Rolling idle/o2 problem(long)

**myburb** · 06-15-2015

I am having trouble with unstable rpm, especially at idle. I can feel it on closed throttle coasting very slightly, more noticeable at idle in gear and more again in park. I have worked a lot on the ve tables and at normal cruise my blm's sit pretty close to 128 and don't move much. I have just installed a wide band o2. A/F at cruise shows roughly 14.1 to 14.9. Idle shows 13.5 to 15.2. O2 voltage is roughly .085 to .850 at idle and .165 to .875 at cruise. I thought maybe the o2 and headers was a problem so I installed a heated narrow band. No change. The idle fluctuation caused the IAC to dance around so I got the IAC to zero and disconnected it. Helped at bit but not acceptable. I have read on forums that P4 pcms have overly active o2 handling. I have set and unset closed loop idle. No change. In the data stream closed loop always shows and it seems as though it is being controlled by the o2 even when I think it should be open loop at idle. Today I disconnected the NB and it drove great. Cruise the WB showed 14.7-14.9, foot off coasting 15.2-15.3, idle 13.7-13.9, no light throttle surge and idle rpm stable. I said it was going to be long. It is perfectly driveable right now in open loop but that won't last for weather changes. I don't speak GM so don't know what a lot of the o2 parameters mean and things are pretty sparse regarding documentation. So anyone have any ideas? My next move is to install one of my mega squirts and let it look after fuel and ignition, trans and IAC on 6395. Is there any way to slow the NB response down and being as my wide band has a second channel is there any way to control with that?

**CFI Z51** · 06-15-2015

Try these one at a time to get a direction .Clear the memory between tests.
For the idle surge you can try zeroing the idle overspeed / underspeed spark advance settings as a test,to eliminate spark as the culprit.
As far as i know the main control for the 02 are the proportional gain settings,you might try reducing these at idle by 50%... as a test.
With headers you may need to increase the Integrator Delay vs. Airflow ,the 3 at the lower end of the table..50% as a test.

Maybe post your bin and a short data log...

**myburb** · 06-16-2015

thanks for the reply Z51, i checked the sparks and they were zero. i then took a large swing at int delay vs airflow. I would say I pretty well didn't notice surge at low throttle, slightly slighty better at idle in drive and about the same as it was in park. I looked at the proportional gains and didn't understand it so left alone. I could change them one at a time and then data log but that would take a lot of runs. I hope to be able to attach a bin and log.
As an added note I too have an 84 Z51.

**Roadknee** · 06-16-2015

I've been learning how the fuel trim parameters work via trial and error on a couple small blocks running the 7427 PCM. I looked at your bin and it's interesting how GM has them set up for a big block. They took a completely different approach compared to how they tuned the small blocks.

A couple things you might try: set the lower limit for slow filtered O2 (scalar) to about 60. It's currently at about 150. Also, there are three tables in fuel trim parameters:

mean rich/lean O2 voltage threshold vs airflow
rich O2 voltage threshold vs airflow
lean O2 voltage threshold vs airflow

Set all the tables equal and try increasing the values in the lower airflow areas. This will cause the ecm to start adding fuel sooner when the O2 swings from rich to lean, and add more fuel the leaner it gets from the target. Try these as a start:

550
550
525
500
475
451
451
451
451

There are similar values in Scalars for idle that you can try increasing as well.

**CFI Z51** · 06-17-2015

"I would say I pretty well didn't notice surge at low throttle" You should be able to tune it out with INT /Delay then , might be able to get it with timing alone.
The factory timing table leaves a lot to be desired ... a lot to be gained in that area... a must do with a bigger cam. spec on yours ? lift and duration at 50'?

For the idle surge try this Bin .
I added 4* timing at idle and and raised idle by 100 rpm, may need more of each...but should help...post a data log.
on next data log: engine at full temp , couple minutes in park , same in gear will be enough .

Stock vette , stock tune ? A tpi fuel pump and 14 psi = new tires.

**myburb** · 06-17-2015

About a month ago I accidentally put a 350 van bin in and it ran pretty darn good. Turned out the bin was set for 61 inj and mine were 72 so almost made up for the displacement difference. It was a little lean but drove really well. Handled idle nicely. Today I put it back in, fudged the injector size to make it happy and did a couple of logging drives. Fixed the ve problem areas and corrected spark bias and all was good. Although there wasn't much I switched all the 4l80e settings from the7.4. From a drivability standpoint it has never been so good and I was able to reintroduce full IAC operation also. Thanks for your help. The 84 has a 489 BBC with a muncie. I have and extra 80e that might find its way in.

**CFI Z51** · 06-19-2015

"Outside the box" i like it

added my notes on idle tuning for the next guy.
"489 BBC with a muncie" ....

**myburb** · 06-20-2015

I have copied the text. I just typed a long note about the on going and I got told I needed to resign in and it dumped my post. Will get to it tomorrow.

billygraves · 06-20-2015

Originally Posted by myburb

I have copied the text. I just typed a long note about the on going and I got told I needed to resign in and it dumped my post. Will get to it tomorrow.

This has happened to me a number of times. I asked for any suggestions - none that worked out of the one written. So, I don't post much. My time is worth more than this frustration.

**CFI Z51** · 06-20-2015

Any thing more then a few lines i type out in note pad and past it in. Saves a lot of

**myburb** · 06-20-2015

For what it's worth Bill I enjoy your 4l80e info.

billygraves · 06-20-2015

Originally Posted by CFI Z51

Any thing more then a few lines i type out in note pad and past it in. Saves a lot of

That will work. I won't go that far and don't feel I should have to do this crap. My time and years are worth something.

**myburb** · 06-20-2015

Being as the 95 350 worked pretty good I looked for 95 454 bin hoping gm had improved things. It was a $31 mask and I had trouble loading it and thought maybe it was the difference in mask and checksum. Turns out the emulator was dying. I burned a chip and data logged and burned again and the idle surge problem is pretty well non issue. Fuel is off sum but according to the wide band the int/blm does a good job of trying to keep all good.

**CFI Z51** · 06-21-2015

I burned a chip and data logged and burned again and the idle surge problem is pretty well non issue. Fuel is off sum but according to the wide band the int/blm does a good job of trying to keep all good

Just hadta turn the dizzy an fiddle with the idle screws.....

mind posting the bin , just curious how much it took .

Now that its close do the same thing in CL ,then add 100 - 200 rpm ....repeat , then blend into open throttle tables.
Then you can start on the OT Spark Table , maybe start with the SS454 table ,iv read it works fine in a burb ,(never done it)just watch for knock.

**myburb** · 06-21-2015

The bin is BMHM. It is a $31 mask so hard to run compare. I will dig out another old laptop and run the $OE along side the $31. I will try and attach the bin. Going on holidays for about 10 so won't be doing much.