Wow... I didn't know the knock sensor doesn't run below 2K RPM. That may explain why the manual says to expect some knock at low engine speeds.

That's good to know, but may I ask where that information comes from?

I switched to 89 for the last few months and the knock went away. I may switch back to 87 soon. I'm also burning about 1qt/2500mi and trying to figure if it's all related.

Thanks!

 

What code does it throw if the knock sensor is doing what it's supposed to do, sensing a knock and retarding the timing while the ping is occuring?
The only knock sensor codes I've seen is circuit codes, for bad grounds, bad sensors, or wiring. I've never seen an "Your engine is pinging but the knock sensor took care of it" code. If it's doing what it's intended to do, why would it set a code? Unless you're saying the knock sensor isn't working, then the only way the PCM would know that is through it's circuit check and would be unaware of the ping.

 

According to the factory service manual, the engine computer monitors the knock voltage to see if it is above a predetermined knock threshold. If it is above that threshold, it reduces spark advance to quiet it down. If it continues to knock, it further reduces spark advance.

This is what the manual says about the computer's monitoring of the knock sensor:

• When Monitored: This monitor runs above 2000 rpm, under open throttle conditions. The Knock diagnostic does not run at idle or during decelerations. The high voltage test runs all the times the engine is running.

It is a little unclear as to what is monitored, and when.

 

I see, I thought you knew of a code that sets when it detects ping. I've been a tech for the last 15 years and have not seen one. I thought they might have changed something on the newer models. I've never seen a code when the spark is being retarded like it's supposed to be.
Most pings I've seen are caused by a lean mix, whether it be from a dirty MAF, which chrysler doesn't use, or a vacuum leak, or dirty injectors, or carbon build up. Overheating can cause a ping too, but that would be visible on the temp gauge.
Are you a tech, too?

 

It wouldn't set a code when detecting knock, as long as it can take care of it by retarding the ignition advance. My impression is it would set a code if it detects a knock voltage it can't fix.
When you say you're a tech, do you mean you have access to tech data beyond that which is available in the JK factory service manual?

 

I work in an independent shop, so we use AllData, which is just regurgitated Factory Service Manuals. Another source we use is IATN.net, which is a forum for technicians to ask for help from other techs. It's a great source to find out info about a problem you see but aren't sure what to check and see if someone else has seen that problem and what the fix was.
But no, there is no knock over threshold code that I've seen. If that were the case, every time you hear a ping, you'd get a code. The driver has no indication that the spark is being retarded if the system is working correctly. When the ping is real bad when you hear it, because then the PCM can't correct it.
So are you a tech, too? You do seem to have a technical background, so I'm thinking you do something of a technical mechanical nature. I read the forums quite a bit, but don't reply often, you always seem to have good answers.

 

I've been working on military and private aircraft since '72. Just retired from the military in '09.
Ever since my first car, back in '68, I have treated each one as a hobby, researching and working on them; and helping friends with their vehicles (I work for food).

I've been researching how the JK handles knock, but find the service manual a little vague in this area. Still looking for some specific data from Chrysler.

 

I see, you have the important job fixing stuff. If I screw up, someone can't make it to work, if you screw up, someone gets killed. I know a guy who is a mechanic for United Airlines, he comes into our shop. He and I got to talking, and I said I don't envy him working on planes, and he said he doesn't envy me working on automobiles and trucks. LOL, to me, Aircraft are far more precise and far less of a margin for error.
I can't tell you specifics on the JK's knock sensor system, but I could tell you about how most of them work.( Other Manufacturers).
Most of the sensors are piezoelectric and send a resonating voltage to the PCM, which then retards timing based on the frequency.
Some supply a ground to the reference voltage sent by the pcm, and when the sensor is affected by a ping, the ground is interrupted.
They are a lot less complicated then you'd think they are.

 

You know, I've been sitting here thinking about any vehicles that were Chryslers that I had to repair for a knock sensor problem, and I can't think of any. I remember they have two wires, whereas most are only single wire, but I can't say I've worked on that system. Usually, when you diagnose something, as you know, you have know how it works or you can't make an accurate diagnosis. So, I guess the fact that I don't know this means I haven't messed with them, meaning it's not prone to problems.
Nissans, I know their knock system pretty well. But that's usually a bad sensor or broken wire harness. LOL, doesn't help, I know.

 

If you have access to any OEM tech data that deals with any problems posted on the forum, don't hesitate to research it and post up what you learn, if you have the time.
If I have trouble finding specifics, I tend to take the item apart to see what makes it "tick".