TBI 350 Target Idle Speed

[maronielb]

Hi i received the new prom chip on Monday but i have not had time to install it yet. Wheather been shitty, i will probably install it this Saturday. One thing i was thinking about the other day is if i do the IAC reset but leave the IAC unplugged then the truck shouid idle at the base idle speed correct? With the IAC unplugged the computer won’t be able to get the idle up to 1k. This will also be a dead give away to is there is a vacuum leak present. With IAC fully extended out and left unplugged the truck would idle at the base idle that was set at the factory since the cap has never been removed on this TBI. Should i try this before i switch the prom chip? Or am i just running in circles

[ralmo94]

With the IAC closed and unplugged, yes it should run at where ever the butterfly stop is set, but that still wouldn't tell you why the ECU is requesting 1k idle speed. It may be a worthwhile test to see if it will idle slow, but if it has a vacuum leak, butterfly open too far, it should still target around 550 to 650 rpm, after warm up.

[maronielb]

Ok i get what your saying, i won’t be able to install the new chip
Until Monday evening but I’ll let you know what happens. I really don’t think it’s the chip i think it’s something to do with the TPS sensor but idk

[maronielb]

OK so I installed the new chip in the computer today and I did not notice any difference in the idle speed or the way the truck ran or drove. I didn’t notice any difference the RPM needle still sits right around 1000 RPM and also ended up breaking one of the little pins on my old chip when I was trying to take it out of the casing so I can no longer use that. So now I’m stuck with the new one which is fine I mean the truck still runs the same exact so I’m sure it’s not gonna be an issue but when I took the old chip out I did notice that it said Delco on it so I’m assuming that that is a factory chip that was never modified or anything like that but yeah, I’m gonna lost now with this with this idle speed. I don’t know what else to do frankly, I mean I can try a new TPS sensor just for the hell of it but other than that, I really don’t know where else to look.

[MO LS Noobie]

Go to beginning of your post, start there.

[Resin]

Do you have your scanner hooked up constantly as you are testing?

Edit: I read more of the thread and answered my own question.

Check for resistance across pins A and B of your ALDL connector.

[maronielb]

You are essentially tell me to adjust the base idle speed manually lol

[maronielb]

I don’t have a multi meter to get resistance, what wouid that do anyway?

[Resin]

Some GM ECUs have a factory diagnostic mode that requests a constant 1000 rpm regardless of other settings. Putting a 10k resistor between pins A and B will activate it. If your wires are worn and shorted to each other that could cause the ECU to enter that diagnostic mode and then it doesn't matter what you do, you will never get it to idle less than 1000 rpm. You can disconnect the IAC and set base timing but the moment you reconnect it, it will go right back up to 1000 rpm.

[TriumphR3]

A and B should have open loop between them, Yes?

[Resin]

Correct. Normal operation is open loop.
0 resistance puts it into a diagnostic mode that will flash the SES light.
3.9k resistance is another mode (I don't remember what this one does at the moment)
10k resistance is end of line factory test mode

[maronielb]

That makes sense but if the wires were shorted together wouldn’t they cause the SES light to constantly flash also aren’t those the pins that you jump to get the IAC to go fully closed?

[Resin]

If there was a short through a thin medium it could add resistance between the wires.
It could also be that someone wired a switch in somewhere to put it in diagnostic mode and they did a shoddy job.
It could also be, since I believe one or the other of the A & B pins is a ground, that the wire is worn and grounding on the body but the connection isn't good enough to be a low enough resistance for the base diagnostic mode. I believe it's something like 500 ohms or below is the threshold.

0 resistance will cause the IAC to close fully. That is part of that diagnostic mode.

[maronielb]

I had the whole dash harness out of the truck and i did not noticed any cut or spliced wiring or any switches, i don’t think the issue is in the wiring to be honest, the computer in the truck is a reman so idk if the computer itself is possible the problem? Or maybe it’s something completely unrelated that’s causing the idle to be 1k and it’s just a coincidence that the computer is requesting a 1k idle when i have my scanner hooked up

[Resin]

I'm not sure, but I had a similar problem and that's what it was. In my case, I had my ALDL connector for data logging connected. To connect to my particular computer, I need to use a 10k resistor. If I don't disconnect the resistor after connecting to my ECU it will idle at 1000 rpm.

I went through a lot of the same steps. I reset base timing, reset IAC so it was at about 15 counts, and set my TPS to .55 volts. After I hooked my data logger back up, it idled at 1000 rpm again and I lost my shit. I tore through every hose and nook and cranny and found a small vacuum leak. I thought I was triumphant having fixed it only for it to continue to idle at 1000 rpm after I started it back up as if to give me the middle finger. That went on for a few days. At one point I had started it and then hooked my data logger up and noticed a jump in rpm. TBIs kind of idle high from cold anyway but I thought it was weird. Once it was warm i disconnected the cable and the idle didn't come down so I shut it off and restarted it to see if it would jump again when i reconnected it. It started to idle normally for the first time. Then I connected it again, and right back up to 1000 rpm.