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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 11-18-2013, 10:45 PM
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bluebee bluebee is offline
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Is there any way to tell whether the piezoelectric knock sensors are activating?

It may be impossible to determine, but, I ask because it would be very useful if we could determine whether the piezoelectric knock sensors have activated in, say, the last FTP.

Is there any register that holds a value that we could read after the fact which would tell us whether the knock sensors have been activating?

If not, can we read an indirect value that implies the same thing?

(For example, how would we determine if the timing had been automatically retarded in the past FTP, due to signals from the knock sensors combined with other factors?)

See also:
- How to understand the piezoelectric knock sensor operation (1)
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  #2  
Old 11-20-2013, 03:19 PM
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bluebee bluebee is offline
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Drat. Do we take the lack of response as a "no"?

If we can't figure out whether the knock sensors have activated, can we at least measure a change in the so-called "fuel trim" (whatever that literally means)?

See also these quotes, taken from an octane thread, which shed some light on the issue of fuel trim adjustment due to octane rating changes:

Quote:
Originally Posted by Burning2nd View Post
almost no one knows that it takes 2.5 to 3 full tanks of fuel for the trim to change...

so running 91 then 87 then 91 does nothing but confuse your car..

you want to test it for real?
you run it for a month or 6 month period... not tank by tank.
Quote:
Originally Posted by Burning2nd View Post
Well First, you have to understand the way the Ignition system works..
Then you have to understand what Knock is.. and how the sensor works

Then you have to look at the O2 system
then the maf and or map..

Then the programing perameters of the EMS (engine managment system)
These bmw's are no different then any other car on the road... they just choose to do something a little differently

Its all tied together.. The computer has a set valve at which it can run with in. when it is out of that valve you get a engine light
as the car runs in defult it stops looking at those imputs (primary's) (HINT HINT HINT>> UNPLUGING YOUR MAF )

the same goes for the secondary system (fuel trim) It has a set peramaeter that it can run with in, And it will adjust by adding air and or fuel and advancing or retarding the timeing.. (NOT the physical time, the electronic timeing) To stay with in that tollerance....

(sorry thats a really quick getto exsplaintion


you run 94 octain all the time.. The car remembers those valves and goes back to it.. when you switch to 87... It still runs like its running 94... The knock sensor detects that difference and adjust's the trim.. But NOT immediately
Figure a good time frame is the same time it takes you to clear you code and be able to pass emisions... in that time frame.. (few hundred miles) the car will adjust its trim... and it will run the same damn way as it was running the 94


so to all these people who put one tank of 87 in and then cried about how poor it was... guess what.... your right, it did run "not as well" but your wrong in thinking it has anything to do with fuel..
you never gave the car the chance to adjust the trims to get back to the same values

NOW IF... AND i say IF.. we where having the conversation 20 years ago.. Id be completly wrong.. But technology has changed...


So now that you have withdrawn the info from me i must go back in to exile
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Last edited by bluebee; 11-21-2013 at 05:55 PM.
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  #3  
Old 06-26-2014, 09:54 AM
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bluebee bluebee is offline
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There was a good discussion of what the knock sensors are doing over here today, in the E46 forum:
> E46 (1999 - 2006) > Gas saving chips

Quote:
Originally Posted by crowz View Post
Umm actually I KNOW what its doing. You can to.

Take your obd scan tool and hook it up and get a live data connection.

Now go to the section in the live list showing timing.

Now go for a drive. Its easier if someone else is actually "driving" and your in the passenger seat monitoring it.

Get them to accelerate. Watch the timing. You will see it climb very fast then hit a wall so to speak. It will jump around a tad bit there. Your seeing the computer react to the knock sensor.

The part you need to change in thinking on this is that the timing never climbs much and will only hit the knock point under heavy loads. When in reality the advance point to cause knocking is hit most of the time at fairly low load amounts. Heres how it worked in the old days so maybe it will make more sense.

In the old days you set a base time when you first set the timing on a motor.

This may be 8 degrees or such on a toyota 18r motor for instance.

Now with that setting its fairly generic and usually wont cause pinging. Remember no computer here. Were talking pure hearable spark knock climbing hills and such. Sounds like the motors knocking real lightly.

Now this setting while ok doesnt doesnt cut it for a correct timing setting. What your interested in is actually TOTAL timing advance. To achieve this you measure timing at 2k rpm's on this particular motor. This reading will show you max advance. What your after is max advance without spark knock. Even doing the timing properly as mentioned you still have a good chance of the driver experiencing spark knock no matter what you do during the total timing adjustment. Since temperature etc can vary. But the goal is to get as much advance out of the motor as is possible while being driveable aka no spark knock. So 90% of the time you still had to pull a few degrees from the final adjustment.

Computerized cars have knock sensors. This allows the computer to adjust the timing very rapidly to hide the spark knock from you. The computers base timing is far higher than you would ever dream of trying on a carb motor.

The designers KNOW its going to spark knock but they dont care since they have it factored into the programming to go to max and back off. It remembers this number and works with it from there. Theres a table that stores these values on most cars programs.

The main point is no matter who you are or how you drive you WILL hit the knock sensor kicking in point everyime you drive you car. How often and how much of course depends on how hard you drive it and the weather in you location.

In modern cars octane raises mpg up till the point you reach what is required to keep it under the knock threshold.

Now the requried octane varies a bunch and how much octane also depends on what your using for base fuel too aka ethanol additives and such.

On my bmw I see a 7 mpg average change between 87 octane and 93 octane over a tank. But we have high temperatures with high humidity and its all hills here.
Quote:
Originally Posted by crowz View Post
Oh also if you want to really see whats happening get auto enginuity. It records all this data as you drive and you can play it back and watch everything and graph everything to see this in action.
Quote:
Originally Posted by crowz View Post
Few more things to throw into this to cover the octane stuff.

When someone asks what is the lowest octane I can run in my car people tell them to read the manual or look for a sticker showing minimum octane required. This of course answers their question and all but did you ever wonder WHY its there and why it requires x octane?

As I posted above the manufacturer runs a much higher base timing with the computer controlled cars. This requires them to make an assumption of minimum octane. If for instance you tune a car to require 91 octane fuel and then put 87 octane fuel in it the car will spark knock. The reason is the base timing is so high that the computer cant compensate for it. It cant pull enough timing to make up for the lower octane fuel. So thats the real reason for minimum octane requirements. The numbers are not there to tell you to run x octane fuel to get the best performance. They are there to tell you to run x octane fuel to avoid damaging the motor.

Yes the car will perform worse going under the recommended octane but the chance of motor damage is really the reason its there.

When doing custom tunes for my LT1 motor in my formula firebird I get my table numbers from doing live data captures which shows spark retarding in the graph it records. Its pretty wild how different fuels effect it.
See also:
- How long does it take a piezoelectric knock sensor to adjust the fuel trim based on the gasoline AKI octane rating of the fuel used (1) & how to better understand the piezoelectric knock sensor operation (1)
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Last edited by bluebee; 06-26-2014 at 10:06 AM.
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  #4  
Old 06-26-2014, 12:25 PM
edjack edjack is offline
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If the knock sensor fails, it will throw one of many specific codes. The DME continuously monitors the knock sensors for correct operation.

The DME controls ignition advance based on real-time inputs from the sensors. It acts within milliseconds to retard the spark.

The DME maintains optimal operation based on programmed fuel and ignition advance maps, and operating conditions. This, if you feed different fuel, it will compensate in real time.

http://en.wikipedia.org/wiki/Engine_knocking

http://en.wikipedia.org/wiki/Engine_control_unit

http://en.wikipedia.org/wiki/Obd-ii#OBD-II
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Last edited by edjack; 06-26-2014 at 12:30 PM.
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  #5  
Old 10-14-2014, 12:27 PM
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bluebee bluebee is offline
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The question came up again, over here today, of how long the timing is retarded when a momentary knock is felt:
> E39 (1997 - 2003) > Gas - 91 recommend [87/89/93] Available Only!

Some answers that came back were:
> E39 (1997 - 2003) > Gas - 91 recommend [87/89/93] Available Only!
Quote:
Originally Posted by MKJS View Post
How long does it take to adjust spark timing? I'm another that doesn't really know, but I'm thinking it could be on the order of > 50/sec.
Quote:
Originally Posted by Burning2nd View Post
it takes about .750 of a full tank for the computer to make the adjustments and run to closed loop
Quote:
Originally Posted by Burning2nd View Post
you have to do it with full tanks..

fill the tank twice with each octain, note the price per gallon, and the ammount consumed,

The car needs at least 1 tank to make the adjustment
so by the 2nd full tank of said octain you are trimmed

and you could do this with a dyno, but there has to be things that must be done first,

first the car must be in 100% OBD perfect condition, No check engine lights.. and car has to have made its (what is it) 200 mile trip b4 dyno.

first dyno run has to be on said octain (perfer a half tank status) (OF 2nd full tank of said octain)
2nd Dyno run has to be on other octain (also half tank status) (OF 2nd full tank of said octain)

you guys follow?

you can just fill it up with 2 gallons, dyno it.. run it out and then fill it with other,

it has to trim it self out, first...

Good project for you "got time" people
Quote:
Originally Posted by sxty8goats View Post
May actually feel a bit slower. Octane isn't a power boost. Octane is an ignition retardant. Engines that run hot (ours) and under high compression can actually cause the fuel air mixture to ignite as it compresses, before the spark of the spark plug. Pre-ignition detonation is engine knock.

High compression engines produce higher horse power. The trade off is you need a fuel that is harder to ignite. Some of this effect can be reduced via the ignition timing. So cars that have knock sensors and injectors can adjust the fuel / air ratio and timing if knock occurs. This is why lower octane fuels will feel like they have less power than high octane fuels in high compression modern engines. The car adjusts to compensate for the knock before you even realize it is knocking.

Octane boosters actually rob a bit of power from the fuel. It causes the fuel to burn slower and that causes a slightly lower 'explosion' event. Bumping the octane rating up more than necessary can have a negative effect on over all power.

During the hot summer months I run high test in my GTO. Once the average air temps drop below 50*F, I have no knock issues and I run 87.
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Last edited by bluebee; 10-15-2014 at 12:11 PM.
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  #6  
Old 01-26-2015, 05:33 AM
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sixpot_simon sixpot_simon is online now
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My experience is that my M54 knocks at WOT on RON91. This is strange, since the gas cap says 91 is within spec (the minimum recommended is 89). The knocking is sustained, so it appears that the knock sensors aren't effective at stopping it.

On RON95, there is no knocking. 91 is fine for pottering around, it only knocks under high load.

As a side note, due to the E39 sound insulation, I find it hard to hear the knocking unless the windows are open.

On second thoughts, my vanos seals are 90k miles old, maybe the problem is because the cam timing is incorrect. So I'll report back once I've done the Beisan seals replacement. Another possibility is early signs of a failing ignition coil (I've had one fail recently, maybe another is on the way out)

Regarding bluebee's question, indeed it would be really useful if we could see when the knock sensors are kicking in. Apparently it's possible to monitor the sensors' output using INPA: https://www.bmwland.co.uk/forums/vie...hp?f=5&t=93512

Last edited by sixpot_simon; 01-26-2015 at 05:55 AM.
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  #7  
Old 01-26-2015, 07:10 PM
spoddye39 spoddye39 is offline
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The best fuel economy and lowest emissions are obtained with the spark advance set to the point just prior to knock - this maximises the burn time. To achieve this the engine management system continually advances the timing until the knock sensor starts activating, then retards it a bit. In this way the spark timing hovers around the point just before knock. The knock sensor is quite sensitive and it 'hears' the onset of knocking before you do, so you should never hear it (unless its being caused by low octane fuel, which is completely different and out of the control of the engine management system). To answer the question then, the knock sensor should be going off all the time, except perhaps at idle.

Last edited by spoddye39; 01-26-2015 at 11:50 PM.
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  #8  
Old Today, 12:05 AM
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sixpot_simon sixpot_simon is online now
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Quote:
Originally Posted by spoddye39 View Post
the engine management system continually advances the timing until the knock sensor starts activating, then retards it a bit. In this way the spark timing hovers around the point just before knock. The knock sensor is quite sensitive and it 'hears' the onset of knocking before you do, so you should never hear it (unless its being caused by low octane fuel, which is completely different and out of the control of the engine management system). To answer the question then, the knock sensor should be going off all the time, except perhaps at idle.
My understanding is that the knock sensor is a failsafe system which just waits until a certain threshold is reached before kicking in, rather than a continuous optimisation of spark advance.

http://p38arover.com/rover/p38a/Engi...43_Systems.pdf
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