This is a scan of my '85, I can't get the idle set. It has no vacuum leaks, holds 17-18 in. but with 1000 rpm idle. Below is the scan I took, the number on the far right was flashing on the screen by each reading.

Prom ID - 51905................................... ......................01
MAF - 13 gr/sec..................................... ....................14
veh speed - 0....................................... .....................
rich lean flag - lean.................................... .................17
TPS - .58..................................... .............................
pwr steer - i high: yes..................................... ............20
MAT - 123.8................................... ...........................10
A/C - requested............................... ..........................23
O2 crosscount - 0....................................... ...............13
coolant temp - 176.8................................... ...............03
rpm - 1000-1075.................................... ....................06
Oxygen sensor - 352-844 m volts..................................0 9
block learn - 90...................................... ...................12
IAC - 91-112 steps................................... .................
inj.pulse - 1.8 ms...................................... .................15
EGR (deleted in chip) - 0%...................................... ....
Loop - open.................................... ..........................18
integrator - 123..................................... ....................08
EGR - off..................................... .............................
knock sensor - 114..................................... ................11

If the knock sensor is high it's probably because I have an oil scraper in the pan which is barely touching the crank.
The engine runs like a bear, just won't idle down to 850 which is what's in the chip. If I do happen to get it down around 850 it starts hunting all over the place. I tried two different IAC's, two different TPS's and a different t body, (all thanks to Jay at Vette2Vette) all of this was done yesterday with no real difference. Any help here is greatly appreciated

 

I'm assuming your chip has already been modified to raise the idle rpm to 850 rpm and lower the BLM lower limit (Normally 108).

I don't have a $1F bin on hand, so I can't say for certain what is standard tuning for $1F.

Is your chip also adding to the idle rpm for A/C on? I see that the A/C is requested. This could explain some of the extra rpm. Does it improve with A/C off or in gear? I didn't notice if you have an auto or manual car.

From the relatively high MAF flow for the listed rpm and BLM value of 90, it appears that you are over estimating air flow and running very rich.

Hard to say what is going on without knowing exactly the target idle rpm that is in your chip.

From the high IAC counts it appears that the ecm is either commanding a high idle, or its not in closed loop PID idle control and is instead running off the IAC park position table.

I suspect you have too much throttle opening and the IAC values from the table plus the throttle opening are resulting in too much air.

90 IAC counts should contribute around 5-6 gm/sec of air flow based on my measurements.

At 13 grams per second, this indicates that the closed throttle on its own is contributing 7-8 grams/second.

This should normally be around 4 gm/sec (minimum air) to fit with the default airflow offset and IAC park position table settings.

You could try backing off on the throttle stop and readjust the tps voltage.

If you're burning your own chips, you may also want to adjust the MAF scalars to reduce the airflow which should bring the blms back up.

 

tequilaboy pretty much covered it. Good call on the A/C requested flag. I'd like to see about 30 seconds of data with timing, load, and target idle info to really nail it down.

 

tequilaboy is exactly on the money. These two statement stood out to me. You can tell that the chip is not stock. The MAF g/sec should be around 10 at 1,000 RPM. I would also suggest to lower the TPS voltage and might as well go through the minimal idle steps.

 

The chip is programmed for an 850 idle speed. I tried turning the A/C on but there was no change in the scan reading or the idle. I can't put the car in gear at this time, I have the rear brakes apart powdercoating the calipers. I was just trying to whip the idle problem while it was apart.
I have tried closing the throttle blades and resetting the TPS but it made no difference at all. This chip is the one that was in the car before the recent engine rebuild.
The car does not seem to be rich at all, no black smoke at any time. Car is an automatic.
Okay, I just went out and fired the engine and let it warm up to 170 degrees. it was idling at 950-1000 rpm. I shut it down, put the code reader in and turned the key back on to let the IAC extend. I waited over a minute before I unplugged the IAC. At this point the engine will not start without giving it some throttle. I went ahead and backed the throttle stop off 1/4 turn and reconnected the IAC. Now it won't start without giving it throttle so the throttle blades are not too far open. Also, I forgot to mention that before I took the rear brakes apart the engine would drop about 400 rpm when I would put it in gear and would not come back up as it's supposed to until I put it back in park, then it would go back to higher rpm idle. This alone is why I'm thinking ECM but since I really don't know I was wanting to see what you guys thought.

 

What is the IAC start up position?

Is it fully open? (144 or 145 counts) Perhaps this been changed in the chip?

I would think the IAC fully open would be enough air (approx 8 gm/sec) to start even with a fully closed throttle.

Have there been any changes to the crank fueling or bigger injectors or increased fuel pressure that would require more air to start?

Maybe the IAC somehow lost synchronization between starts with all the connecting and disconnecting etc.

 

Don't know what the start up position of the IAC is but I can get that tomorrow a.m.
There is NO changes to anything on this engine except for rebuilding the shortblock to a 383. As I stated there has always been a slightly unsatisfactory idle with the engine but nothing like it is now. Nothing was touched or changed when it was taken apart either, it was put back together just the way it came apart. I might add that I have tried 2 different IAC's used and a brand new one. Each time I changed ANYTHING I did the full minimum air idle procedures with the exception of timing which is at 6 degrees advance. Fuel pressure is set at 45 lbs with vacuum off, it holds at 40 with vacuum on.

 

Anytime you screw in or out on the TB stop screw it changes the voltage at the TPS. Once the IAC is fully extended and disconnected it will be difficult to get it to idle to start with, but should smooth out. After adjusting the stop screw, check the TPS voltage as it will change. The more open the throttle plates the higher the TPS voltage.

Once done, key on, make sure the TPS voltage is correct and when opening the throttle plates...the voltage increases smoothly to WOT. It should be >4.0v at that time. Also, I have read where the EST needs to be disconnected when doing the minimal idle adjustments, but no evidence that it helps or not.

Have you checked for vacuum leaks or the AIR system?

 

As stated, every time I made any changes I re-adjusted TPS. TPS maxes out at 4.4 wot. No AIR system on car, 17-18 in. vacuum at idle.

 

Sorry, must have read past all of that. It would be nice to have a spare stock chip to try out.

 

No big deal, I have been through so much trouble I can't remember what I did write and what I left out.
I do have the Hypertech that was in the car, that will be my next move before buying another ECM.

 

I pulled the custom chip out and stuck the Hypertech in, got the same results but on a lesser scale because of the lower programmed idle in the H-tech. Still starts, idles high for a bit then drops down around programmed idle then starts hunting all over the place. It idles smooth at the higher rpm. BTW I did re-do the min. air settings on the IAC and TPS with EST disconnected.
If ANYBODY can think of ANY reason for me not to get a new ECM tomorrow I'd sure like to hear what you have to say. Keep in mind the fact that the idle drops about 400 rpm when I put it in drive and stays there until I put it back in park/neutral. This is what makes me believe the whole thing is the ECM.

 

I'm sure you have checked for vacuum leaks, but I will give you a description of what mine did.

On my 86, when the engine was started cold, the idle would good then get low and back and forth about every 4 sec. After closed loop the idle would stablize and idle smooth. I could also hear a hissing noise when the idle dropped in open loop. I found the vacuum leak and fixed it. Then when I started the engine cold the idle was smooth as is closed loop.

Have you checked the TB throttle plates or worn throttle bore?

I'm not sure a different ECM will help, but is always handy to have a spare.

 

My idle NEVER stabilizes. It isn't too bad until the engine gets warm then it drops down in rpm and the race is on, up, down, etc. By dropping down I mean 950 to 1000 at best. I used cigar smoke blown into the vacuum port next to my cold start injector, the only place smoke came out was the mouth of the throttle body. I can even put my hand over the t body and blow into the vacuum line and I can feel the pressure build so there is no vacuum leaks, I'll bet $$ on that. Before, the idle would stay about the same whether in gear or not, now it drops almost 400 rpm when in drive or reverse. BTW, the t body is almost brand new, maybe 400 miles on it.
I won't have a spare ECM, they take the old one for core, that's the biggest reason I'm leery of getting another one.

 

I really think you have idle mixture problems based on the behavior and the strangely low 90 blm value.

When the idle is going up and down, the calculated load will also cycle up and down, but out of phase with the rpm.

As rpm drops, the load goes up. This results in even more fuel being added causing rpm to drop even further. Anti-stall spark advance eventually kicks in and may prevent a stall and temporarily raise rpm, but the cycle continues.

The spark advance and IAC will also be contributing to the oscillation.

You need to stabilize the idle mixture. I would start by reducing the fuel pressure or MAF calibration table #1 and maybe even reduce the the % change to fuel air ratio vs. load table to get the mixture under control.

Double check the chip to make sure a high park/neutral offset isn't being added (50 rpm is normal).

Note: I thought the 85 fuel pressure specs were between 34 and 40 psi. The higher pressure regulators came later.

I don't think you have an ecm problem. It sounds like a tuning problem to me.

I know this can be a pia to sort out.

 

I'm afraid I'm way out of my league here, I bought the scanner so I could provide a reading to someone who understands all of what you say. I can reduce the fuel pressure okay but have no idea what is meant by reducing the MAF calibration table#1 or how to reduce the % change to fuel air ratio vs. load table. This is all foreign language to me.
Obviously I can't check the chip either. The reason I have higher fuel pressure is because I have an adjustable regulator. I really appreciate what you're trying to do but I guess I'm going to have to try and find someone to straighten this mess out. Thanks so much for your trouble.

 

Sorry, my bad. The changes I was discussing are some of the tuneable parameters within the chip or bin file that could contribute to your car's behavior.

Since you already have a reprogrammed chip, I was thinking that you had a copy of the bin file and could review it with one of the common editors like TunerPro, TunerCat etc.

Try and get a copy of your bin file from your tuner and download an editor and $1F definition file from moates.net. At least this would give you an idea of what you currently have in the car.

Even if you can't get a copy of your bin, it wouldn't hurt to get a demo version of one of the editors just to see what is available for tuning.

 

I've been trying without success to get hold of the individual who burned the chip but haven't had any luck.
What about the 400rpm drop when I put it in gear, isn't that controlled by the ECM? This is the same chip I have been using and the rpm never used to drop like that.

 

I have a 85 with a 383 with TF heads, 30lbs. inj. 268 comp.cam and my fuel pressure is 40psi. no vac. and 35 with vac. drop the pressure it should smooth out a bit. As for the higher idle when in park are neutral there is an offset in the bin called " IAC Park/Neutral Offset" Stock its set to 50rpm so when in park or Neutral this is added to your Target Idle RPM Vs. Coolant Temp. table. My target idle at 68deg c. is 650rpm so my idle in park is 700rpm. I'm new to all this still I've only had this vette 1 year now but i've been through everything you've described. But when you hook up a scanner to the diagnostic port the idle is rised to 1000rpm and I think the timing is also advanced 9*. Do you still have the stock injectors? I dont remember seeing what size they were?

 

Just want to say thanks to tequilaboy, RRT vette, Apex85Blue, and byebyeL98 for all your suggestions, I got in contact with ski_dwn_it and he got me straightened out. He instructed me on a different way to set the idle and it now idles with only a 20-30 rpm fluctuation. Jesse is the one who burned my chip so he had a better handle on my situation. It is really hard to diagnose something without being right there but you guys tried and I am indebted to you.