Controlling idle timing.

[Redline Motorsports]

I am dismantling a tune in a 05' C6. The car is a H/C car that was tuned a couple years ago by another shop. We just did a couple minor upgrades so the car is back in our shop. The car has always had an odd idle in which the car rocks from side to side (and its not the cam). While logging data I noticed the timing is oscilating between -5 to 23 degrees and the rocking follows the timing swing.

I noticed that the the base timing table for idle had 13 in the heart of the idle area and the high and low octane tables where in the mid 20's. We went and bumped the idle base timing to 18 and it seemed to clean it up a bit but was still pretty erratic. I started snooping around the spark tables and couldn't find anything that looked like it would command these values or modify the base timing. What I did find was B5132 for idle limit. The value was -7. For the hell of it I changed it to 18. Once the car was started the timing didn't budge from 18 and the car idled very stable. It did however raise the idle from the desired 900 to about 1050.

Just starting to get into the LS2's and not sure of what else in these new tables effects the final timing value. I did also check the idle overspeed corrections which are at 1.00.

Any thoughts? I ran out of time tonight to keep playing so I though I'd throw it out there!

Thanks

Howard

[oztracktuning]

Maybe experiment with up nearer to 30 deg timing. This will make the MAP lower and smooth things out. If its a biggish cam it needs more timing at idle.

[Redline Motorsports]

This car has been like this for two years and the customer really doesn't mind. It really bothers me more them because I don't think its right.

Something is causing the rapid timing swing...........

[oztracktuning]

Its not timing correction of rpm is it?
B5935 B5936
Is it idling lean? i have found generally that cammed cars idle better a little rich.

[Redline Motorsports]

See the attached log file. The car is idling at 14.3-14.5 solid.

[oztracktuning]

have you tried open loop at 13.8 or so. mine isnt happy if it gets intop the mid14s

[Bruce Melton]

The fuel trims are way positive and it looks like the idle speed is bouncing with the timing. Adjacent cells very different in timing table?
I too, know nothing about the new LS2 tables-

[oztracktuning]

Is it idle rpm correction timing? its best to reduce these to smaller numbers as much as possible before it starts to hunt at idle

[Redline Motorsports]

The fuel trims are way positive and it looks like the idle speed is bouncing with the timing. Adjacent cells very different in timing table?
I too, know nothing about the new LS2 tables-

That was the point of the post..............the bouncing timing....

Oz,

I looked at the timing correction factor and it was at 1 I believe. I am going to take a better look at this tomorrow and see what other tables I can mess with!

[oztracktuning]

These tables B5128 B5129 B5130 B5131 B5132 ?

[GMPX]

The tables oztrack lists will be causing the problem, but as was also suggested, just lower the numbers, don't zero it.

Cheers,
Ross

[Redline Motorsports]

Can you guys please explain me this with those tables;

B5128

If the actual idle speed is below the desired idle speed value and the transmission is in gear, then this table is used to make adjustments to spark timing.
The RPM error values are in relation to the difference between actual idle speed and desired idle speed.

Is this saying that the column values (rpm) is the difference between the commanded and actual rpm? If so I can see where a cammed LS2 with a target idle of 950 will want to naturally swing above and below this target due to the dynamics of cams overlap. Based upon this; if the rpm error is..........112 then the PCM does what with the 13 degrees in the next column? Is says that this table will be used to make adjustments at those condition. Does it add this to the base timing in B5104?

If this is the case then B5129 (low side) should be the amount of timing removed from B5104. Yes?

If this is correct the amount of swing is crazy! If we narrow the plus and the negative sides of the desired idle speed adjustment, it should theoretically stop the mood swings! This is also why Ross states to not make them zeros but close the limit range...

I might be repeating myself but that's what I need to do to absorb it!:bash:

Thanks guy!

HT

[dfe1]

The idle correction parameters of the LS2 system are similar to those in the LS1 PCMs. I've found that with an aggressive cam, the systems fall into a vicious cycle of applying corrections through both spark timing and air flow, and they wind up fighting each other. Each time idle drops too low, the system alters spark and air flow which results in too much compensation and the idle becomes too high. It corrects again, and idle gets too low and the cycle starts all over again. Reducing spark correction in B5128 through B5131 to between 1/2 and one degree, up to about 112 rpm (difference between actual and desired idle speed) should solve the problem. Engine rpm will normal move 25-50 rpm with a performance-type cam, so you have to allow for that kind of rpm swing. I don't agree with having to run excessively rich at idle unless the cam is huge. Up to about 235 degrees (at .050) with 114 degrees of lobe separation, you should be able to get good idle quality in the 14.4 to 14.7 AFR range.

[Redline Motorsports]

The idle correction parameters of the LS2 system are similar to those in the LS1 PCMs. I've found that with an aggressive cam, the systems fall into a vicious cycle of applying corrections through both spark timing and air flow, and they wind up fighting each other. Each time idle drops too low, the system alters spark and air flow which results in too much compensation and the idle becomes too high. It corrects again, and idle gets too low and the cycle starts all over again. Reducing spark correction in B5128 through B5131 to between 1/2 and one degree, up to about 112 rpm (difference between actual and desired idle speed) should solve the problem. Engine rpm will normal move 25-50 rpm with a performance-type cam, so you have to allow for that kind of rpm swing. I don't agree with having to run excessively rich at idle unless the cam is huge. Up to about 235 degrees (at .050) with 114 degrees of lobe separation, you should be able to get good idle quality in the 14.4 to 14.7 AFR range.

Dave,

Well stated. This particular car was a good opportunity to final figure out how these parameters work. I also agree it does turn into a vicious cycle once it starts.

Howard