knock sensor

[02 rado]

 

my check engine light has been on for a few months now and i finally had the chance to bring it to the dealer to have it checked. the knock sensor was bad and was replaced. very expensive job. what does the knock sensor do??? any info would really be cool. thanks!!!! 5

[mmmikkke]

The knock sensor tells the computer when detonation is occurring.

 

Detonation is the uneven burning of fuel. Normally, you want the flame to propogate smoothly across the combustion chamber starting at the spark plug.

 

Low octane fuels are less opaque to the combustion process, and so they are notorious for permitting spark knock (detonation) in vehicles designed before computer engine controls. They permitted spontaneous ignition elsewhere in the combustion chamber in advance of when the smoothly propogating flame would have arrived. The leading edge of the burn produces light photons, and lower octane (less opaque) fuels permitted these photons to travel through the unburned air/fuel mixture to the other end of the combustion chamber where they caused a 2nd burn of the mixture to begin. Result, is a knock, or what we call detonation.

 

Higher compression increases knock likelyhood also. As compression goes up, the inclination for the charge to burn unevenly increases, due to a higher density of photons at the leading edge of the burn. More photons -> more chance of a photon causing spontaneous ignition at the opposite end of the combustion chamber.

 

This uneven burning places higher loads on engine components that would otherwise be the case, and you hear a knock as the engine components experience the shock due to the irregular burn. So knocking is the enemy, because if knocking occurred all the time, your engine would sound like a diesel, and your rod bearings, pistons, etc., would wear out very quickly.

 

To maximize fuel efficiency, the computer tries to have your engine run just at the hairy edge where it's about to knock. When the computer senses a knock at the knock sensor, it retards the spark just a bit, or makes other engine adjustments, to keep you just at that hairy edge and not in detonation.

 

I'd guess that with your knock sensor out of commission, your computer made your engine run in safe mode, and it probably got poor fuel economy and didn't have much power. With it fixed, you probably have more power and get better fuel economy. Am I right?

[Indy]

The knock sensor tells the computer when detonation is occurring.

 

Detonation is the uneven burning of fuel.  Normally, you want the flame to propogate smoothly across the combustion chamber starting at the spark plug.

 

Low octane fuels are less opaque to the combustion process, and so they are notorious for permitting spark knock (detonation) in vehicles designed before computer engine controls.  They permitted spontaneous ignition elsewhere in the combustion chamber in advance of when the smoothly propogating flame would have arrived.  The leading edge of the burn produces light photons, and lower octane (less opaque) fuels permitted these photons to travel through the unburned air/fuel mixture to the other end of the combustion chamber where they caused a 2nd burn of the mixture to begin.  Result, is a knock, or what we call detonation.

 

Higher compression increases knock likelyhood also.  As compression goes up, the inclination for the charge to burn unevenly increases, due to a higher density of photons at the leading edge of the burn.  More photons -> more chance of a photon causing spontaneous ignition at the opposite end of the combustion chamber.

 

This uneven burning places higher loads on engine components that would otherwise be the case, and you hear a knock as the engine components experience the shock due to the irregular burn.  So knocking is the enemy, because if knocking occurred all the time, your engine would sound like a diesel, and your rod bearings, pistons, etc., would wear out very quickly.

 

To maximize fuel efficiency, the computer tries to have your engine run just at the hairy edge where it's about to knock.  When the computer senses a knock at the knock sensor, it retards the spark just a bit, or makes other engine adjustments, to keep you just at that hairy edge and not in detonation.

 

I'd guess that with your knock sensor out of commission, your computer made your engine run in safe mode, and it probably got poor fuel economy and didn't have much power.  With it fixed, you probably have more power and get better fuel economy.  Am I right?

 

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Excellent explanation. I'm impressed. I have a knock sensor code up also ever since I washed my engine and the connection corroded. I'm hoping I can just clean it up rather than replacing it.

[02 rado]

thanx guys awesome explination. i appreciate your time and expertise.

everything is working just fine now. i'll let you know about the fuel economy.

[Wingnut]

I'm not sure that your mileage will increase. My light has been on for a while now (same code) and my mileage hasn't gone down, nor have I expreienced a loss of power. (See my topic on pulling a really big boat)

 

I really need to get in there and replace it. I have the new sensor, and new intake gaskets ready to go...I'm just being lazy

[agoody68]

I'm not sure that your mileage will increase. My light has been on for a while now (same code) and my mileage hasn't gone down, nor have I expreienced a loss of power. (See my topic on pulling a really big boat)

 

I really need to get in there and replace it. I have the new sensor, and new intake gaskets ready to go...I'm just being lazy

 

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There ar actually 2 knock sensors. I'd replace both of them at the same time. That is what I did. Might save you from doing it again. Also there is a bulletin to put silicon around the rubber seal grommets to keep water out of the knock sensors. That is why they go bad, water gets down in there and rust them out. Don't ever power wash your engine.

[Rich Randall]

Hi guys, so I f I understand, I have to remove the intake manifold, is this right, I know I can't find this sensor. Please help me.

[Burrito]

About how much did it cost to have the knock sensor replaced?

 

After researching why I have spark knock, I think this its' my best bet to have the knock sensor replaced. I bought my truck used and the dealer power washed the engine. I most likely have a corroded knock sensor.

[Burrito]

Well I took my truck in and the shop is gonna install 2 new knock sensors for me. The bill will be $311. Hopefully that's the only thing causing the spark knock. I'll let yall know how it works out.

[Indy]

I washed my engine in August or September and shortly after that the knock sensor code came up. I was going to rip off the intake and see if I could clean up the problem. Before I got around to it the sensor dryed out or whatever and the light has not come back on and the truck is running great with no knock at all.

[Burrito]

I washed my engine in August or September and shortly after that the knock sensor code came up. I was going to rip off the intake and see if I could clean up the problem. Before I got around to it the sensor dryed out or whatever and the light has not come back on and the truck is running great with no knock at all.

 

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Consider yourself lucky because mine knocks so bad i sounds like a diesel. That's why it's getting fixed as I type this.

[Haulin' 8.1]

I got a truck in yesterday that sits up for weeks/months at a time, a 2003 GMC 1500. It had a knock sensor code so we pulled the intake off with plans to replace it. Turns out a rat had made the under intake, valley pan area it's nest and decided that two wires comeing out of the floor wasn't going to work. So it chewed them off all the way to the back of the intake. We have to make a new harness because you can't get a new one, only pig tail connectors. The worst part is the rat feces, those suckers are too lazy to go outside and take a dump. Oh and at one point a while back she brought it in and there was a dead frog under there, stunk like crazy. I guess the rat took it up there to eat later.

 

Well if you part it or store it, use moth balls.

 

Vernon

[Burrito]

I got both knock sensors replaced and the knock is gone. There is a slight knock whenever I'm real heavy on the pedal but the mechanic said it might take a little time for the computer to relearn everything. One of the sensors was fairly corroded and the other wasn't, but they replaced both since they already had everything cracked open. The job cost $310.