Idle overspeed/underspeed question

[BADFNZ]

Is the value listed in the idle overspeed/underspeed table the max that the PCM can use or the value that it will use?

For example, if I have 5* advance in the underspeed for -300 rpm, will the PCM automatically command a 5* increase in spark if a 300 rpm deficit is seen, or will it just have the authority to use up to 5* to correct it?

I figure it makes a big difference one versus the other. With one, you might want to keep the number lower so the spark doesn't vary as much. With the other, you might want to make the number bigger so it gives the PCM more leeway in choosing the correct value.

[SSUte01]

It is the value the PCM will use for under/overspeed conditions. Generally speaking with cammed setups I tend to 'soften' the effects at low RPMs and have it more aggressive at higher RPMs as spark is definitely the best way for correcting rpm delta on Gen3s in particular the IAC setups.

[BADFNZ]

Cool thanks.

When I turn "Spark Idle" on using the VCM Controls (which keeps the spark constant), the idle gets a lot worse. So I feel my PCM is using spark as a crutch to keep the idle. I'm wondering if this is ok or if there is an underlying issue.

[sjgrimsley]

That's normal. The PCM varies the spark to keep the idle rpm in check along with the IAC. It uses the spark because it reacts much, much quicker than airflow.

[edcmat-l1]

Cool thanks.

When I turn "Spark Idle" on using the VCM Controls (which keeps the spark constant), the idle gets a lot worse. So I feel my PCM is using spark as a crutch to keep the idle. I'm wondering if this is ok or if there is an underlying issue.

That's normal. The PCM varies the spark to keep the idle rpm in check along with the IAC. It uses the spark because it reacts much, much quicker than airflow.

While this is true, the more accurate the fueling is, the less the timing will need to "swing" to keep the idle in check. Ideally, you should be able to shut off your fuel trims, and shut off the idle spark control, even the IAC, and still have a pretty damn stable idle. Think about a carb and distributor. No fueling changes, no timing changes, not even an IAC, and they still idled stable.

[carlrx7]

While this is true, the more accurate the fueling is, the less the timing will need to "swing" to keep the idle in check. Ideally, you should be able to shut off your fuel trims, and shut off the idle spark control, even the IAC, and still have a pretty damn stable idle. Think about a carb and distributor. No fueling changes, no timing changes, not even an IAC, and they still idled stable.

damn, thats why I couldn't get my car to idle with spark control off, is because i never turn off the fuel trims.