Limp mode ? WTF…….

Been surfing YouTube, mostly reviewing how to test all the electronic stuff. There is a few good videos on how to test the MAF and others which is the next and last step if everything I’m currently working on doesn’t work.

But what the hell ? I’ve never heard or read anything like this since I’ve had my 4Runner……

3.slow gang talks about the knock sensor putting the 3VZE into….Limp Mode ????? If the sensor goes bad AND the only replacement that will work is $250 ? Or more dollars and it has to be from Toyota.

I’ve had my 1990 since 2014, up until last year never an issue.

While having my engine out I discovered that my knock sensor is mounted on the left side head, at least I’m guessing that’s what it is because in the block valley under the intake there is a sensor but no wires that go through the head to the sensor, it’s just plugging the hole.

So is any of what they say true ? I’m sure some of you know which video I’m talking about.

I’ll post a photo of mine later today, they mention something about S10 conversion ?

 

Back in 96 my 4runner went into limp mode when a sensor went out. Can't remember which one it was but if it was the knock sensor, then that would make sense.

 

I thought knock sensors had to be mounted to the block. Not sure how it can work on the side of the head. But not too knowledgeable on them either. 3rd gen's 3400 have them under the intake on the block and there are 2. They are expensive also...but I'd never trust anyone but TOYOTA for them as tearing everything off the top of the engine to replace them is a PITA.

 

Did you pull codes to verify it's a knock sensor? There's quite a few writeups on using a gm knock sensor, also relocating the knock sensor to the lifting eye on the motor.

https://www.yotatech.com/forums/f116/cheapest-knock-sensor-fix-code-52-a-254275/

 

I’m just kinda blown away that it can put the engine in “LIMP MODE”. I didn’t know going that far back that they could even do that ? I mean it’s 32 years old.

As far as my 4Runner where it’s located is how I got it, so the previous owner must have done the mod I’m guessing and I have what….40,000 miles on it. I highly doubt that’s an issue for me. But I’m also shocked that supposedly the only good replacement cost that much ! Holy Crap !

I was gone most of today but I’ll still get a picture and post it tomorrow just in case anyone is interested. And when we pulled codes on mine it was 25 I think, which is lean condition.

 

Ford introduced its Limp Mode when it went to SEFI (Sequential Electronic Fuel Injection) on its engines. First year for that on the Mustang 5.0 was 1986. Earlier I think for some premium car lines in the Lincoln marquee.

Other manufactures did it about the same time, usually earlier (Ford drags its feet). Dunno about TOYOTA.

So yeah, it was a thing before the 90's.

 

Yeah I guess so negusm, it just caught me off guard because I’d never heard of this before and here’s where mine is located.



It is mounted to the rear drivers side engine hook.

 

It is mounted to the rear drivers side engine hook.

Hi:
Mounting the knock sensor to some external part of the engine is a hack amateur way of trying to put out the check engine light.
The proper place for the sensor on the 3 liter V6 is in the valley of the block. It sits in a drilled and tapped boss on the block. There is no hole into the engines insides
It is electrically attached to the outside world with a short specialized cable that snakes up between the head, and both the lower, and upper intake manifolds.
It is this cable that is the usual failure point that will turn on the CEL with a code 52 on second gen 4 Runners.
The cable is a dealer only item ($17-$25) and is the cause of the problem in 95% of the time, the sensor is very robust, and rarely fails.
The cable is a PITA to replace, and is the reason why the hacker, too lazy to do the job correctly, installed another sensor on the hook.
The sensor on the hook can't do its job correctly, so whoever did the hack wasn't fixing anything, just putting his engine at risk.
Are you sure you saw code 25, and not 52?
A 52 will definitely put the car in limp mode. The ECU will pull timing out of the engine during a 52 to preserve the engine mechanically.
This is done to detect, and prevent pre ignition, which can be very destructive over a period of time to the engine.
If the code was definitely a 25, then your problem lies elsewhere than the knock sensor wherever it is located.

Good Luck
Art

 

Art,

Hell of a post man, you nailed it bigtime. So I towed my truck back to our mechanic because I was pretty sure I missed the timing somehow. Sure enough I was of 2 teeth on one cam. He also said the my MAF sensor was garbage, $270 new from China because at the time it’s all I could find.

He also said one of my injectors was wide open ? These are the factory injectors I had cleaned and flow matched @ LCE Engineering, so he fixed that too.

It just so happened he had another customer with a 1991 4Runner, no# 6 cylinder blown head gasket and water in the oil. So he borrowed that guys MAF and put it on my truck and it seemed to run fine. I ended up buying it from him for $100. If anyone wants a 1991 4x4 4Runner with a blown head gasket I know where it’s at, he wants $900 and has a clean title. 4Runner otherwise is pretty nice.

Anyway my check engine Light was still on, I asked why the light is still on if the MAF supposedly fixed my problem. He said we didn’t pull any codes because I still have a long way to go, whatever that means. So I paid him $895 dollars for the work done and brought the 4Runner home last night. I could tell something still isn’t right and that check engine light glowing in my face STILL !

I couldn’t sleep so got up, put my headlamp on @ 2:30 this morning, went out opened the hood with the intention of checking the timing, yesterday they timed it by ear, never put a light on it.

So antifreeze was all over the drivers side of the engine, they forgot to tighten the clamps on the throttle body cooling hoses…..nice ! Thank you.

My battery cables were completely loose, resistor missing from my EGR delete, plug wires not put back properly also. So I took care of that stuff then fired up the 4Runner, started right up, put the timing light on it then installed the jumper in the diagnoses ports and engaged the timing light.

The timing mark is still maybe an 1” and a half past the timing mark plate, it ends at 15 degrees, so that far above. I then go look at the dash and the check engine light is flashing…….1-2-3-4-5 pause 1-2. Code 52. You nailed Art !!!

Knock Sensor

That explains why it’s advancing the timing, well according to what I’m reading.

So why it worked where it is for so long is a mystery to me but my ECM is definitely picking it up now, what really is upsetting me for lack of stronger words…if our mechanic had to pull the plenum to fix an injector, had he read the code he was already 1/2 way into it and could have fixed the knock sensor while he was that far in, even if we had to order a new cable and waited a few more days.

Now I’m scared to drive it any further, it’s not moving again until I fix it myself. Which means tearing the top end off AGAIN ! I’m going to order the short cable from Yotashop, the one that goes from the block valley through the head, new intake and plenum gaskets and put this thing back the way it was supposed to be.

Trouble is my back is so bad, from all the wrenching already this is going to be really hard on me. And I need to look and see if there’s a way to test my knock sensor that’s sitting in the block now to make sure it’s good before I buy a replacement or put the engine back together.

Boy I’ll tell you this has been one of the toughest most expensive projects I’ve ever done. Can you tell I love my 4Runner.

That was a great post Art, I was having trouble figuring out how to respond. But it just fits right in now and you really nailed it. Thanks Man.

 

bthp2223,
The knock sensor has a piezo electric crystal inside it which will give off a pulse of voltage when the engine is knocking from the pre ignition of the combustion charge on any or all of the 6 given cylinders in the engine. That is why it needs to be in the center of the motors block so that it can sense any of the cylinders in the engine when they ping and create the tell tail mechanical thud which will cause the sensor to give off a pulse of voltage to the ECM. The ECM will advance the ignition until it gets a voltage signal form the sensor and then it backs off the ignitions advance to keep the advance as high as it can without destroying the motor by causing pre ignition. This is why the code 52 when indicated puts the ECM into limp mode because it cannot detect engine knocking either because the sensor is broken or more than likely the wiring is severed to the sensor. It is a single wire feed to the ECM but there is some shielding on the piece of wiring that goes from the sensor to the connector on the wire harness that goes to the injectors and TPS and all the other ones on the back of the 3VZ-E. So replacing this wire is a good idea given the age of it and the way it has to worm its way past the intake manifold and head to find passage to the sensor in the block.

Also I applaud the passion you have for your 4Runner. Many of us are right there with you on this love affair!

 

Thanks Parker, great description of the knock sensor and I might add it uses very specific parameters for this engine, that’s why just any old knock sensor will not work. As mentioned I don’t know how it worked for almost 7 years and passed smog the way it was but it did work. I posted a picture in my other thread, Wednesday the intake manifold went back on and today I’m going to try and get the injectors, fuel rail assembly all put back together, then it should be smooth sailing after that. My back can only take so much.

I hope by Sunday I’ve got her ready to fire up and will report back. And love, love love is in the air ! It’s springtime this has to work….praying I can tell you guys she’s done and purring again…..!

 

The Factory Knock Sensor and new connection wire worked like a charm. Gotta Thank Yotashop for having the correct part and the Fast Shipping.

Expensive it is but huge difference. You can’t even tell my 4Runner running @ idle it’s so smooth.