Whose ready for a troubleshoot a doozy

[3s1kill]

Having some issues. Thanks Ai for the notes write up on what I've tried. Because everyone in my family are mechanics but I decided to go into I.T. I sort of know what to check lol. But I'm still lost. Besides this below, I'm gonna borrow a code reader to see what the CEL is reporting.

Note it's been atleast -5 for two weeks.

I'm basically looking for a sanity check on what I've tried and what others have experienced.

Vehicle:
2006 Toyota 4Runner V8 (2UZ-FE)

Issue:
Cranks strong but will not start. No stumble, no cough.

⸻

How it started:

Ran fine, no issues. Parked it for a few days and Battery was drained after trunk light was left on. Battery is less than 2 years old. Charged overnight with a CTEK 5.0 (repair mode reached final stages). Engine cranks strong and consistently.

⸻

Symptoms
• Strong crank, no start
• No fuel pump noise (understand Toyota doesn’t always prime at key-on)
• Tachometer does NOT move while cranking
• CEL present at key-ON
• Immobilizer ruled out (tried both keys, normal dash behavior)

⸻

What’s been checked / replaced
• Battery: Confirmed charged, cranks strong
• Fuel pump relay: Swapped with known good relays (no change)
• Crankshaft Position Sensor: Replaced with new sensor
• Old and new sensors both test ~1.8kΩ (within spec)
• Sensor seating: Confirmed fully flush, not cocked, no missing O-ring
• ECU power: CEL present at key-ON
• Immobilizer: Ruled out

Had timing belt and other components replaced 20k miles ago by Toyota. Current mileage is 114k.

⸻

Key diagnostic findings
• Tach briefly moved earlier in diagnosis, then stopped completely
• No tach movement now = ECU not seeing crank signal
• Crank sensor itself tests GOOD (old and new)
• Harness-side ground test at CKP connector:
• One pin = OL (normal signal wire)
• Other pin = resistance values jumping around (NOT stable ground)
• Jumping resistance indicates bad ground path / broken conductor, likely internal wire break from cold + age

⸻

Environment
• Temps around -5°F for extended period
• Wiring extremely stiff / brittle
• CKP harness is very tight with minimal slack near the connector

⸻

Current theory (most likely cause)

Crankshaft Position Sensor ground wire failure in the engine harness, not the sensor itself.
Likely an internal wire break upstream in the loom that worsened when the harness was disturbed.

This explains:
• No tach signal
• No spark / no start
• Strong crank
• Sensor testing good
• Issue worsening after cold soak and movement

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Planned next steps
• Back-probe CKP ground from rear of connector (paperclip method) when warm
• Temporarily force ground to battery negative to confirm diagnosis
• If confirmed: repair ground wire / add dedicated ground or have shop repair harness properly

⸻

Looking for input on:
• Known CKP ground splice locations on 2UZ-FE
• Common harness failure points on 4th gen V8s
• Anyone seen similar cold-weather CKP ground failures

 

[Drcoffee]

Have you checked continuity on the fuel pump circuit? Did you check the cam position sensor too? Are you getting power to the relays in the fuse box for fuel and ignition? When you crank it do you smell fuel at the back of the truck exhaust? Need to find out if you have fuel and spark before tearing into the harness.

 

[Cezar]

Props for such a thorough post.

Wish I had anything to add but I will say on my v6 I had mice eat my crank position sensor wire and it presented the same way where it would crank but not start.

 

[Drcoffee]

Props for such a thorough post.

Wish I had anything to add but I will say on my v6 I had mice eat my crank position sensor wire and it presented the same way where it would crank but not start.

In cold climates, animals will look for warmth. Thats a good guess. -5F. Glad its not me.

 

[3s1kill]

Props for such a thorough post.

Wish I had anything to add but I will say on my v6 I had mice eat my crank position sensor wire and it presented the same way where it would crank but not start.

Thanks, the easier I make it for people to read the higher success rate of getting a response lol.

Also, I didn't think about a critter getting in there. I really hope that's not the case.

I did borrow a fancy code ready from my brother. Looks like a big ipad, hopefully I can get more information for everyone.

 

[3s1kill]

Have you checked continuity on the fuel pump circuit? Did you check the cam position sensor too? Are you getting power to the relays in the fuse box for fuel and ignition? When you crank it do you smell fuel at the back of the truck exhaust? Need to find out if you have fuel and spark before tearing into the harness.

I have not checked fuel pump or not even sure how to do that. I didn't move relays around in the fuse box under the hood and tried to start it. Didn't seem to do anything. I just swapped matching color relays with another and checked for burnt fuses.

When I crank it I don't recall smelling fuel but I'll check that next time I'm out.

I agree, I'm really trying to find the easiest things to test and replace before tearing stuff apart.

 

[3s1kill]

In cold climates, animals will look for warmth. Thats a good guess. -5F. Glad its not me.

Oh man I really hope not. I don't see any small critters often. Idk if I ever seen a mouse around here lol. We do have rabbits, like a lot of them but they're too big to fit between the skid plate

 

[rdruss]

Oh man I really hope not. I don't see any small critters often. Idk if I ever seen a mouse around here lol. We do have rabbits, like a lot of them but they're too big to fit between the skid plate

I believe there's a fuel Schrader valve on driver side above valve cover in a steel line for checking fuel pressure and then you could pull a plug and coil and check for spark, as was mentioned, you need these two for a start.

 

[3s1kill]

Got the code scanner from my brother and tested it. Here’s the results and stuff I replaced and tried. Still no start.


2006 4Runner V8 – No Start / Continuous Crank Update

Symptoms:
• Cranks with key
• Starter keeps cranking after key release
• Stops only when key turned to OFF
• No start since Dec 13
• Aftermarket remote start installed

Scan Tool: Autel MaxiSys Elite (full system scan)

Initial Codes:
• ABS: C1249 (Stop lamp switch circuit open), C1223
• Body ECU: B1241 (switch circuit diagnosis)
• Engine ECU: No codes

What was fixed:
• Replaced and adjusted brake light switch
• STOP/TAIL fuse verified good

Results:
• Brake lights now work normally
• ABS/VSC codes cleared and did NOT return
• Engine ECU still clean

Current Issue:
• Starter still cranks after key is released
• Body code B1241 remains

Conclusion so far:
• Brake switch fault was real and is fixed
• Engine sensors (crank/cam) ruled out
• Continuous cranking points to ignition switch electrical circuit or starter relay logic, not ABS or engine ECU

Next step:
• Battery disconnect overnight (BCM reset)
• If symptom remains, inspect/replace electrical portion of ignition switch

 

[Drcoffee]

Im cautious about AI but it keeps asking if the oil is full. LMAO

Its clear you have power, everything works, but engine wont fire.

Are you out of gas or frozen water in fuel line or bad fuel pump?
The ecm is not seeing the engine is turning, cam or crank sensor
No fuel present
No spark, may smell raw gas while cranking
Disconnected sensor plug somewhere or rat squirrel damage
No oil

 

[SocalSam]

"...Likely an internal wire break upstream in the loom that worsened when the harness was disturbed..."

Props for such a thorough post.

Wish I had anything to add but I will say on my v6 I had mice eat my crank position sensor wire and it presented the same way where it would crank but not start.

Mice ate through an injector wire close to the connector but luckily left just enough to fit a patch wire. When I was cleaning up the nest, I found a chewed wire underneath the engine bay fuse box. That patch was not so easy. Wire insulation were dried and crackling especially near the engine. Harness will send my T4R to the junkyard before the drive train gives up. Try grabbing the wire harness to the fuse box and shift it slightly while cranking. Yes, very shade tree,

As am owner of a 20 y.o. Mercedes, immobilizer system gives me great anxiety. Our cars have enough immobilizer to be troublesome. There is an ring on the ignition cylinder (I think) that excites the key chip which sends code to immobilizer module. Immobilizer code sends authorization to ECM and we have start. Authorization may not be present. If ECM has been swapped and not VIN programmed, there may be a limit on how many starts is allowed. I've read our system will brick after 10 starts. Immobilizer module is near upper vents and passenger airbag and requires dash removal. Please note, I have not looked at immobilizer in our cars.

I'm sure you have checked this. Failed injector should cut spark. There may be something, a fuse or broken wire that has cut signal to all injectors which has resulted in no spark.

 

[SocalSam]

****SORRY FOR WHAT APPEARS TO BE DOUBLE POST. EDIT BUTTON IS NOT WORKING.****

"...Likely an internal wire break upstream in the loom that worsened when the harness was disturbed..."

Props for such a thorough post.

Wish I had anything to add but I will say on my v6 I had mice eat my crank position sensor wire and it presented the same way where it would crank but not start.

Mice ate through an injector wire close to the connector but luckily left just enough to fit a patch wire. When I was cleaning up the nest, I found a chewed wire underneath the engine bay fuse box. That patch was not so easy. Wire insulation was dried and crackling especially near the engine. Harness will send my T4R to the junkyard before the drive train gives up. Try grabbing the wire harness to the fuse box and shift it slightly while cranking. Yes, very shade tree.

As am owner of a 20 y.o. Mercedes, immobilizer system gives me great anxiety. Our cars have enough immobilizer to be troublesome. There is an ring on the ignition cylinder (I think) that excites the key chip which sends code to immobilizer module. Immobilizer code sends authorization to ECM and we have start. Authorization may not be present. If ECM has been swapped and not VIN programmed, there may be a limit on how many starts is allowed. I've read our system will brick after 10 starts. Thanks to Car Car Nut, if the ECM is bad, mystery no start is a possibility. ECM capacitors that have a finite lifespan. Immobilizer module is near upper vents and passenger airbag and requires dash removal. Please note, I have not looked at immobilizer in our cars.

I'm sure you have checked this. Failed injector should cut spark. There may be something, a fuse or broken wire that has cut signal to all injectors which has resulted in no spark. A live data scanner should detect injector pulse and spark signal. Again, I have not dealt with this so on shaky ground here.

 

[MikeinNH67]

I assume that high end scanner is bidirectional, command the fuel pump on and listen for it. Also, try giving it a huff of something flammable up the intake to see if it will at least fire. You can also pull a CoP (coil on plug) to check for spark. You have to narrow it down. One thing to note, do not use any aftermarket parts for sensors as you will add possible problems. Toyota or Denso only.

 

[3s1kill]

It's been so cold I had to take a brake but we a few days in the teens.

Just to rule it out, all the spark plugs were changes with Denso Iridium 3297.

I also visually inspected every fuse in the engine bay and under the dash. None were burnt out.

Again, I swapped relays in the fuse box in the engine bay (like for like). Example Brown with another brown and blue for blue. I assume not every relay would die so swapping them to test if it would start.

Another just in case my fuel gauge wasn't busted or reading wrong, I added 4gal of gaa to it from a gas can.

Last thing which I thought was odd is last week it was -10 out and I tried to start it using Starter Fluid. Nothing, not even a cough. When it warmed up to the teens I went out to try again and it started for a few seconds and died. Weird, so I tried starter fluid and it actually coughed and ran for a few seconds and died.

Fast forward to today and were 15deg out. I try to start it a it fireams up to my surprise. I got out quickly to dance and then it died again. Roughly 3-5 maybe? So I get back in and now it won't start still.

Not sure if that means anything. To me it's very strange on why it starts briefly after just sitting there for several days. A part of me wants to think something legit frozen or frozen fuel line or something.

Someone tell me I'm bit crazy lol. So tomorrow I'm gonna hook up the scanner again and record the live data. Then try to start it for the first time and see if I capture anything useful.

 

[MikeinNH67]

Firing on starting fluid tells a ton. Compression and spark are there but no fuel. It's possible the computer is killing the fuel injectors but I would think it would also stop spark. I'd be going to the fuel pump next. Look at the video below, it will show how to get access to the pump harness so you can check for voltage and ground when it's commanded on. Heck, you can open the little door and try tapping on the pump to see if it comes alive. Icing is a possibility but not as common as it was 40 years ago.

https://www.youtube.com/watch?v=IBMWcjb9bro

 

[Zero0575]

If it starts with starting fluid, I would be checking for fuel pressure, or at least fuel flow. Hook a pressure gauge up or disconnect the fuel feed line and put it into a plastic bottle or something, then crank. See if fuel comes out.

Note: Our 4runners don't prime the fuel pump with ignition on, only when you crank the engine.

 

[3s1kill]

Firing on starting fluid tells a ton. Compression and spark are there but no fuel. It's possible the computer is killing the fuel injectors but I would think it would also stop spark. I'd be going to the fuel pump next. Look at the video below, it will show how to get access to the pump harness so you can check for voltage and ground when it's commanded on. Heck, you can open the little door and try tapping on the pump to see if it comes alive. Icing is a possibility but not as common as it was 40 years ago.

https://www.youtube.com/watch?v=IBMWcjb9bro

Appreciate it and I’ll reference the video and check when it warms up.

I’m getting codes for the MAF and IAT now lol. I think it has something to do with me unplugging the MAF and moving the wires out of the way to get to the spark plugs. I really hope the cold and me bending the harness didn’t cause any breaks.

I also tried this ecm reset thing. No dice.
https://www.youtube.com/watch?si=qUqPHrFgDBCNIH7v&v=ZHR0FYSluzU&feature=youtu.be


Imgur error codes from today.
https://imgur.com/gallery/2006-4runner-v8-codes-01-03-2026-7np8Imb