mikesundrop
Newbie
Hey guys,
I've got sort of a nitpicky issue/question that's as much for my own knowledge as anything. I've searched around and I haven't found what I'm looking for.
Does anyone know how the ECM logic works to control the IAC on a TBI? Which inputs does it use and what all does the ECM do?
Here's how I assume it works:
1. ECM reads coolant temp and picks an appropriate target RPM.
2. ECM sends a signal to the IAC to move the pintle and increase airflow, while it also increases injector duration, raising RPM. Closed loop using the tach signal (RPM.)
3. As the engine warms up, the IAC closes down and injector duration is decreased.
4. All changes are based on temp/RPM curves that define air and fuel flowrate in response to coolant temp and A/C on/off signal.
Does anyone know whether I'm completely off base here? Here's why I want to know. I just swapped in a TBI 350 in my 94 K1500 (previosly 4.3.) I swapped in the complete engine, all sensors, and ECM from a good running truck. It runs like a top in general. When started cold it idles up to about 1600 RPM for a minute or two, then drops to about 1100 RPM. It stays there until the truck gets all the way to 180 degrees coolant temp before settling in at 600 RPM. It's so smooth you could forget it is running.
The problem is that when I rev the engine at all, it comes down very slowly. From an idle, rev it up to about 2500 RPM, let off it goes through the same progression as cold start - 1600 RPM for a few seconds, 1100 for close to a minute, then settles back down. It makes the truck refuse to slow down at the bottom of the rev range. I have to slip the clutch because it doesn't want to drive below 1600 RPM. Poking around in a parking lot is kind of a pain. Like I said, nitpicky.
I'm confident that it's mechanically sound and that the high idle is what the ECM is intending to do since I just went through everything during the swap. I've swapped the IAC with a known good one with no change. I've also waited until the truck warmed up and unplugged the IAC. It runs a TON better like that, doing exactly what I tell it right off the 600 RPM idle. If I wasn't afraid that the ECM assumes the IAC is working and increases fuel flow, I'd probably put a switch in line and flip it when the truck warms up.
From what I know, I don't see any reason why revving the truck would change the IAC setting. Unless maybe it uses the MAP sensor and TPS somehow in its calculations? Any ideas? Thanks and sorry about the long post.
I've got sort of a nitpicky issue/question that's as much for my own knowledge as anything. I've searched around and I haven't found what I'm looking for.
Does anyone know how the ECM logic works to control the IAC on a TBI? Which inputs does it use and what all does the ECM do?
Here's how I assume it works:
1. ECM reads coolant temp and picks an appropriate target RPM.
2. ECM sends a signal to the IAC to move the pintle and increase airflow, while it also increases injector duration, raising RPM. Closed loop using the tach signal (RPM.)
3. As the engine warms up, the IAC closes down and injector duration is decreased.
4. All changes are based on temp/RPM curves that define air and fuel flowrate in response to coolant temp and A/C on/off signal.
Does anyone know whether I'm completely off base here? Here's why I want to know. I just swapped in a TBI 350 in my 94 K1500 (previosly 4.3.) I swapped in the complete engine, all sensors, and ECM from a good running truck. It runs like a top in general. When started cold it idles up to about 1600 RPM for a minute or two, then drops to about 1100 RPM. It stays there until the truck gets all the way to 180 degrees coolant temp before settling in at 600 RPM. It's so smooth you could forget it is running.
The problem is that when I rev the engine at all, it comes down very slowly. From an idle, rev it up to about 2500 RPM, let off it goes through the same progression as cold start - 1600 RPM for a few seconds, 1100 for close to a minute, then settles back down. It makes the truck refuse to slow down at the bottom of the rev range. I have to slip the clutch because it doesn't want to drive below 1600 RPM. Poking around in a parking lot is kind of a pain. Like I said, nitpicky.
I'm confident that it's mechanically sound and that the high idle is what the ECM is intending to do since I just went through everything during the swap. I've swapped the IAC with a known good one with no change. I've also waited until the truck warmed up and unplugged the IAC. It runs a TON better like that, doing exactly what I tell it right off the 600 RPM idle. If I wasn't afraid that the ECM assumes the IAC is working and increases fuel flow, I'd probably put a switch in line and flip it when the truck warms up.
From what I know, I don't see any reason why revving the truck would change the IAC setting. Unless maybe it uses the MAP sensor and TPS somehow in its calculations? Any ideas? Thanks and sorry about the long post.
