Should timing jump around a lot at idle?

[turboffr]

Just curious. Car runs good but I see idle timing swinging from as low as 2 to as high as 20. I suppose it doing that to keep the idle steady. Tune attached in case anyone wants to check it out.

[Shrek]

In a modified vehicle using an E38 ECM timing swings at idle of that magnitude are often indicative of an airflow issue - either the base idle airflow is too low, or the VE at idle is not accurate - or both.

[DSteck]

Really radical cams and light clutches can make it more erratic. You can adjust your over and underspeed spark to reduce the swings. Post the cam specs.

[turboffr]

Mild cam....222/230 .597/.609 115+2 in an '08 auto

[edcmat-l1]

Post a scan log @ idle.

[turboffr]

Post a scan log @ idle.

here's one from today. beginning of log has idle

[DSteck]

Give it a little more idle timing. I'd run 15* from 0 to 0.36 g/cyl and 0 to 1000rpm. Blend it out from there. I'm also not a fan of just adding a blanket amount to the final idle airflow. I'd play with that a little bit.

[turboffr]

Give it a little more idle timing. I'd run 15* from 0 to 0.36 g/cyl and 0 to 1000rpm. Blend it out from there. I'm also not a fan of just adding a blanket amount to the final idle airflow. I'd play with that a little bit.

ok, I'll try it with the idle at 15. I really didn't need to do much to get the cam to drive good so I'm just circling back on this now. As far as final idle airflow, what am I looking for to get that dialed in. You're correct, I just added 1.5 g as a starting point. Thanks.

[gn2beatu]

A ported throttle body that has been overported in the blade area and has too much gap can cause this too. It will pull timing to drop the idle.

[Shrek]

... I'm also not a fan of just adding a blanket amount to the final idle airflow ...

Agreed ... this is where tuning, rather than guessing comes into play. A cam upgrade will typically reduce VE somewhat at the lower end, there will be a point of inflection in the midrange, followed by increases in VE on the top end.

Once the VE table itself is representative of the engine's true airflow dynamic, then the idle behaviour can be accurately adjusted. As with many things in life, there are multiple pathways to success.

I prefer to correct the idle airflow - which will bring the spark advance back into line (13 to 15 degrees works well). After the idle airflow is accurately setup, the idle correction parameters (Proportional / Integral) and spark advance can typically be reduced to prevent unwanted idle swings, and promote a natural lope to the idle.

[DSteck]

A ported throttle body that has been overported in the blade area and has too much gap can cause this too. It will pull timing to drop the idle.

That's not the issue.

[Mez]

I agree with Shrek. Usually when timing goes +- that much, the issue is somewhere else, most likely its one of the air flow tables. The PCM is adding/subtracting timing to compensate for incorrect air tables.

Another place to check is a mechanical issue like vacuum leaks, ignition misses on one or more cylinders, etc.

Lastly, as DSteck said, set your idle advance around 15 degrees. If you set it in the mid-20 degree area, its closer to the maximum torque point and just 1 or 2 degrees will cause the idle RPM to swing around dramatically. Keep the idle timing low will make the idle RPM less sensitive to timing changes but it depends on the cam. A more radical cam that yours generally requires a higher idle RPM, thus vacuum, and a little more timing.

One more thing to try is spark over and under speed tables can be pulled back a little in the 16-32 rpm cells as a bigger cam will naturally cause the idle to jump a little. What you don't want is over correction or as I call it chasing its tail. The idle RPM oscillation up and down rhythmically is a sign the over/under speed tables are too aggressive.

[turboffr]

thanks guys. I'll bump up the timing a bit and work on the idle airflow table. I don't think it will take much to nail it.