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Old 11-04-2018, 02:50 AM   #1
blunted_brick
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Knock sensor voltages

I've searched on knock sensor voltages but haven't found much info specific to the EA388 gen 3. Does anyone know what the knock sensor voltage for a detonation event would be in the neighborhood of?

Attached a plot of knock sensor volts after oil temp reached 180 F.
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Old 11-04-2018, 02:20 PM   #2
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I don't think sensor volts necessarily equate to KR data. The sensor sends data to the ECU, and the ECU interprets the data to decide if it's detonation, or some other knock. I've found when trying to equate the 2 on past vehicles, there's never a pattern you can identify.
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Old 11-04-2018, 04:11 PM   #3
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I thought that sensor voltage correlated to KR but then slowly realized they did not connect to one another on my logs. I could have KR or a misfire and the sensor voltage wouldn't show any pattern.



The only pattern I noticed with the voltages is:
1-2.8krpm= .5-.8 v
3k - 4k = .8-1
And then 4k+ can go up as high as 1.5 v


I think it is just the power voltage that the sensor is getting. It naturally gets more as the car generates more electric current at the higher rpms. But this does not show what the sensor is actually reading. Which they are more like special microphones than voltage based/trip-based.
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Old 11-05-2018, 07:12 PM   #4
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As rpm climb it should climb to 1.5-1.7V. If you see a spike in the upwards curve then the sensor is getting higher engine noise and most likely detonation/knock is present
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Old 11-05-2018, 08:40 PM   #5
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Found a pic I had saved back in the days. See how the one cylinder outputs more.

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Old 11-05-2018, 11:28 PM   #6
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Quote:
Originally Posted by George Smooth View Post
Found a pic I had saved back in the days. See how the one cylinder outputs more.


So lets say that my cylinder 1 looks similar to those on a graph. How do we solve this? Is it gas? Bad sensor, etc?
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Old 11-06-2018, 12:59 AM   #7
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So lets say that my cylinder 1 looks similar to those on a graph. How do we solve this? Is it gas? Bad sensor, etc?

This is the $xk question. I've got a rough running car with choppy exhaust note and a vibration most evident at WOT and < 2krpm. It doesn't seem to be coils or plugs, so next suspects are dirty valves and injectors. Really just wanted to look at knock sensor voltages as a sanity check, but none of my voltages seem to be remarkably different from the others, so it's inconclusive. Next step is compression test, which is conclusive...


Edit: To clarify, my thinking was that a large knock sensor voltage offset or out-of-family behavior in one or more cylinders might indicate, say, piston slap or loss of compression. When the car is started cold, it sounds... loose

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Old 11-06-2018, 03:38 AM   #8
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Quote:
Originally Posted by blunted_brick View Post
This is the $xk question. I've got a rough running car with choppy exhaust note and a vibration most evident at WOT and < 2krpm. It doesn't seem to be coils or plugs, so next suspects are dirty valves and injectors. Really just wanted to look at knock sensor voltages as a sanity check, but none of my voltages seem to be remarkably different from the others, so it's inconclusive. Next step is compression test, which is conclusive...


Edit: To clarify, my thinking was that a large knock sensor voltage offset or out-of-family behavior in one or more cylinders might indicate, say, piston slap or loss of compression. When the car is started cold, it sounds... loose
Mine is experiencing something similar. Choppy exhaust and my knock sensors will trade going way other than the other three. However, I can't seem to solve it. I did a compression test and all four came up 190. Plugs are good. Coils are good. No clue.
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Old 11-07-2018, 12:22 AM   #9
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Quote:
Originally Posted by Twist View Post
Mine is experiencing something similar. Choppy exhaust and my knock sensors will trade going way other than the other three. However, I can't seem to solve it. I did a compression test and all four came up 190. Plugs are good. Coils are good. No clue.

Sounds like we are in the same boat. If it isn't spark or compression, it's gotta be dirty valves or bad injectors. Or a bent rod or some other bent reciprocating component...
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Old 11-07-2018, 12:35 AM   #10
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Quote:
Originally Posted by blunted_brick View Post
Sounds like we are in the same boat. If it isn't spark or compression, it's gotta be dirty valves or bad injectors. Or a bent rod or some other bent reciprocating component...
Wouldn't a bent rod cause lower compression in that cylinder?
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Old 11-07-2018, 01:42 AM   #11
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Wouldn't a bent rod cause lower compression in that cylinder?

Theoretically speaking, no, not necessarily. But a bent rod would in all probability cause vibrations in WOT conditions. Hopefully this is not the case, but if compression test and decarbing intake valves and replacing injectors don't do it...
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Old 11-07-2018, 11:27 PM   #12
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I've been testing this more and the car must have some massive knock . Not sure if this is from a bad batch of gas (I mix e20 on a 93 map to avoid this) but my cyl 1 and 3 voltages are sometimes as high as .5, .6 v over the other two with cyl 3 being the worst. I don't have Cobb so I can't see knock only timing retard. The car doesn't seem to retard much but it's advance isn't too great either.

Not sure what to try next other than switching to brisk er12s plugs for a bigger spark .
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Old 11-07-2018, 11:58 PM   #13
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Quote:
Originally Posted by Twist View Post
I've been testing this more and the car must have some massive knock . Not sure if this is from a bad batch of gas (I mix e20 on a 93 map to avoid this) but my cyl 1 and 3 voltages are sometimes as high as .5, .6 v over the other two with cyl 3 being the worst. I don't have Cobb so I can't see knock only timing retard. The car doesn't seem to retard much but it's advance isn't too great either.

Not sure what to try next other than switching to brisk er12s plugs for a bigger spark .

Twist, a couple questions:


1) What peak voltages are you seeing on your noisiest cylinders?
2) Do you see large differences between voltages at idle?


My situation changed this morning with APR DP and stage 2 s/w after compression test was reading 145 psi on each cylinder - which seems a bit low, but they were all consistent - so i blame a bias error on the pressure gauge (knocking wood)

Engine doesn't feel strangled any more but maybe the (much louder) exhaust note and broader torque band are drowning out unpleasant sensations. Will update once i've run a tank or two through - if it's still running rough, the next step will be decarb and injectors.
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Old 11-08-2018, 02:12 AM   #14
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Quote:
Originally Posted by blunted_brick View Post
Twist, a couple questions:


1) What peak voltages are you seeing on your noisiest cylinders?
2) Do you see large differences between voltages at idle?


My situation changed this morning with APR DP and stage 2 s/w after compression test was reading 145 psi on each cylinder - which seems a bit low, but they were all consistent - so i blame a bias error on the pressure gauge (knocking wood)

Engine doesn't feel strangled any more but maybe the (much louder) exhaust note and broader torque band are drowning out unpleasant sensations. Will update once i've run a tank or two through - if it's still running rough, the next step will be decarb and injectors.

At idle the worst one is still SLIGHTLY higher. They are all about the same at cruising speeds and then around 3k rpm is when the gap starts widening. Alot. And then by 3-4k rpm they can be a whole .5 V off. The highest I've seen is low 1.7s.
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Old 11-08-2018, 01:38 PM   #15
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Thanks for the info, will see if i can take some data over the weekend.
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Old 11-08-2018, 06:00 PM   #16
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Are we all sure that knock sensor voltage is the same thing as knock? I know George said it is but mine will switch cylinders and doesn't make any sense if it is real knock... Are we sure it's not just the current voltage passing through the sensor? Naturally it would go up when revs climb..
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Old 11-16-2018, 10:24 PM   #17
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Did about two dozen pulls earlier this week and learned:

1) no obvious correlation between knock sensor voltages and timing pulls
2) no obvious correlation between knock sensor voltages and misfires
3) no obvious correlation between timing pulls and misfires
4) no preferred cylinder for timing pulls at WOT
5) no preferred cylinder for misfires at WOT*

The final item gets an asterisk as my #2 seems to get many more misfires at low rpms when cold, when it sounds like knock city... the Mk6 didn't rattle like this, but also didn't pull like this! Hopefully the rough running can be solved with intake valve cleaning and replacing injectors at the same time. If that doesn't fix it, then i'll just have to live with it until it breaks.
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