Hotrod6616

I'm sure this has been asked but I can't find the answer or numbers I'm looking for. Maybe it should be asked in the c6 section but I figured I'd start here. I have a ls2 crate motor in a 66 nova and I bought a plug and play harness that came with the ls7 maf. Well ever since I've got the car running it does not like to idle normal. It hangs up around 1200 to 1400 rpms and sometimes it will come down and sometimes it will not. I figured out the computer was set up for ls2 maf readings and I have adjusted the tables some and it's better but still not right. Does anyone have any numbers for the maf that should get me close? Or is this a trail and error type deal. I've been reading a lot of people have troubles using his maf sensor but when they're right they work great.
Also the computer does not have vss sensor hooked up, paperwork says it's not needed to work but I think the car might not know what to do after driving for a bit.

Motor is basically stock with some headers and air intake don't to it.

tblu92

You are having what they call a "hanging idle " The MAF table is used to correct fuel trims at idle or P/T driving---The only way to know if you need an adjustment to your MAF table is to have a tuner data log your fuel trims. They can either be positive or negative --make the corrections accordingly
Typically a hanging idle is corrected using OTHER tables----Most often the "throttle follower table" is used----Available both for in gear and P/N--To correct a hanging idle You would ADD a % to the follower table
Other tables also can influence a hanging idle----The "throttle cracker table" would be used 2nd if the follower didn't work----3rd you can make an adjustment to the IAC settings----Removing a % would correct a hanging idle---
Only a tuner with experience can sort out exactly where to make the fix---
Timing can also correct some idle issues--A tuner can data log timing at idle --If the timing is fluctuating a lot that could be the issue as well----Best is to have it only move around 3-4* at idle IF it is moving around more than that a tuner can narrow down the gap and it will idle smoother and better
PS : Does this happen all the time ? Engine started cold only OR only when warmed up ? This would indicate a whole other fix--As when started cold the en gine runs in "open loop" but after warmed it stays in open loop for just seconds and quickly changes to "closed loop" In open loop the fuel is commanded solely by the open loop table--In closed loop the fuel is constantly being corrected to 14.68 AFR using data from the front 02's and corrected via the fuel trims--

Hotrod6616

Thanks for the reply. I should have mentioned the car likes to do it when it's warm. It will do it when it's cold but 85% of the time it will return back to normal idle a few seconds later. Also the car is a stick shift car and the computer does not know park or when the car is in gear, I'm sure this is causing some problems as the computer doesn't know when to idle or when it's driving down the road. We have adjusted the p/n tables though some and it did not make a difference. We have adjusted the follower table with no luck either. The only table that made any difference was the maf table. The maf was set up for an ls2 style maf and the maf I received in my plug an play kit was ls7.

The open loop and closed loop does make sense now that you mention it. We will have to do some data logging and see what we can find if anything.

Hotrod6616

Being in closed loop it's trying to maintain that 14.6 afr and if the maf is reading too much air the fuel tables would be trying to correct to maintain that 14.6 correct?

tblu92

Ok when the engine is dead cold the fueling is set entirely by the "open loop normal table" until it reaches approx 140* coolant temp--After warmed and the engine is above 140* it switches to a timer rather that coolant temp to switch to closed loop--Generally this is about 20 seconds--With mods especially cam and LT headers the open loop loop or starting AFR is very rich---So you have to "trick" the ECM into a AFR --leaner that stock to get it to start and idle normally
As an example on my hds/cam/LT 98 car I had to command a very lean AFR in open loop just to get it to start and idle correctly---I commanded an AFR of 16.10 in the start up/ idle fueling areas just to get it back to the stock 14.68 AFR---You also have to add more air in the " DESIRED AIRFLOW TABLE" to keep it idling normal---
This is a complicated process--A mailorder tuner can only guess at what will work--The only way to get it spot on is to get a tuner who knows how to data log idle and P/T fueling and has the expereince to know how to trick the ECM into accommodating all your mods