be the startup airflow!

Edit: here's a snapshot of mine:

Capture.JPG

-D

[Jay@HAP]

post a log of it doing it. What are your IAC counts when the engine is warm?

[mowton]

The secret to Gen III startup tuning is to make sure you put the correct airflow values into the correct buckets. This is so the PCM can use and decay the necessary airflow adders during the correct phase of startup.

Startup begins with Friction adder (get the metal on metal parts initially moving) in concert with Cranking (there is actually airflow as well as fueling used to spin the engine to get it started), once the cylinders fire off (rpm goes from 250'sh to 900+) you enter the Startup phase and should be up to desired idle rpm)...now the idle desired rpm will control the required rpm slowly ramping down to the operating temp steady rpm. As stated above, each of these will have a set value, a decay value and a delay value. Larger cams are going to need more initial as well as longer delays. The decay can actually be reduced as you are starting with a higher value so the stock decay values will leave you with longer periods of these adders which may not be necessary. If you increase the initial value, decrease the decay by the same percentage.

In the middle of all of this is the RAF (minimum airflow) which sets the lowest base airflow value to which all the previously mentioned values as well as AC, Fan etc. arflow values are combined. You can see that if you don't put the right airflow values into the right bucket, you won't be able to attain a stable startup/idle condition. One common eror is a novice tuner will continually increase the RAF to get the car started, but then experience hanging idle/cruise control, because the needed ai which should have been decayed out previously is still thee once you reach operating temp.

Hope this helps...the entire process and details are covered in our tutorial as well as tuning tips.

Ed M