Low boost KR problem

[Lhorn]

Wondering if anyone can help me figure out why I get KR (up to about 4 degrees) on only about 10# boost. Basically a stock setup. This KR problem has plagued me from day 1. With boost limited to 10#, it's an intermittent problem. Going WOT from freeway speed I get blips on the knock guage at least half the time, actual KR readings on the SM about 40% of the time and sometimes get no KR (about 1/2 the time). I usually let up. Once it went from .9, 1.x, 3.x,back to zero. My o2 are 800's - 830's generally at WOT. My LM-1 however is reading pretty rich - generally AFR less than 10.5, at times dipping into the 9's.

SM numbers at idle:
AF 6
L8 50
bat 14.1
Int 128
BL 118
Clt 157
ATS 68
RPM 650
TPS 0.44
IAC 34
CC - cycles appropriately
MAL - 0

Some things I've tried and some observations:

This seems to be occuring independant of my octane - I recently ran full 100 octane Torco fuel, now a mix of that 100 + 91 octane; still knocked on 100 octane.

No difference with my stock OEM MAF and a T+ and LS1 MAF combo

Checked my coil with ohm meter - all readings between 11-12K

Fuel pressure rises at least 1 lb for every lb of boost.

Tried both a TT 5.6 (for stock injectors), stock chip and another chip for stock injectors

Tried 2 different ECM's

Fixed all exhaust leaks as far as I can tell. No header leaks at all

Fixed all vacuum leaks.

I think my motor mounts are stock but look great and engine doesn't rock much

Dp not hitting frame etc.

Knock sensor checked and torqued to only 14 ft/lb

New plugs/wires

Injectors cleaned

Car drives fine, has steady vacuum readings, and passes Ca smog easily.

Any thoughts would be greatly appreciated. Especially things to check or try, and an explaination on how Im getting knock yet my O2's are fine and my AFR on my WB are actually pretty low.

Thanks.

 

[Razor]

The knock is false. The question is whats causing it.

All your numbers look ok.

Stupid stuff like MAF hitting altermator, loose TQ converter bolts, broken transmission mount, etc.

Does the knock sensor go off if you rev it in idle. Like idle to 3500 rpm's?

Does it go off if you roll into the gas like from 60 to 80 mph?

Does it go off if while driving you apply brakes and gas at the same time.. like at 40 MPH spool the turbo keeping the speed at 40 MPH?

You need to figure out what makes it want to knock. If its strickly boost related check the turbo. If its RPM.. then its valvetrain.. if its speed.. then mounts.

Had a customer chase a knock issue for 6 months. His exhaust was banging on the rear axle.

 

[AZMotorgod]

Okay, time for some newbie advice (I am facinated by this post).

Razor-

How can you get knock from the exhaust hitting the rear axle, down pipe hitting frame, motor mount issue, etc...?

(or in other words- How can you get knock from one particular metal component making contact with another?)

Aren't metal components making contact with one another all the time?

 

[Lhorn]

The knock comes in whether I floor it or whether I more gently roll into it. It does seem boost related to me. I've only seen it happen at about 9-10 # boost or more. I've never seen it reving the engine in neutral, but haven't tried the other maneuvers you mentioned. Also forgot to check the torque convertor bolts. I'll try those.

 

[TurboDave]

The knock comes in whether I floor it or whether I more gently roll into it. It does seem boost related to me. I've only seen it happen at about 9-10 # boost or more. I've never seen it reving the engine in neutral, but haven't tried the other maneuvers you mentioned. Also forgot to check the torque convertor bolts. I'll try those.

As was stated, it's definately false knock since you still have the problem even when running high octane race gas.

Something is "hitting" something else and making noise. Sometimes it's the downpipe hitting the upper A arm. Or any one of a number of other things when the engine moves when you get into it.

The knock sensor will pick up any noise created by metal to metal contact when the engine "moves" because of torque over.

Your knock sensor could have also been over torqued, causing it to be overly sensative to ANY noise. 14 ft/lb is proper.

 

[AZMotorgod]

Aaah, that makes sense (thanks TurboDave).

 

[quikv6]

cam...

Do you have the stock cam? I had the same symptoms as you:
Low boost knock-started at about 8lbs of boost and rose progressively. I tried race gas...still low boost knock. I posted about it about 5 years ago. Everyone agreed it was false. I checked everything and everywhere looking for signs of contact...to no avail.
The car ran great when it wasn't in boost...super smooth, no noises, no ticking, great oil pressure, etc.
I gave up earlier this year and brought the car to Cotton's. Within hours, he figured out my stock cam was wiped. (#3 exhaust lobe...I think that's the typical spot).
Needless to say I wasn't happy about the cam, but I was glad that it was found. Now I'm not exactly sure how a wiped lobe actually causes knock...but it certainly did, and am grateful to the Cotton's crew for finding it, since I was trying to chase this seemingly "false" knock for years.
Got a new cam, and the car runs like a champ. 25psi...no knock.

 

[Razor]

Okay, time for some newbie advice (I am facinated by this post).

Razor-

How can you get knock from the exhaust hitting the rear axle, down pipe hitting frame, motor mount issue, etc...?

(or in other words- How can you get knock from one particular metal component making contact with another?)

Aren't metal components making contact with one another all the time?

Knock sensor listens for high frequency metalic sounds.. think of a tuning fork..PINGGGGGG.. Anything that bangs and produces that sound is picked up as knock. Like two pieces of metal contacting each other. Not rubbing aginst each other. So in the case of the tail pipe.. think of it like a hammer hitting the pipe.. the noise travels down the pipe... knock sensor picks it up..

 

[Razor]

Do you have the stock cam? I had the same symptoms as you:
Low boost knock-started at about 8lbs of boost and rose progressively. I tried race gas...still low boost knock. I posted about it about 5 years ago. Everyone agreed it was false. I checked everything and everywhere looking for signs of contact...to no avail.
The car ran great when it wasn't in boost...super smooth, no noises, no ticking, great oil pressure, etc.
I gave up earlier this year and brought the car to Cotton's. Within hours, he figured out my stock cam was wiped. (#3 exhaust lobe...I think that's the typical spot).
Needless to say I wasn't happy about the cam, but I was glad that it was found. Now I'm not exactly sure how a wiped lobe actually causes knock...but it certainly did, and am grateful to the Cotton's crew for finding it, since I was trying to chase this seemingly "false" knock for years.
Got a new cam, and the car runs like a champ. 25psi...no knock.

I've seen a wiped out cam also create this. But you can tell by doing a compression test on the motor.. and by doing a visual of how much each valve moves when the engine is spun over

 

[Lhorn]

Thanks for the replies. I'll do a compression test tonight. I have nightmares of wiped cam lobes. I've taken off the valve covers in the past and thought that the rockers seemed to move about the same. It's probably a bit hard to tell until the lobe goes pretty flat though.

Do you guys run the engine with the valve cover off (pretty messy but I've done it on my SBC), disconnect the power to the ECM and crank the engine with the starter or put a big ratchet on the alternator pulley and rotate by hand? Plugs out of course.

Perhaps getting ahead of myself, but if you find a wiped lobe is it still pretty safe to just put in a roller without a rebuild. The motor is pretty fresh stock rebuild.

 

[Lhorn]

Ok, did a compression test.
160-164 on all 6 cylinders.

Pulled both valve covers and turned engine over and feel that all rockers moved similarly. Even got out a caliper and grossly measured how far the rocker lifted (measured from the edge of the head) and they were all within a couple of millimeters.

Any other thoughts?

I'm gonna get under the car again and look for things hitting other things, but because it's been happening at a specific boost, I'm guessing it's not from that.

Any ideas why my o2 are decent, but my AFR as measured on the WB looks really rich at WOT (it's about 14.5-14.7 at idle)? Could the computer think it's lean although it's really not and be dumping in fuel? What could cause this? Exhaust leak upstream of the o2 sensor? I had the headers off very recently and they looked fine. I replaced my gasket (including the one under the turbo) with copper gaskets and as far as I can tell, no significant leaks.

I'm stumped.
Thanks

 

[Lhorn]

Checked torque converter bolts last night. They are tight.

Rev to 4000 rpm in park - no knock.

Checked under car for downpipe hitting frame, control arm, etc. Nothing looks even close to hitting and motor mounts don't look even a bit soft/saggy.

I'm running out of things to check.

 

[XLR8]

Going WOT from freeway speed I get blips on the knock guage at least half the time, actual KR readings on the SM about 40% of the time and sometimes get no KR (about 1/2 the time). I usually let up. Once it went from .9, 1.x, 3.x,back to zero. Thanks.

Does it knock when starting from 0? It could be the KR is coming when the transmission downsifts from OD to Drive.

 

[Lhorn]

Thanks for the response.
Most of my testing currently has been from a slow freeway roll. I cruise at about 50mph and either mash the gas or more gently accelerate to WOT. As I mentioned, I don't always get knock but when I do it can occur under either scenario. I know that I've also gotten knock when mashing the pedal from a standstill. Seems like it gets to full boost (currently limited to about 10#) or a couple of seconds, then I start to get blips on the knock gauge or SM. When I've stayed in it, sometimes it climbs a bit (like from 1 to 3 to 4 degrees) and sometimes it seems to go away.

I'll be honest most of the time I have a hard time telling when the transmission is shifting. Mine shifts pretty smoothly and because I'm looking at the SM while trying to keep it out of the ditch, I'm not sure I'd know if it was knocking on a shift. I have alway put it in OD, so maybe the first thing to do is put it in 3rd so it doesn't shift as much. From 50mph on the freeway, should I accelerate very gently so that it doesn't downshift and just stays in 3rd?

It's raining today so no testing unfortunately.
Thanks