Possible false knock source

[BillC]

A friend tipped me off to a possible source of false knock. In the Feb 2003 issue of Chevy High Performance magazine (page 114, I think), in the questions & answers section at the back, the author mentions that their test engine didn't like the JE pistons they had just installed. Apparently, the JEs are short skirt (compared to the TRWs they pulled out), and they think the piston rock/slap was setting off the knock sensor. Unfortunately, they didn't say what model/version piston they were using.

Anyone hear of this before? Does this sound reasonable? I'm wondering if this might be the source of the false knock I've been chasing since the Nats. It would really suck if this was it, because fixing it would require another rebuild (and different pistons).

 

[leroy]

There must be some other people running JE pistons. I wonder if any of them have had problems?

I've been involved in quite a few projects chasing false knock. On one vehicle, we never found the problem, and did everything but change the pcm out. First you have to make sure it's false and there are several ways of determining that:

Run higher octane fuel and see if it goes away.

Bolt some copper tubing to the head, and hook a stethoscope to it. Drive down the road and listen. This is Barry's idea, and works great. You get lots of looks.

Hook a tape recorder or sound card to the knock sensor. Play it back and see what it sounds like.

Now let's say you have determined it is definitely false. Here are some solutions (I'm sure there are others.):

Switch out knock sensors.

Re-torque knock sensor. Stock torque is 14 ft-lbs. Loosening desensitizes it.

Use additional teflon tape on knock sensor threads.

Run shielded wire to the knock sensor. Let me know if you want more information on this.

Relocate the knock sensor. Let me know if you want more information on this also.

Replace memcal.

Replace ecm.

I'm sure you've been down most of these paths, Bill. Maybe this post will help others with similar problems.

Jim

 

[BillC]

I've already tried a lot of those things.

Run higher octane fuel and see if it goes away.

Tried, no difference.

Bolt some copper tubing to the head, and hook a stethoscope to it. Drive down the road and listen. This is Barry's idea, and works great. You get lots of looks.

Haven't tried this, but I have spent a lot of time with a mechanic's stethoscope, listening to the engine. Since I can make it false-knock by blipping the throttle, I can listen while making the false knock occur (confirmed by watching the knock counts increase on the scan tool). Never heard anything beyond normal engine noises, even when the computer thought it was knocking.

Hook a tape recorder or sound card to the knock sensor. Play it back and see what it sounds like.

I'm currently trying to borrow an oscilloscope, to take a look at the signal -- haven't heard back from my friend yet. Anyone have a functional one for sale, cheap?

Now let's say you have determined it is definitely false. Here are some solutions (I'm sure there are others.):

Switch out knock sensors.

I now have three knock sensors, one original and two new, each of which gives exactly the same indications.

Re-torque knock sensor. Stock torque is 14 ft-lbs. Loosening desensitizes it.

Tried, no difference. If I made it any looser, it would fall out.

Use additional teflon tape on knock sensor threads.

No difference with this, either.

Run shielded wire to the knock sensor. Let me know if you want more information on this.

Haven't done this yet, but I don't think its RFI, since the false knock happens at specific, repeatable conditions. I also checked the routing of the wire, and it's clear of all the major noise sources (ignition wires, coil, etc.)

Relocate the knock sensor. Let me know if you want more information on this also.

Tried moving the sensor to the front of the intake manifold, made the situation far, far worse (was virtually undrivable, from all the knock retard). Unfortunately, my L35 heads don't have the knock sensor holes, so I can't try it there. Maybe I'll have to drill some holes....

Replace memcal.

Tried this, no difference.

Replace ecm.

Don't think it's the ecm, because I'm also running Opie's knock light (which attaches directly to the memcal), and it matches the Datamaster data exactly.

A couple more pieces of data:

First, the false knock is temperature dependent. Above 90 degrees F, it doesn't do it at all. As the outside temp drops, it gets worse and worse. In the 80s, I get an occasional flash (from the knock light). In the 70s, it'll flash fairly regularly. Below that, it'll flash almost all the time while cruising on the highway. This kinda fits the piston hypothesis, because you would expect the piston clearance to go down as everything gets warmer.

Second, it only does it at part throttle, never while in boost. Doesn't matter if I just blip the throttle or if I'm cruising down the highway. In fact, if I'm steady-state cruising, I've had it flash for up to ten seconds at a time.

 

[leroy]

This sounds similar to a truck I worked on, except moving the knock sensor cured it. How did you mount the knock sensor when you moved it?

Jim

 

[BillC]

This sounds similar to a truck I worked on, except moving the knock sensor cured it. How did you mount the knock sensor when you moved it?

Jim

When you moved the sensor, where did you put it? How did you install it? Any special tricks?

When I tried moving the sensor, I made a 1/4NPT to 3/8-16 adapter, and screwed it into the bolt hole at the front of the intake manifold (on the passenger side, near the a/c bracket).

At the time, I just assumed the extra false knock was because my self-guiding roller rockers were noisy, since I've heard they have that reputation. However, I really didn't hear anything unusual with the stethoscope from the rockers. Been thinking about trying a Jesel shaft-mount setup, but that is serious $$$$ and I'm not sure it would do any good.

 

[leroy]

That sounds like the same bolt hole I used, so I guess I'm not much help again, Bill.

I'm not sure you can rule out RFI. Couldn't RFI cause specific, repeatable noise? This would also explain why moving the knock sensor made it worse. (You just added a longer antenna.)

The piston theory sounds reasonable also, except I think you could hear that noise with a stethoscope....maybe even without a stethoscope.

Jim

 

[BillC]

I'm not sure you can rule out RFI. Couldn't RFI cause specific, repeatable noise? This would also explain why moving the knock sensor made it worse. (You just added a longer antenna.)

However, if it were RFI, I would expect it to be more constant. For instance, if I'm cruising down the highway at a constant speed, the knock light will start flashing for a few seconds, then calm down, then at some random time later, start flashing again for a few seconds. If it were RFI, I would expect it to stay flashing once it started. Plus, the flashing goes away when it gets warm out.

The piston theory sounds reasonable also, except I think you could hear that noise with a stethoscope....maybe even without a stethoscope.

That's the really annoying thing about this -- I'm not hearing ANY noise that sounds even remotely like knock.

 

[UNIsackV6]

The piston theory sounds reasonable also, except I think you could hear that noise with a stethoscope....maybe even without a stethoscope.

That's the really annoying thing about this -- I'm not hearing ANY noise that sounds even remotely like knock.[/quote]

A few things a few I'm sure you've checked and one I haven't seen mentioned here yet.

1) I have heard of engine tie down chains causing false knock...

2) Bad engine mounts can cause false knock...

3) Cam walk? I see you have L35's what pressure springs do you have on the heads? The knock didn't start when you put the new heads on did it?

Food for thought, keep us posted....

Ryan
'92 Ty
SuperSlowCauseMarbleSaysSo

 

[ed hess]

without getting off topic here.

how did you go about extending the knock sensor wire to relocate. Is there a connector to be able to extend it. otherwise its gotta be a bitch to get in there to solder another wire onto it to extend it?

ed

 

[BillC]

A few things a few I'm sure you've checked and one I haven't seen mentioned here yet.

1) I have heard of engine tie down chains causing false knock...

Don't have a tie down chain anymore -- took it off when I installed the Poly motor mounts with Jeff's mount pads.

2) Bad engine mounts can cause false knock...

The mounts only have about 2000 miles on them (to the nats and back, plus some other driving around). I checked the mount bolts, and they all seem tight. Also, the mounts don't seem to be melted or otherwise damaged.

3) Cam walk? I see you have L35's what pressure springs do you have on the heads? The knock didn't start when you put the new heads on did it?

I've had these heads on for about 12k miles (when I installed the current motor). I've had a little bit of false knock ever since the new motor went in, but it was just the occasional hit, not nearly as bad as it is now.

The problem didn't really seem to get bad until after I replaced the factory hood with a carbon fiber cowl-induction hood. I've been wondering if maybe the distributor (factory original) might be temperature sensitive, since it would get most of the incoming cold air. So, I'm going to try installing a MSD billet unit (even if the dissy isn't the problem, it's a good excuse to get something I already wanted ).

One other piece of data inplicating the distributor: when I replaced the pickup coil a few weeks ago, I cleaned and lubricated the distributor shaft. The false knock seemed to be greatly reduced at first (for about 20 miles), but it came back after I did a WOT launch (surprised the crap out of my brother :lol: ).

how did you go about extending the knock sensor wire to relocate. Is there a connector to be able to extend it. otherwise its gotta be a bitch to get in there to solder another wire onto it to extend it?

When I extended my knock sensor wire, I had to cut it and solder in a new piece. You're right, it was a real pain getting in there to solder and to apply heat shrink.

 

[myclone]

Mind if I ask what brand/type of roller tip rockers youre using? The Comp cams roller tips are noisey IMO and I wont use them just for that reason.

Then again the dist being the culprit makes sense too.

 

[just a 6]

False knock

False knock

I wonder if we have the same problem that the 1G DSM's have, a lot of first generation DSM cars have false knock, and so far nobody have been able to cure it. My though is the location of the sensor, the Chevy V8's have the sensor mounted on the side of the block, so dos Saab Turbo's, all the DSM cars and Volvo Turbo's. The non turbo cars with OHC V6 have the sensor mounted in the galley between the heads. On our V6 block i think you can remove the block drain plug on the side of the block and the knock sensor will fit there, i thought of mounting my knock sensor there. Also on Volvo and Saab Turbo cars the wire from the knock sensor is shielded all the way to the contol module, what ever it is PCM or stand alone module (Saab 9000 turbo).

Saab have a stand alone system called APC (Automatic pinging control) I installed a system from a 9000 Turbo on my DSM (Eclipse GSX), it have 3 items: Control unit, knock sensor and 3-way valve, when the system sense knock it reduse the boost psi instead of pulling timing, and the control box is adjustable. So you can adjust the sensetivity of the knock sensor, i'm thinking of removing my stock knock sensor and use a Saab system instead of the GM factory.

I got my APC from a pick-&-pull junk yard for $25 , they had no clue what it was and charged me for a knock sensor :lol:
The hook up is three wires, ignition, ground and rpm. If you decide to get a system from the yard, make sure you get the whole wire harness due to that they are using shielded wires. Saab also have a adjustable fuel cut sensor, going by boost pressure. Only adjustable to 18psi but for people that running less then 18psi it's a very good safty thing. Basicly it's wired on the fuel pump feed and when you hit more boost then what ever boost it's set on, it cuts the power to the fuel pump. But as soon as the boost drops the pump comes back on. No more over boosting by misstake with one of those.

And it have to be a system from a 86 or newer Saab 900 Turbo or 9000 Turbo.

 

[BillC]

Mind if I ask what brand/type of roller tip rockers youre using? The Comp cams roller tips are noisey IMO and I wont use them just for that reason.

I'm using the Crane Gold self-aligning, full-roller, 1.6-ratio rockers. I have heard that all of the self-guiding roller rockers are noisy, but I didn't get the problem until well after I installed the rockers (which was about 8-9k miles ago). In fact, I didn't notice any difference in the amount of false knock when I replaced the Speed-Pro 1.5 non-guided roller rockers (that the machine shop installed) with the ones I'm currently using.

just a 6
-- I have heard that Saab has a new high-tech knock-detection system. Apparently, it measures the resistance across the spark plug gap every time the spark plug fires. From what I read, detonation changes the amount of ionization in the chamber, and this system reads the detonation directly (rather than listening for the accompanying noise). Do you know anything about this system? What vehicles is it on? How hard would it be to adapt to our trucks?

BTW, our ECM also reduces boost in response to knock. It just starts with ignition timing first. But, the boost reduction delay is adjustable in the chip, so you could have it reduce boost immediately if you wanted.

 

[just a 6]

I have to look in to that, but i suspect that it's built in the PCM, the old APC system was a seperate system very easy to adapt to other vehicles.



[quote="BillC
just a 6
-- I have heard that Saab has a new high-tech knock-detection system. Apparently, it measures the resistance across the spark plug gap every time the spark plug fires. From what I read, detonation changes the amount of ionization in the chamber, and this system reads the detonation directly (rather than listening for the accompanying noise). Do you know anything about this system? What vehicles is it on? How hard would it be to adapt to our trucks?

BTW, our ECM also reduces boost in response to knock. It just starts with ignition timing first. But, the boost reduction delay is adjustable in the chip, so you could have it reduce boost immediately if you wanted.[/quote]

 

[just a 6]

This is what i found sofar, as i suspected it's in the PCM. Very sofisticated:

Sophisticated powertrain performance. Highly sensitive and precise throttle control, adaptive torque delivery and smooth power on/power off characteristics from new Trionic 8 engine management, latest version of Saab's own 32-bit in-house system

[quote="Bill]

just a 6
-- I have heard that Saab has a new high-tech knock-detection system. Apparently, it measures the resistance across the spark plug gap every time the spark plug fires. From what I read, detonation changes the amount of ionization in the chamber, and this system reads the detonation directly (rather than listening for the accompanying noise). Do you know anything about this system? What vehicles is it on? How hard would it be to adapt to our trucks?

.[/quote]

 

[turbodig]

I've already tried a lot of those things.

First, the false knock is temperature dependent. Above 90 degrees F, it doesn't do it at all. As the outside temp drops, it gets worse and worse. In the 80s, I get an occasional flash (from the knock light). In the 70s, it'll flash fairly regularly. Below that, it'll flash almost all the time while cruising on the highway. This kinda fits the piston hypothesis, because you would expect the piston clearance to go down as everything gets warmer.
.

This is pretty much classic piston slap noise... thermally affected.

I'd try the laptop sound card thing, in lieu of a scope to hook up to it.
One thing, though, you'll have to attenuate the output of the knock line
somehow, either through a resistor from the knock line to ground, or
an input level control on the sound card. Once you've got it recorded,
you should be able to pick up the sound that's tripping the knock circuit.

As to fixing it, you'd have to either make a notch filter somehow, or try
to fix the noise source.

You might also try finding donor ESC filter modules from different
applications... maybe an LT1, or a 3.8 buick, or ?. Gotta still use the
SyTy calpak, but the ESC should tolerate a little fiddling.

Later,

Dig