Idle and EGR issues

[OpTorch]

SC400/LS400 PCM IDLE & EGR Control Workaround – Step-by-Step Guide (For Educational Purposes)

Author: Harrison Douglass | LifeAfterWideOpenThrottle on YouTube
Vehicle: 1991–2000 Lexus SC400 / LS400
Location: Santa Rosa, California (Vehicle has been in Northern California its entire life)
Topic: Interim Manual IDLE and EGR Control Fix for PCM (Powertrain Control Module) Failure

---

Important Disclaimer:

Before trying any workaround, you MUST properly diagnose your vehicle using an oscilloscope. The IDLE and EGR systems are stepper-motor driven circuits controlled by the Powertrain Control Module. Incorrect assumptions or skipped diagnostics can lead to wasted time, money, or safety issues. Always follow factory service information for accurate results.

---

Background:

This specific issue occurred on my personal Lexus SC400, which spent its entire life in Santa Rosa and Northern California. Many early Lexus SC400 and LS400 models suffer from common Powertrain Control Module failures, often caused by leaking or exploding capacitors inside the PCM. When this happens, the IDLE circuit, EGR circuit, or both may fail, leading to hard starting, high emissions, or stalling.

This guide details an interim manual control solution I personally used before purchasing a remanufactured PCM from Tanin Electronics (highly recommended for proper long-term repair).

---

What You’ll Need:

✅ 12-point single-phase rotary dial (remove limiter; Amazon sells them)
✅ Multimeter (for continuity/voltage checks)
✅ Oscilloscope (for proper diagnosis)
✅ Wire cutters/strippers
✅ Soldering tools or quality connectors
✅ Electrical tape or heat shrink tubing
✅ Wiring diagrams for SC400/LS400 IDLE and EGR circuits

---

Step-by-Step Manual IDLE and EGR Control Setup:

1. Remove Dial Limiter:
Modify your 12-point rotary dial so it spins 360° continuously, with no stops.

---

2. Identify IDLE & EGR Circuit Wires:
At your Powertrain Control Module connector:

• Locate the 6 wires controlling IDLE and EGR stepper motors:

  • 4 power control wires (two per motor half)

  • 2 ground wires

---

3. Map the Dial Connection Points:

• The dial has 12 steps; determine correct wire order for simulating stepper motor phases.

• This may require testing or trial/error.

---

4. Wire Grounds:

• Connect the 2 ground wires from the Powertrain Control Module to the dial’s ground terminal(s).

---

5. Wire Power Circuits:

• Connect the 4 power wires from the Powertrain Control Module to the dial in the proper sequence.

• You’ll create 3 full cycles of these 4 wires across the 12 dial points.

• (NOTE: Depending on your specific issues, you may need multiple dials – one for IDLE, one for EGR, or both.)

---

6. Secure Connections:

• Solder or use solid connectors.

• Insulate all wiring with heat shrink or electrical tape.

---

7. Test Dial Function:

• Rotate dial and verify proper wire energizing with multimeter.

• Confirm stepper motor simulation works as expected.

---

8. Install & Test on Vehicle:

• Reconnect Powertrain Control Module wiring.

• Start the engine.

• Adjust the dial manually to control the idle air control valve (IAC):

  • Clockwise = increases IDLE

  • Counterclockwise = decreases IDLE

You MUST manually adjust IDLE as the engine warms to maintain proper vacuum for the brake booster.

---

9. Monitor Performance:

• Ensure stable IDLE across temperatures.

• Watch for stalling or brake vacuum loss.

---

Critical Safety Reminder:

⚠️ Manual IDLE control is required anytime the vehicle is running.
⚠️ If you fail to lower the IDLE as the engine warms, brake vacuum will drop, causing poor brake performance and serious accident risk.

---

Final Notes & Recommendations:

✔️ This is a TEMPORARY workaround to get the vehicle running in emergencies.
✔️ For permanent repair, I highly recommend a remanufactured PCM from Tanin Electronics – they resolved my issue entirely with upgraded components.
✔️ These steps are based on my personal experience – not every SC400/LS400 issue will match mine.
✔️ Always diagnose thoroughly with an oscilloscope before attempting this fix.

---

Contact & Credits:
If anyone needs more help, feel free to reach out to me:

• Harrison Douglass | LifeAfterWideOpenThrottle (YouTube)

I want to help others keep these amazing Lexus V8 platforms on the road, either the right way or through reliable interim solutions like this.

[OpTorch]

Just now, OpTorch said:

SC400/LS400 PCM IDLE & EGR Control Workaround – Step-by-Step Guide (For Educational Purposes)

Author: Harrison Douglass | LifeAfterWideOpenThrottle on YouTube
Vehicle: 1991–2000 Lexus SC400 / LS400
Location: Santa Rosa, California (Vehicle has been in Northern California its entire life)
Topic: Interim Manual IDLE and EGR Control Fix for PCM (Powertrain Control Module) Failure

---

Important Disclaimer:

Before trying any workaround, you MUST properly diagnose your vehicle using an oscilloscope. The IDLE and EGR systems are stepper-motor driven circuits controlled by the Powertrain Control Module. Incorrect assumptions or skipped diagnostics can lead to wasted time, money, or safety issues. Always follow factory service information for accurate results.

---

Background:

This specific issue occurred on my personal Lexus SC400, which spent its entire life in Santa Rosa and Northern California. Many early Lexus SC400 and LS400 models suffer from common Powertrain Control Module failures, often caused by leaking or exploding capacitors inside the PCM. When this happens, the IDLE circuit, EGR circuit, or both may fail, leading to hard starting, high emissions, or stalling.

This guide details an interim manual control solution I personally used before purchasing a remanufactured PCM from Tanin Electronics (highly recommended for proper long-term repair).

---

What You’ll Need:

✅ 12-point single-phase rotary dial (remove limiter; Amazon sells them)
✅ Multimeter (for continuity/voltage checks)
✅ Oscilloscope (for proper diagnosis)
✅ Wire cutters/strippers
✅ Soldering tools or quality connectors
✅ Electrical tape or heat shrink tubing
✅ Wiring diagrams for SC400/LS400 IDLE and EGR circuits

---

Step-by-Step Manual IDLE and EGR Control Setup:

1. Remove Dial Limiter:
Modify your 12-point rotary dial so it spins 360° continuously, with no stops.

---

2. Identify IDLE & EGR Circuit Wires:
At your Powertrain Control Module connector:

• Locate the 6 wires controlling IDLE and EGR stepper motors:

  • 4 power control wires (two per motor half)

  • 2 ground wires

---

3. Map the Dial Connection Points:

• The dial has 12 steps; determine correct wire order for simulating stepper motor phases.

• This may require testing or trial/error.

---

4. Wire Grounds:

• Connect the 2 ground wires from the Powertrain Control Module to the dial’s ground terminal(s).

---

5. Wire Power Circuits:

• Connect the 4 power wires from the Powertrain Control Module to the dial in the proper sequence.

• You’ll create 3 full cycles of these 4 wires across the 12 dial points.

• (NOTE: Depending on your specific issues, you may need multiple dials – one for IDLE, one for EGR, or both.)

---

6. Secure Connections:

• Solder or use solid connectors.

• Insulate all wiring with heat shrink or electrical tape.

---

7. Test Dial Function:

• Rotate dial and verify proper wire energizing with multimeter.

• Confirm stepper motor simulation works as expected.

---

8. Install & Test on Vehicle:

• Reconnect Powertrain Control Module wiring.

• Start the engine.

• Adjust the dial manually to control the idle air control valve (IAC):

  • Clockwise = increases IDLE

  • Counterclockwise = decreases IDLE

You MUST manually adjust IDLE as the engine warms to maintain proper vacuum for the brake booster.

---

9. Monitor Performance:

• Ensure stable IDLE across temperatures.

• Watch for stalling or brake vacuum loss.

---

Critical Safety Reminder:

⚠️ Manual IDLE control is required anytime the vehicle is running.
⚠️ If you fail to lower the IDLE as the engine warms, brake vacuum will drop, causing poor brake performance and serious accident risk.

---

Final Notes & Recommendations:

✔️ This is a TEMPORARY workaround to get the vehicle running in emergencies.
✔️ For permanent repair, I highly recommend a remanufactured PCM from Tanin Electronics – they resolved my issue entirely with upgraded components.
✔️ These steps are based on my personal experience – not every SC400/LS400 issue will match mine.
✔️ Always diagnose thoroughly with an oscilloscope before attempting this fix.

---

Contact & Credits:
If anyone needs more help, feel free to reach out to me:

• Harrison Douglass | LifeAfterWideOpenThrottle (YouTube)

I want to help others keep these amazing Lexus V8 platforms on the road, either the right way or through reliable interim solutions like this.

I have personally had an accident with the interim fix, and i only recommend it as an interim fix for this reason. please use this as a tool. and not a permanent repair. most idle issues can be solved with this issue. unless there is an actual concern with your throttle body, idle air control valve, or a vacuum leak is present. However this fix is only recommended for people with a no idle? high idle condition at random times. if you vehicle is ALWAYS at high idle. PLEASE address any vacuum leaks or IAC(idle air control) valve issues first.

ALSO, as i pointed out. to verify if this fix is right for you, you MUST use an oscilloscope to verify that you IDLE/ EGR signal is not working as intended. Your signal should look like a digital signal, with squares present in the display presented to you. If you see fluctuating triangles of any kind on your scope, (when back-probing the PCM) you most likely have a PCM issue that is affecting your vehicle. Good luck to those who find this post. The best place to reach me is Instagram, @Harrison._.D

[OpTorch]

2 minutes ago, OpTorch said:

I have personally had an accident with the interim fix, and i only recommend it as an interim fix for this reason. Please use this as a tool. and not a permanent repair. Most idle issues can be solved with this issue. Unless there is an actual concern with your throttle body, idle air control valve, or a vacuum leak is present. However this fix is only recommended for people with a no idle, or high idle condition at random times. if your vehicle is ALWAYS at HIGH IDLE. PLEASE address any vacuum leaks or IAC(idle air control) valve issues first.

Random issues, with your idle or EGR. or a code 71 (On OBD1 Models, not aware of code designation on OBD2 models, however i do know the issue is present on those as well. (Due to faulty capacitors used by Toyota/Lexus in this era)

ALSO, as i pointed out. To verify if this fix is right for you, you MUST use an oscilloscope to verify that you IDLE/ EGR signal is not working as intended. Your signal should look like a digital signal (with DIGITAL SQUARES present in the display presented to you, AS 0 volts to X volts). If you see fluctuating triangles of any kind on your scope(Erratic signals, inconsistent, when back-probing the PCM) you most likely have a PCM issue that is affecting your vehicle. Good luck to those who find this post. The best place to reach me is Instagram, @Harrison._.D