Why Do I Have Engine Knock

[knowledge]

Hello I was wondering if someone could educate me as to the possible causes for the engine knock I am getting. I am not asking to be spoon fed the answer, just help in getting to the bottom of this without ruining the engine I just spent the last couple months rebuilding.

( Top engine refresh with slightly bigger cam and injectors as well as ported heads )

First of all I am throwing

[PCM] P0327 - Knock Sensor Circuit Low Voltage (Old) (History) (Current) (Immature)

But my understanding is that means its not sending any signal and should not be causing my PCM to register a false knock or even a legitimate knock.

The knock happens very briefly when I jump on and off the gas while its in neutral or when I was going up a fairly steep hill.
Also, the PCM is programmed to only retard the spark 6* max but yet when I was going up that hill my timing was 16* less than what it is at idle under no load. So somehow the PCM was retarding it another 10*? How could that be?

Also, I have about 26* spark advance at idle in park. Is that way too much. That's what the stock spark table called for at that RPM and MAP values but is that way off for a slightly bigger cam and injectors?

My limited understanding is the only causes of knock are spark timing is off, cylinder is overheating, bad gas or I am running lean.

Are there any other possible reasons.

Also being I very new to this. What can I do to ensure that the computer is not doing its best to "correct" my tuning issues and therefore not letting me see the true problem.

Any thoughts no matter how inconsequential they might seem, could be the key that gets me going in the right direction. So please don't be shy.

Thanks

[wyochimneysweep]

Burst knock maybe? U could try zeroing that table and see if it's there and maybe a log

[knowledge]

Hi wyochimneysweep,

Glad to see you!!

I am not sure which table you mean exactly. I can see three that probably need changed

Engine->Spark->Advance Main Spark Table

Engine->Spark->Advance Idle Spark Advance

Engine->Spark->Retard Min Spark Advance

It looks like at speeds of over 14mph the main spark table is disabled.

Well what is used to determine spark at that point. And why if its disabled at 14mph do we need a Idle spark advance for closed throttle.

Then there is the Min Spark advance table. Which I could alter to make sure the advance never gets below a certain advance as well.

My question is why when my max knock retard is 6*, did my spark advance drop to 10* at ~2400 RPM going up hill when it is ~24* at 1000 RPM in neutral. I had very little vacuum ( 92 kPA ) up the hill as opposed to 40 kPA at 1000 RPM in neutral. I can see where the one table dictates that it cannot go below 10* at that low of a vacuum.

But what in the world would make it think it needs to be any lower than 24* in the first place if the knock on retards it 6*. Lets just say even if I am knocking like crazy I wouldn't expect it to go any lower than 18*, so how the heck did it get down to 10*

Also, to note that I hooked up a vacuum gauge on my intake and it held a steady 16 inches of Hg at 2500 RPM in neutral. I did this to test to see if my cats were all clogged up. My understanding is as long as it holds steady there is a good chance I am not getting a ton of back pressure from backed up cats.



Is it possible I just need to alter the spark tables or do I have something more serious going on?

[planethax]

Engine-> Spark-> Retard-> Burt Knock
Disable/zero out the Hi/Lo tables and relog
Pcm will pull as much as 12 in Hi table and 14 in the Lo table

You will also want to fix your knock sensor (Pcm may be pulling more just because you have this error? )


refresh me, does this vehicle have 1 or 2 knock sensors?

[wyochimneysweep]

Should have only one I believe

[knowledge]

Hello planethax and wyochimneysweep

First of I really appreciate the input.

Second there are two knock sensors. One on each side. And the replacement and new pig tails should be on my porch as we speak but finding time to put in in is another story even though its not difficult I have lots of other distractions that need attended to first unfortunately.

Just so I can learn something. What is the purpose of disabling the burst knock table. What does it do in the first place and would that probably explain why I get knock when I jump on the gas and let off real quick in neutral.

Is this something I will permanently keep disabled or is it just to help me diagnose what the actual problem is?

Also, would someone mind briefly explaining to me what in the heck the cylAir Delta threshold is as well as what VSS means.

I am guessing it means vehicle speed something and the cylAir delta is a the change of air in the cylinder but change of what exactly.

Would the burst knock values supersede the maximum knock retard table or is it the other way around?

Are those "OR" conditions or are the "AND" conditions meaning do both need to hold true to enable or just any one of them. For example do I need to be going >7mph and RPM >1200 or does it only take one of those to be true to enable the burst knock retard.

Finally, this is more of a basic engine question but aside from cats being backed up ( working on knowing for sure if they are or are not ) what else would cause my PCM to think it has to retard the timing so much going up hill under load?

Thanks again for the help

[longboxdime]

Hi IATs will pull a lot of timing

[knowledge]

I checked and it does look like the IAT table will pull up to 5* depending on the temp however I don't think my IAT every gets too bad.

The only time I see it go over "room temperature" is when the engine was already run but is not actually running now. Then I have seen it go to like 113 or so but as soon as it gets air running through it again it goes back to room temp.

While I don't think that is my main problem it sure isn't helping and I will do my best to mitigate its effects.

But that brings out another question. Do certain tables take precedence over others or do the all effect the final outcome?

[wyochimneysweep]

Burst knock retard is under changing conditions the ECM predicts what it might see for knock and reduces it with the burst knock table. I haven't been on my laptop to view your files yet. Sorry.

[knowledge]

Hi wyochimneysweep,

No need to apologize. Ill still be here whenever you get a chance to check it out. Plus your help is more than worth the wait

I guess burst knock is the result of jumping on the gas and the computer is trying to momentarily compensate for the additional air getting into the cylinder before the fuel injectors can catch up. Basically compensating for the brief lean fuel mix by way of retarding the spark, right?

Playing devils advocate....

Would burst knock be a constant factor over 30 to 45 seconds of holding the throttle body at more or less the same position??

Also to note, based on what I see burst knock would have been disabled because my MAP sensor was reading > than 70kPa. Which I was at 91 or so kPa going up the hill.

Which brings up another question.

Do these ECMs literally sample all the sensors a bunch of times a second so that more or less the ECM only does its thing based on the most current sampling and there is little residual impact from previous readings.

For my example in particular. I certainly could have had burst knock retard working against me when starting up the hill but as soon as my MAP went > than 70kPa it would have been disabled. Or would it have continued because it already entered that mode and conditions never improve enough to take it out of burst knock?

[wyochimneysweep]

A lot have a time delay that over time reduces what the ECM did so u don't have a huge change all at once. Ramps timing back in after it retarded it.

[Gh0st]

I got that error on my GTO after I pinched the knock sensor harness when putting my stock LS6 intake back on. After I reseated the intake, the error went away. Check your harness to make sure it isn't broken or being pinched.

[knowledge]

Hmm well I see the knock retard decay table but what the numbers mean are rather vague. "The higher the number the faster the recovery." its is .150 out of 1.0 across the entire table

Also, in the brief amount of time I had, I took off the knock sensor connector from the knock sensor and plugged it into a brand new knock sensor and it is still throwing the same code. So barring the knock sensor needs to be installed to be grounded or something like that I am thinking that I broke the wire or something in the process of rebuilding the engine.

My next step is to readjust the valves and then next I guess start disabling the burst knock. Which I can do easily by disabling it on my ECM.

I am a little concerned with disabling safety features though being I have never had this engine running right. Am I at risk for destroying anything?

[wyochimneysweep]

You shouldn't really have a problem with no load or light loads on the engine. It's under high load that will do damage. I do think the knock sensors need to be grounded. I could be wrong through

[acehaag]

I had a broken wire in my knock sensor harness and it would pull a ton of timing any time I hit the throttle. Get the code fixed first then try to tune.

[knowledge]

Well Ill replace the pig tail first and then the sensor if it still is throwing the code. It would serve me right if that is what is pulling the timing because I decided to over look the DTC code yet again. its not like those codes are important or anything....


I still am trying to figure out why my vacuum dropped so much. Though I am not sure if the drop in vacuum caused the spark retard or the spark retard caused the drop in vacuum. But I should know more disabling the burst knock and then maybe upping the spark advance min table.

Not being disrespectful in any way but isn't that kinda just putting a bandaid on the symptoms and not treating the root cause. Or is it really as "simple" as getting my knock sensors to work and then as long as I am not getting knock I can advance the timing as much as I want, hopefully preventing me from ever going down to 10* and such low vacuum.

Damn sisters wedding. I can't wait start eliminating my issues and I have to go sit through that crap. As soon as I can I will implement the advice I received here and report back.

I am sure I will still post questions though as this consumes most of my waking thoughts...... so check back often please

[knowledge]

So one thing I noticed and I can't be sure this is even the case but my last time I ran the engine I didn't get any knock retard at all.

Granted I only ran it for a like a minute and in was in neutral as I just was checking my fuel pressure and knock sensor but I didn't get any knock while I jumped on and off the gas really quick. Which I know would make me register knock on a warm engine. It caught my eye that I had zero knock the whole run and intentionally tried to make it knock and it never did.

So my question is, did the knock not register because it has some crazy ass thing were it doesn't activate until the engine is warm or whatever.

Or is it because the ECM is making it run rich because it a cold start and therefore I am just running too lean??

Much rather think about this than pay attention to boring wedding....

[knowledge]

Ok so replacing the connector took care of the knock sensor DTC. Which I swear made it knock more. That is, until I disabled the burst knock correction. Which definitely helped out but it definitely wasn't the only thing going on.

I am still back to is it the spark retard causing the vacuum drop or vice versa.

Whats strange is when I start out, its rough but has enough power to go up a fairly steep hill without me needing to worry about whether I can make it to the top or not.

So why after I make it to the top of the steep hill, more or less coast down the backside of even steeper hill get to the bottom and begin up a slight but constant grade do I begin to notice power loss and even more so when going up a hill with even more grade.

The thing is both of the hills after coasting down the big hill were nothing compared to climbing the first hill right out of my driveway before I coasted down. But that's when I lost considerable power, vacuum and spark advance

[knowledge]

FYI when you see me go like 4 to 6 miles per hour before and after the "road test" that is me backing up my drive way and coasting down like from 0:00 to 5:15 and then from 8:15 until 12:45 trying to see what is causing what

**The road test is between 5:15 and 8:15 the rest is me messing around idling, revving it in neutral or backing up my driveway and coasting down

Otherwise from about 5:15 to 5:55 I am going up a fairly steep hill for about 125yards. Feels a little rough but not too bad.

Then From about 5:55 to 6:45 I am going ( coasting mostly ) down a pretty steep hill for about 300 yards. No problems noticed because its down hill

From 6:45 to 7:05 Blow through stop sign at bottom of big hill and I am beginning going up a slight but constant grade approx 150 yards. Notice it seems to begin to run rougher

From 7:05 to 7:25 continuing up slight but constant grade approx 50 yards. Almost kinda thinks it starts acting better until I blow through another stop sign and begin to go up the steeper grade.

From here until I reach my drive way it is running pretty damn bad to the point I am not 100% sure I am going to be able to make it into my driveway

From 7:25 to 7:55 I am going up a steeper constant grade approx 50 yards kinda flattening out for the next fifty yards then I am at the foot of the failrly steep hill I started this test run with.

After blowing through a stop sign at 7:55 continue up that hill 50 yards and in to driveway at about 8:15 where I back up and coast down a few times to see vacuum spark and knock relation

[knowledge]

Torrent of questions.....

Is loss of vacuum causing me to loss spark advance or vise versa?

Is the knock retard deterioration/decay too slow and the knocks just keep piling up?

Does it take that long for my cats to back up?

Why is my IAT temperature so high?

During the initial test run up and down my driveway before disabling burst knock it would knock as soon as I hit gas and let out clutch.
Could that mean knock isn't due to cat back pressure?

Why when ebrake is on and I let out clutch there is no knock but when I hit gas and let out clutch results it will knock right away?

Why do I have more power and it runs smoother up a steeper hill in the beginning than I have up a much less steep hill although its over a much longer distance?

Do I alter the spark min table to stay well above 15*

When I disable burst knock will it still show me if it is knocking or not and just not do anything about it?

Why rockers are noisier after doing test run but even the very warmed up didn't do make as much noise as before?

Any thoughts are welcome.