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Old 10-16-2022, 10:22 AM #1
BretMann BretMann is offline
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Is Knock sensor replacement really critical vs octane boost?

After just over a year of my truck sitting as I found the time to fix minor issues, it's all back together and running ok. However, I now get the P0325 code for the single knock sensor, which is an intake system-out replacement job PIA. The Hanes manual states that replacing the knock sensor is really important because detonation will destroy the engine. My understanding is yes, this is true but it depends on how bad the knock is and a little from time to time is nothing to worry about. The manual states that it's also common for my truck when stepping into the gas pedal in 1st gear, which I can confirm, especially with the A/C on and regular gasoline.

In my gentle test run now, I didn't hear it knock at all and I expect it to be a little rough at times as the computer re-educates itself. The check light is continually on, not blinking (the more severe level).

As I understand it, the knock sensor doesn't prevent knock, it listens for it and when it's detected it then regulates the spark timing - so some knock must happen anyway initially. Would adding an octane booster now to my 3/4 full tank of regular, then run on premium later, be a safe work-around until I have the time the replace the knock sensor?

Thanks.
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Old 10-16-2022, 02:07 PM #2
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Is there any chance that mice chewed your sensor wiring and built a nest on top of your transmission causing you to not be able to shift while your rig was down for over a year? How were you able to drive it if you cant shift gears? Is it stuck in one gear?
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Old 10-16-2022, 03:15 PM #3
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Although it was kept inside the garage, it is possible mice got to the knock sensor wire. I'll check on that today.

In the other post I said I was able to shift gears again after extending the clutch master plunger rod about 1/4".
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Old 10-16-2022, 04:51 PM #4
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The knock sensor harness takes a lot of heat cycling where it’s located and is usually what fails vs the sensor itself.

I don’t have a 3rz, but low rpm knocking isn’t normal and isn’t caused by a failed sensor because the sensor is ignored under 3k rpm anyway, you should be able to drive around on regular gas with the ac on and not have any knocking. Octane booster won’t do anything.

Are you running the old gas that was sitting in the tank while it wasn’t being driven?
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Old 10-16-2022, 05:53 PM #5
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At it's core, knock sensors are just microphones that send out an audio wave which the ECU reads like an oscilloscope; If the waveform is less than "perfect" - whether that be from knock itself, bad connection, faulty parts - then a code will be thrown. Believe me, it's VERY picky on how clean it is. Strangely enough too the knock sensors don't even start listening until like 3000-3500rpms, somewhere in that ballpark.
Running high octane gas in your non-interfering, low compression engine won't do a thing (unless you had a supercharged V6 - they have an issue with pinging, especially on the low end of the tach.) The only fix is to either replace the sensors or replace the harness, and since you have to get to the same point to do either one you might as well shoot for both. There's a couple redneck fixes floating around but I didn't find any of them helpful when my knock sensors crapped out.
The job is definitely a PITA on the 5VZ but I've never seen it done on a 3RZ, I can't imagine it would be worse.
Mind you everything I'm saying is in relation to the 5VZ, but I'm gonna take a wild shot in the dark and say the same knock sensors were used on both engines. In either case you should be able to find an example of a proper waveform in the FSM and hook your sensors up directly to an oscilloscope to compare them. But again, you're gonna have to take down the engine to get to this point anyways
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Old 10-17-2022, 12:50 PM #6
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3rz knock sensor is right below the intake manifold. Removing the intake is a 5VZ thing, where it is deep in the V of the motor.

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Old 10-17-2022, 04:05 PM #7
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It should be a significantly easier job with the 3RZ as opposed to the 5VZ.

That being said, I realize the effect of time and money restraints and not being able to do the job instantly. As a failsafe the ECU will pull all timing advance so the risk of pinging is basically 0. It also means the engine will be underperforming and drive sluggish until the code is cleared. Putting in an octane booster won't hurt anything but neither will using the gas you've already got in there.
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