Slightly lumpy idle with headers - rich??

[Boost]

On my '03 4.8 Silverado, with cheap shorty headers and a leaking muffler, MSD plug wires and newer NGK TR55s gapped properly, it misses slightly but regularly during a hot idle with the a/c on. I suspected an ignition miss because the wires rub on the headers, but SCAN is not picking up any significant individual cylinder misfire. When I temporarily decrease the AFR, the exact condition exaggerates, confirming my suspicion that it's a rich burble (?). But I thought headers make it leaner?
I plan on logging and simultaneously monitoring cells. But can I have a clue as to what it may be or what I should tune to lean the idle, if that is in fact what's needed here? Thanks!

[joecar]

Headers will typically make it leaner in the torque band, and richer at idle.

See if you can prevent the plug wires from touching the headers.

The PCM may or may not pick up on a misfire.

[Boost]

Thanks, that makes sense but could I ask what table should I lean out for idle? And what is a / the determining factor of whether a misfire registers in the PCM or not? (besides tiny or huge miss)

Also, for learning purposes:

What is the logic / theory behind the rich during idle only with exhaust mods? Does it have something to do with the MAF not being as accurate during low engine speeds or something else?

[Boost]

By increasing slightly the VE during idle speeds, can I fix the idle richness due to headers?

[bmax]

By increasing slightly the VE during idle speeds, can I fix the idle richness due to headers?

Boost,



Adding to the VE table will add fuel.


You need to use a wideband to get it dialed in right.

Brad

[Boost]

Thanks Brad, I know I need a wideband and do not plan on doing any serious tuning without it. But since my truck runs well all around except the idle richness, to adjust that one simple thing that might make a noticeable difference in economy based on how long I idle with the a/c on sometimes, I wanted to carefully lean the idle just a tad, even if blindly. Anyway part throttle and idle adjustments are possible though not optimal with just narrow band sensors, no?

Also would really like to learn just for theory, why if headers improve scavenging and therefore improve VE, why would telling the PCM about it add fuel as you pointed out. Or do headers worse idle airflow but improve the rest? What gives, sorry I am confused because I don't grasp it yet. Thanks a lot!

[bmax]

I can't give a very technical explanation but, the VE table tells the pcm how much air to expect so it can calculate how much fuel to mix in to get the desired AFR.
So more values in air mean more values in fueling. And a richer mixture.

You could try reducing the VE table in the idle areas by a percentage and see if it is better.

Good luck

Brad

[joecar]

...
What is the logic / theory behind the rich during idle only with exhaust mods? Does it have something to do with the MAF not being as accurate during low engine speeds or something else?


...
Also would really like to learn just for theory, why if headers improve scavenging and therefore improve VE, why would telling the PCM about it add fuel as you pointed out. Or do headers worse idle airflow but improve the rest? What gives, sorry I am confused because I don't grasp it yet. Thanks a lot!Yeah, it seems to me that headers would scavenge at any engine speed, but I'm told that there is a a trade off between gas volume and gas velocity at low engine speed... but I don't know the theory, yet.

Can you post your tune file and some logs, include these pids:
VSS
TP
RPM
DYNCYLAIR or DYNCYLAIR_DMA
DYNAIR
MAF (SAE or GM)
MAP (SAE)
DYNAIRTMP_DMA
IAT
ECT
EQIVRATIO or AFR
SPARKADV
KR
LONGFT1
LONGFT2
HO2S11 (not HO2S12 or HO2S22)
HO2S21 (not HO2S12 or HO2S22)
MFTOTAL

Keep pid channel count to 24 or less (don't exceed 4 packets) for fastest sample rate.

[Boost]

This may seem like or may be a silly question but is it more inmportant to keep the PID / channel counts down for speed, or to log more detailed and accurate data??

[Boost]

Thanks!

[joecar]

This may seem like or may be a silly question but is it more inmportant to keep the PID / channel counts down for speed, or to log more detailed and accurate data??To collect data at the fastest sampling rate... on the LS1 PCM, this is 10 Samples/second at 1-24 channels, and reduces to 5 Samples/second at 25-48 channels (which is quite slow, this will miss HO2S voltage swings).

[Boost]

Ok thank you. I noticed that I had 20 PIDs with 5 channels, so I guess the channels do not increase proportionally with the number of PIDs selected, rather it's which PIDs or how many channels' PIDS are selected?

[joecar]

Some pids take 2 channels (e.g. RPM, MAF, DYNAIR, IBPW1/2), and some take 4.

1-24 channels fits into 1 frame... 25-48 channels fits into 2 frames.

[joecar]

I'm still looking at your log file, I haven't determined anything yet.

[Boost]

Thank you so much for looking at it, I noticed while logging that one bank's LT trim was double the other's.... not sure if that's common or normal due to wear or thermal characteristics. I would also like to mention that the truck's economy is not way off considering 105k miles and my agressive driving, plus some idleing. I really appreciate your reviewing it, however since I would love to improve the tune even if by 1%. Thanks for years of advice LOL

[joecar]

I noticed in your log that when throttle moves from 6% to 8% MAP changes from 38kPa to 45kPa and dyncylair changes quite a bit...

I'm wondering why did the throttle bump like that...?

[Boost]

I noticed in your log that when throttle moves from 6% to 8% MAP changes from 38kPa to 45kPa and dyncylair changes quite a bit...

I'm wondering why did the throttle bump like that...?

After it warmed uo to 185 F I turned on the a/c and eventually revved it a few times??????