7747 Won't Enter Closed Loop At Idle

**SBAdam** · 02-22-2021

I have read hundreds of posts on this forum and it has been really helpful, but I think I have a new problem...

Vehicle: '85 Toyota FJ60, 4-speed manual
Engine: 89 C1500 5.7L TBI, all stock except that AIR system has been removed
ECM: 7747 from the 89 C1500
BCC: ASDW, no modifications

Problem: Failed smog due to high CO at idle (0% CO at 2500rpm). I think my problem is that the ECM is not staying in Closed Loop at idle

What I have done:
1. Verified sensors and actuators (MAP, TPS, IAC, EGR, CTS & ground) are nominal
2. When I raise idle (either with pedal or adjustment screw) to >850rpm the ECM will enter closed loop. When I reduce rpm it drops back into open loop. Target idle in the bin is 600-650 depending on CTS. I think the reason it enters CL at higher rpm is due to the "RPM above which open loop idle is forced off" (825rpm).
3. Verified O2 sensor (singe wire installed in OEM manifold) is moving between 0.2-0.8mV in closed loop
4. Confirmed Park/Neutral switch input (B10 to ECM) is OPEN = Drive
5. Checked for vacuum leaks (not 100% sure there isn't a leak somewhere). MAP is seeing 20-24" at idle
6. Rebuilt throttle body
7. Verified fuel pressure (14psig)
8. Tried biasing the TPS to 1.5V at idle to try to force closed loop (juicing the "Maximum TPS for Open Loop Idle" parameter") (didn't work, does this ECM set 0% throttle at key on? I saw conflicting info)
9. Lots of other stuff that didn't make a difference
I read in one posting that "older TBI trucks didn't really enter CL at idle" but it's hard to figure out what that really means.
I am at the point where part of me wants to just drive the idle speed to 900rpm to force closed loop and try to pass smog, but... the engineer in me needs to figure out wtf is going on.
Thanks for any help, again this forum has been a huge help to me!
Adam

**tayto** · 02-22-2021

has o2 been replaced? i've replaced 1 wire with 3 wire o2s. 3 wire is heated and will get onto closed loop sooner. i also only use ac delco here. i assume you have a cat for emission purposes.

**SBAdam** · 02-22-2021

Thanks for the reply. I did replace the O2 sensor. I think the new sensor is Bosch (which I now know is not the preferred brand. I think the sensor is working, though, since I can force closed loop by increasing the rpm. Granted, that is my interpretation of what is happening and I am not an expert.
I am open to using a heated sensor, but have read that the single wire in the oem manifold position (like I have) should be good.
Yes, I have a brand new CARB certified catalytic converter that seems to work great when the vehicle is running in closed loop (passed the 2500rpm emissions test with flying colors).

**dave w** · 02-22-2021

Attached is the ASDW chip file.

Screen shot below shows there are several Open Loop and Closed Loop parameters.

Considerable trial and error tuning required to get closed loop working to pass emission testing.

dave w

7747 Open_Closed loop.jpg

**In-Tech** · 02-22-2021

Have a look at the ASDZ idle closed loop settings. There are many GM files that go open loop at idle and none with any mileage have a prayer of passing emissions until that is fixed. I fix this quite often here in California, Dave W has you on the correct path, keep in mind all variables have to be "set" for closed loop to enable and stay enabled. Always convert to a heated sensor. As for the junk ass AC-Delco o2 sensors, I have thrown plenty away and always use Bosch or NTK, so, each to his own.