Turn off all IAC functions.

**donf** · 12-30-2018

It really sounds like you have a problem, but are wanting to short cut it. Only after you have done a complete diagnostic, checking things like IAC steps, cts and tps would consider making any changes. You may be able to hack up the settings to where the idle adjustment is broken enough to give a fixed speed under all conditions. There are a lot of things that can cause the problem you are describing. Its common to have a worn IAC loose steps meaning it's not where the computer thinks it is. Also the wiring on the manifold CTS is so faulty most of the times the sensor comes with a new pigtail. Intermittently altering the cts temp voltage can affect the idle. To fix it right, you are going to have to do some digging. Someone may have a copy of the idle diagnostic steps in the factory gm manual, but purchasing my own manual used off ebay is one of the first things I do for my GM cars, if I can find them reasonable. pay attention to the details, I can't tell you how many thousands of times a customer would insist they had checked everything, and thrown on multiple new parts and really it was fixed within and hour or two with careful step by step diagnostics. The best tip I can give you is pay attention to the small stuff when checking for problems, so many times people get in a hurry and miss the problem when it's right in front of them. I have done it myself.

**chris989** · 12-31-2018

Don,
You are right, My guess is a faulty wire of sensor. everything is new under the hood but I have had plenty of defects out of the box. The next time it is idling right, I need to log and make a spreadsheet. Then log again when it is acting up and see if i can find the variable.

My TPS and MAP never give me erratic signals that i have caught. THe TBI set up is so simple, I can not believe i can not find the source of my problem.

Thanks,
Chris

**chris989** · 01-27-2019

Hello again,

I have been fooling with Tuner Pro and loggin some data. Nothing in my data shows a MAP or TPS issue and the truck is running smooth. I have concluded closed loop for AFR is not the same as closed loop for idle?? My truck goes closed loop easily with the heated O2 sensor but i feel the idle logic is OL. Attached is a screen shot of the scalars that make very little sense to me.

Open loop RPM theshold is on at 1600RPM and of at 1500??
closed loop coolant cold timer at 200 seconds?
Why is there a cold and warm coolant threshold? why not one setting where it goes from OL to CL?

What do the timers delay?

Any help would be great.idle logic.jpg
CL coolant warm timer?

**stew86MCSS396** · 01-27-2019

I think you're stating it backwards. Yes with those parameters, it appears you have OL idle bin. If you read enough on this forum, some say they don't like OL idle and prefer ECM controlling with CL idle. I think the tuning strategy for using OL idle is if you're running a radical cam that produces low vacuum.

**chris989** · 01-28-2019

I am trying to disable the open loop idle or have it go closed loop earlier. Today it is back to high idle. I am sure it has something to do with the steps added in OL idle. The truck is in CL but the idle still goes open sometimes.

I will try to zero them one at a time when the motor is cold and see if i can affect a change

thanks,
Chris

**sturgillbd** · 01-29-2019

Make sure your P/N switch is sending the input to the ecm. Also make sure your vss is operational. If I remember correctly, the 7747 needs both for the idle to calm down. A friend put tbi on a 1975 jeep with a sbc. We ended up grounding the P/N input to the ecm because he had a manual trans and the idle would flare up in the woods as you describe. He would end up shutting engine off and restarting to stop the high idle. He didn't have a vss connected and it seems like those ecm's need it. Just something else to check...

Brian

**chris989** · 01-29-2019

Originally Posted by sturgillbd

Make sure your P/N switch is sending the input to the ecm. Also make sure your vss is operational. If I remember correctly, the 7747 needs both for the idle to calm down. A friend put tbi on a 1975 jeep with a sbc. We ended up grounding the P/N input to the ecm because he had a manual trans and the idle would flare up in the woods as you describe. He would end up shutting engine off and restarting to stop the high idle. He didn't have a vss connected and it seems like those ecm's need it. Just something else to check...

Brian

BINGO,,,, I just went back and looked at the raw data and the raw MPH is zero. It is interesting what you said about turning off and restarting. I notice that it will be high and then i turn it off and restart and life is good.
I originally had a 400 turbo but switched to a 4l80. The trans is run by an MSD controller so the original set up is not hooked up.

What did your friend do about the VSS signal?

Thanks for the lead, I will look into how to get the proper signal from the 4l80.

Chris

**stew86MCSS396** · 01-29-2019

Originally Posted by chris989

I have a 89 v2500 burb,
5.7, New GM crate "stock piston/heads"
Lunati 10180700
.437"/.455"
250/256.
Headman full length headers,
Edlebrock intake.
4l80- w/small block convertor
456 gear

Later today I will log a file and upload.

Is the C3 ecm problematic? Is there a better unit i can replace it with?

Ahhh and I thought this was a one off ecm that would run the 4l80E. Duh!

**sturgillbd** · 01-30-2019

We just grounded the P/N pin and it helped a lot. With you having the 4l80, you can hook up the drac and send the signal to the ecm. Should be able to parallel the VSS signal from vss to the drac and the controller. You may be able to get the signal for the controller directly from the drac. What VSS signal does the MSD controller require? 40 PPR or 2 PPR? See http://gearhead-efi.com/gearhead-efi/doc/DRAC2.pdf for a pinout of the drac. Newer models are similar.

**chris989** · 02-01-2019

Originally Posted by sturgillbd

We just grounded the P/N pin and it helped a lot. With you having the 4l80, you can hook up the drac and send the signal to the ecm. Should be able to parallel the VSS signal from vss to the drac and the controller. You may be able to get the signal for the controller directly from the drac. What VSS signal does the MSD controller require? 40 PPR or 2 PPR? See http://gearhead-efi.com/gearhead-efi/doc/DRAC2.pdf for a pinout of the drac. Newer models are similar.

Got under the truck tonight and realized i have no wire at the trans for VSS. My year has the green box under the dash. All original, so it is getting the VSS from the speedo. I have been searching for a testing procedure for the box and come up with a lot of nothing. Anyone know how to test this thing? The only thing in the ECM that uses MPH is the EGR and OL idle threshold.

**chris989** · 02-04-2019

interesting observation today,

I am trying to just use open loop for now, Raising the temp for closed loop to stop it from going closed.

Even with all IAC step removed from the available tables/scalars I still have 39 steps at 700 rpm (cold start). If i put the truck in gear it will add to 58 steps to and 700 rpm. Put is back in neutral and the steps will not decay and I idle at 875 cold.

I see a bunch of decay coificient bla bla but i do not understand what they do.

Any ideas,,,,

Also,, I am getting MPH to the ECM, I just did not look far enough down the log sto see where i finally started to drive.

**CDeeZ** · 02-05-2019

I haven't read every single post in this thread yet. However, I temporarily disabled the IAC error code in my 7427 $0E setup while I was dealing with other BS on this build and working out a ton of other bugs....... Just didn't want to mess with it yet lol. I think I was getting the IAC error code becuase I had not yet set the MIN AIR as per TBI...... I had to pedal it to get it started and keep my foot on the gas a little bit after starting it but otherwise, I noticed no problems...

I have since re enabled the IAC and set MIN AIR to about 20 counts. I'm seeing maxed out IAC values on a cold start now, 160 steps is max I think??? Not sure if this is normal, anyone know??

**chris989** · 02-05-2019

I have since re enabled the IAC and set MIN AIR to about 20 counts. I'm seeing maxed out IAC values on a cold start now, 160 steps is max I think??? Not sure if this is normal, anyone know??[/QUOTE]

My OEM bin has 146 steps max cold.