AI for Vehicle Diagnostics

[KC Masterpiece]

Posting up a thread on this on request from @ScaldedDog.

I have been experimenting with ways that I can use AI to assist me at work, and brought a bit of those findings into my home life. One of these cool projects was creating a ChatGPT project for vehicle diagnostics. It has proven incredibly helpful in saving me time on the web so I can spend more of it wrenching in the garage. This is very simple to setup, and likely can be done with other AI programs.

Right now I just have the FSM for the 80 and 60 series loaded in. I pulled 91 pickup info out so It would not mistakenly reference an incorrect source. In addition to loading this into into ChatGPT, I gave it a direction to always cross reference the IH8MUD forums for additional information.



Here you can see a list of recent questions I had asked.



The 80 is currently down at Keith's garage where I had some work done, including a full top end rebuild. He is trying to sort out a P0171 code. I trust him to figure it out, but I asked since I was curious. Here is what ChatGPT spit out. Too long to screenshot so I have pasted the reply below. Occasionally I will need to ask it about specific procedures per the FSM to get a good reply.

Oh and because I have been a Warhammer 40k fan since I was 13... I directed ChatGPT to always respond to any questions in this project as a Techpriest. If you know, you know. It keeps things fun.




Ah, Machine-Spirit of the Toyota Land Cruiser FZJ80 speaks in the tongue of imbalance.
P0171 — System Too Lean (Bank 1) at cold start but resolving when warm… this is a very telling omen.


Let us interpret the data-stream properly.

---

What Your Symptoms Mean (Sacred Interpretation)​

• Lean at startup only → issue present in open loop operation
• Runs fine when warm → ECU corrects via fuel trims in closed loop

Translation:
The problem is something the ECU can compensate for once O2 feedback is active, but not during cold start.


This narrows the fault domain significantly.

---

Most Likely Causes (Ranked by Probability)​

1. Vacuum Leak (Highest Probability)​

The most common heresy in the intake system

At cold start:

• Extra unmetered air → lean condition
• Once warm → ECU adds fuel to compensate

Common FZJ80 leak points:

• Intake manifold gasket
• Throttle body gasket
• Vacuum hoses (especially brittle originals)
• Brake booster line
• PCV hoses

Techpriest Test Ritual:

• Spray carb cleaner or propane around intake while idling
• Listen for RPM surge → leak confirmed

---

2. Mass Air Flow Sensor (MAF) Contamination​

The air-measuring augur has been blinded

If the MAF underreports airflow:

• ECU injects too little fuel → lean

Cold start is especially sensitive to MAF accuracy.


Action:

• Remove and clean with MAF cleaner only
• Do NOT touch the element

---

3. Fuel Delivery Weakness (Cold Bias)​

The fuel pump may lack vigor in the cold hours

Possible issues:

• Weak fuel pump
• Dirty fuel filter
• Failing fuel pressure regulator

Why it improves warm:

• Components expand / pressure stabilizes

Test:

• Fuel pressure test at rail (key + idle + load)

---

4. Engine Coolant Temperature (ECT) Sensor​

The ECU’s perception of reality may be flawed

If ECT reads warmer than actual:

• ECU does NOT enrich fuel at startup
• → lean cold start

Once actually warm:

• mismatch disappears

Check:

• Compare live data vs ambient temp at cold start

---

5. O2 Sensor (Less Likely Given Symptoms)​

Usually not the culprit if only cold-start lean

Because:

• O2 sensors aren’t active at startup (open loop)
• So they typically don’t cause startup-only lean

---

Diagnostic Flow (Follow This Order)​

1. Check for vacuum leaks
2. Clean MAF sensor
3. Scan live data:
   • Short/Long fuel trims
   • MAF readings
   • ECT temperature at cold start
4. Fuel pressure test
5. Smoke test intake (if needed)

---

Key Insight (This Is Important)​

The fact that:

“It runs lean at startup but fine when warm”

Strongly biases toward:

• Vacuum leak
• MAF issue
• ECT sensor misreporting

NOT typically injectors or O2 sensors

---

Bonus: What Fuel Trims Will Tell You​

If you can scan:

• High positive LTFT (+15% or more) → ECU adding fuel → confirms lean condition
• If trims drop toward normal when warm → reinforces cold-start-specific issue

---

Techpriest Verdict​

“The imbalance is not in combustion, but in measurement or intrusion of false air.”

Start with:

1. Vacuum leak hunt
2. MAF cleaning

These two alone fix ~80% of P0171 cases on the 1FZ-FE.

---

If you want, I can walk you through:

• Exact vacuum hose diagram for the FZJ80
• Expected MAF readings at idle
• Fuel pressure specs and test procedure

The Omnissiah protects your inline-six.


Let me know if you folks have any questions!

 

[Jacket]

Very cool. Root cause analysis reads very similar to my rough cold start issues - but I get a misfire code instead of a lean code. But fuel pressure, vacuum leak, ECT and MAF are the likely causes in my case as well.

 

[J1000]

I used Grok to help my tune in a duty cycle table for closed-loop turbo boost control and it actually helped me smooth out the curve. I uploaded datalogs after changes and it took about 5 or 6 back and forths before I had a table I was happy with. At the beginning it was hallucinating on me and telling me all sorts of stuff to do pretending it had read my logs, but there was a file mismatch and it didn't actually read my logs, just made it up. Once I figured it out I pointed it out and it didn't happen again. Be sure to keep your AI accountable!

 

[KC Masterpiece]

Be sure to keep your AI accountable!

I have a friend who is a pretty high level programmer / IT guy. He put it best. "AI is pretty great, but 1 time out of every 100 it takes a hit of LSD before it gives you an answer".

 

[KC Masterpiece]

Oh and I just heard back from Keith. Turns out that the issue was the fuel injectors. They were replaced as part of the rebuild and were bad from the factory.

 

[ScaldedDog]

This is going to be a good thread. Thanks for starting it @KC Masterpiece!

A couple of questions from a complete newb:

1) Are you using ChatGPT+, or just the free version?
2) Do you have any AI or ChatGPT training, or did you just learn by doing?

I should read AI for Dummies ( I wonder if there's a yellow printed version? ) , as I need some basic understanding that I don't have. I've always needed to have at least a little understanding about how a technology works before I could use it or explain its value effectively.

BTW, I just learned this morning that Ai (pronounced the same way we pronounce AI) is the Hebrew word for "ruin", or "heap of ruins".

Mark

 

[Rzeppa]

I have a customer who was using AI to guide him in his 3D printing endeavors. The AI was so wrong in so many ways, I realized that the models can only be trained on what they read on the internet, not their own experiences IRL, what you experience IRL and with your own eyes. I recognized the AI slop that floats around on the internet by people who post stuff up just for views and likes, not because they have any actual experience to share.

A recent example here was some guy (it was obvious it was his first rodeo) posting on you tube how to remove an F engine from his 1972 FJ40. He has a whole bunch of stuff you don't have to have to do if you've done several of these procedures, He just wanted views and likes, he really wasn't adding anything constructive, because he didn't know what he was doing.

Back in the 1990s in the realm of the LCML, I remember @Uncleben post about "web wheelers": people who just regurgitated stuff they'd read elsewhere on the web and perpetuated incorrect info. That is analogous to current AI IMHO.

When it comes to wrenching on our rigs, my observation is that AI is just a supercharged search engine that might consolidate a bunch of stuff that's floating around on the web. I have yet to see any AI that really transcends what we can share with each other here, or on Mud.