Chasing idle issues. Idle steps dropping.

[CptFalcon]

Hello, my vehicle is a 98 Sonoma with a 383 sbc 24X with 0411 running it. It has a single plane with Holley 4150 tb and a rather large cam (XR288HR). The truck runs pretty good but I cant get the idle to stabilize, there are absolutely no vacuum leaks first off. Ive been messing with the running airflow tables, idle screw and iac, attempting to stabilize the idle. It just wont seem to pick a spot and stay there its almost as if ignition timing or some other factor is controlling it. It will start normally when cold with 310 steps then work its way down but usually it will settle then keep dropping to 0 then the idle maintains. I close the throttle screw and the steps come back up to about 110 then settle at 40-60 then drops to 0 again. Its almost like chasing its tail. I cant get it to pick a spot and stay there, this isnt a rapid fluctuation either it takes several minutes to go up and down. Thanks

[bk2life]

Hello, my vehicle

you have to tune the VE, Spark table, PE, and MAF before you can tune idle.

Your ve looks lazy, your spark table needs help, and i know a SBC will tune different than an LS engine, but, you have things you need to tune first.

And your not logging really anything that will help you tune

get it into closed loop, and start with tuning your VE... raise your PE to at least 70 kpa, and fix the PE ratio you have no PE coming on, ( no 'secondary squirt of fuel if it was carbed)

[SiriusC1024]

PE isn't like an accelerator pump. Transient tables are for that. The extra fuel from power enrichment doesn't provide power. It allows for power. Enrichment slows the burn rate and cools combustion to allow for more spark advance and cylinder pressure.

That's why COT (catalyst over temp) commands such rich mixtures. Cooler exhaust gas to protect the matrix from melting. If you've ever seen my tunes and wonder why I leave COT on, even during tuning, it's because if the sustained temp is high enough to melt the cat then it's probably doing damage to the exhaust valves.

There's also the fact that combustion isn't homogeneous. A lean spot in the flame front during stoichiometric might not be a problem until high load. PE ensures no lean spots.

I wish the dev's would include in the custom OS's an enrichment vs time table like in the factory 3800 tunes. More enrichment during sustained load to mitigate heat. Perfect for boost. You can get the idea of what I mean about the function of PE here:
enrichment vs time 3800.png

[edcmat-l1]

First, your injectors obviously aren't "stock" so what are they? Need to start there.

Second, you aren't logging anything fuel related so you really have no idea what it's doing. You need to log STFT bank 1 and 2, B1S1 and B2S1.

That's just for starters.

Your TPS voltage looks too low. That'll set a code.

Adaptive idle looks too soft. That'll make your IAC try harder, which always leads to idle instability.

[CptFalcon]

First, your injectors obviously aren't "stock" so what are they? Need to start there.

Second, you aren't logging anything fuel related so you really have no idea what it's doing. You need to log STFT bank 1 and 2, B1S1 and B2S1.

That's just for starters.

Your TPS voltage looks too low. That'll set a code.

Adaptive idle looks too soft. That'll make your IAC try harder, which always leads to idle instability.

Thanks for the input. Injectors are Deka 80's because I plan on adding a Procharger in the near future.

I had it in open loop because I was in the middle of tuning with my wideband which I know is kind of backwards since you usually do fuel trim tuning first. I will put it back in CL and do a log and see where it ends up.

Good call on the adaptive idle I will mess with that next. I added +/- 5 degrees on the overspeed underspeed tables and it seemed to help some with the overall idle. Not sure if that was the right move or not.

[CptFalcon]

So I put it back in closed loop today, I only had time to do a really quick drive around the block. The VE needs work for just the little logging I did but isnt far off. I really dont understand though why my O2's are cycling nicely but the fuel trim bars look weird with bank 1 not even moving. I can see the individual STFT banks on the channel reading fine. Fuel trim reset did not fix this, I confirmed my O2's are not crossed as well.


:Edit: I watched the replay of the scanner file and I can see the bars reading just fine, I swear they were not showing up while I was logging, must be a glitch in the program.
I also just seen my TPS is till reading low I will get that adjusted, whats a good range to keep it at for idle? I read 0.60v max.