3.5EB False Knock

[SALEEN961]

I've been dealing with false knock on cylinders #1 and #3 for a while now, and it has been an issue both stock and tuned. It's easiest to duplicate around 2000-3000RPM at 0.3-0.6 load, but I occasionally see it at WOT too. When it occurs, I see cylinders #1 and #3 pull 3-5° of spark while the other four cylinders are sitting at 0KR or adding timing. Removing 5°,10°, or 15° of spark makes no difference, increasing the DI blend doesn't help, 100oct fuel didn't help, and running E50 hasn't made any difference either.

I'm not seeing anything loose that would cause false knock, but I recently started hearing a rattling/grinding noise that coincidences with the false knock now that it has gotten slightly worse. The noise seems to be coming from the area around the passenger side catalytic converters and turbocharger, but it's very hard to pinpoint as the noise resonates through the exhaust system.

I pulled the cats out of my truck and ran a borescope through them, no damage or anything unusual was observed, the heat shields are tightly clamped, and there are no signs of anything rubbing or hitting that I've been able to find. Both turbos seem fine and have the same minor amount of play in the wastegates that you would expect to see.

After reassembling everything and making sure every nut and bolt is tight, the noise and knock are both still there. I tried adding additional clamps to all the heatshields and peening around the catalyst material on the passenger side to hold it in place more securely, but it didn't help. I'd really appreciate some advice from anyone else who has run into false knock on their 3.5EB, particularly if you found something unusual on the passenger side of your truck. I'm tempted to completely replace the cats just to rule them out. I really don't want to spend $2,300 on something that isn't a guaranteed fix, but I'm also tired of waiting for the problem to get worse.

2021 F150 3.5EB 4WD with only 19K miles.

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[Riddick]

I don't have first hand experience with the 3.5 but have played with several Coyotes over the years.

From the sounds of it all of your exhaust bolts are tight. Have you ensured you have enough clearance around all of your exhaust pipes? Sometimes under load when the engine flexes the exhaust can move and rub against the body of the vehicle. This creates a resonance in the exhaust causing false knock.

Here is a cool trick I learned a few years ago. Take a rubber mallet or your fist and tap the exhaust every foot or two and listen for rattles, start as close to the turbo as possible. If you have an issue it's generally going to be around a connection point. If anything is rattling that will also cause false knock.

I know you said you already checked all of your piping under the hood but double check and make sure it's not close to anything. If your intake pipe or charge pipes are rubbing against the frame or body of the truck you will see false knock.

Another suggestion would be to check the actual knock sensors. First make sure they're properly torqued and second make sure the connector is not resting against the block or anything on the engine, you want it clear from everything. I have seen knock sensors in an orientation where the actual plug was resting against the block.

As a last resort you could have your tuner desensitize your knock sensors. This is obviously risky, but as long as you have good fuel you should be good to go.

Best of luck.

 

[SALEEN961]

I don't have first hand experience with the 3.5 but have played with several Coyotes over the years.

From the sounds of it all of your exhaust bolts are tight. Have you ensured you have enough clearance around all of your exhaust pipes? Sometimes under load when the engine flexes the exhaust can move and rub against the body of the vehicle. This creates a resonance in the exhaust causing false knock.

The clearance around the exhaust pipes is a little tight in some areas, but I have verified that it isn't hitting anything. I checked for signs of rubbing and had my wife load the driveline while I watched everything from underneath.

Here is a cool trick I learned a few years ago. Take a rubber mallet or your fist and tap the exhaust every foot or two and listen for rattles, start as close to the turbo as possible. If you have an issue it's generally going to be around a connection point. If anything is rattling that will also cause false knock.

I tried this with rubber mallets, brass hammers, steel hammers, and blocks of wood; but I haven't been able to reproduce the noise by hitting the exhaust. So far the only things that reproduce the noise are driving my truck in the troublesome RPM and load ranges or snapping the throttle over 2500RPM.

Another suggestion would be to check the actual knock sensors. First make sure they're properly torqued and second make sure the connector is not resting against the block or anything on the engine, you want it clear from everything. I have seen knock sensors in an orientation where the actual plug was resting against the block.

Physically checking the knock sensors isn't a bad idea, prior to the knock retard getting worse and the noise becoming audible, doing that was towards the top of my list. The knock sensors are buried underneath the intake and all 4 fuel rails (4.3hr job), so getting to them isn't much fun, but I might have to go this route if I don't find anything elsewhere.

When I first got my truck there were a few extra nuts left laying around and making noise. I wouldn't be horribly surprised to find an extra nut or bolt laying next to a knock sensor or stuck out of sight behind a turbo or exhaust manifold. I half expected to find something from the assembly line laying inside my passenger side catalyst.

 

[HammaMan]

Have you tried giving it an exorcism? Chasing gremlins is no fun

 

[Samson16]

Are those turbine blades and a bypass valve?

 

[astro_fusion]

Maybe try a Mechanic stethoscope if you haven't? You might need some noise cancelling headphones too. Could help pin point the rattling/grinding noise that might be related to the false knock. I'd try and isolate the exhaust somehow, like a wedge or strap to preload it so it can't resonate as much just for testing. The exhaust on my truck is so noisy cooling down, there's got to be a mess of thermal expansion oddities.

 

[HammaMan]

Given it's on the same bank likely from the same sensor, perhaps its faulty? FOD is also a likely culprit as you've touched on, possibly even a loose fastener somewhere in the neighborhood allowing something to rattle.

I couldn't see tearing into the motor down to the sensor and not replacing it, but putting a wrench on every fastener near it seems to be a likely-required diagnostics approach. Tapping with a hammer is a given as you've done, another option is to try using the side of a variable speed sander to impart varying frequencies of vibration into components. That combined with a mech's stethoscope may help. Granted if the sensor itself is faulty you're not chasing anything.

Hell it could be eddy currents even causing false readings.

Ai generated, it's long but may lead to ideas....

1. Piezoelectric Element: At its core, the knock sensor contains a piezoelectric element. Piezoelectric materials generate an electric charge in response to mechanical stress, like vibration or pressure changes.
2. Detection of Vibration: When the engine operates, it produces various vibrations. Engine knock creates a distinct vibration frequency, different from normal engine operation. When this abnormal vibration occurs:
   • The piezoelectric element inside the sensor picks up these vibrations.
3. Signal Generation: The vibrations cause the piezoelectric material to generate a voltage signal. This signal corresponds to the intensity and frequency of the knock.
4. Signal Processing:
   • The generated signal is sent to the engine control unit (ECU).
   • The ECU processes this signal. Advanced algorithms help distinguish between normal engine noise and actual knocking.
5. ECU Response: Upon detecting a knock:
   • The ECU might adjust the ignition timing to retard it. Retarding the ignition timing can reduce or eliminate knocking by changing when the spark plug fires relative to the piston's position.
   • It might also enrich the air-fuel mixture or decrease boost pressure in turbocharged engines, as these actions can also mitigate knocking.
6. Feedback Loop: The system operates in a feedback loop. If the adjustments made by the ECU resolve the knock, the sensor will detect the absence of knock vibrations, and normal operation resumes. If knocking persists, further adjustments might be made.
7. Protection: By continuously monitoring for knock, the sensor helps protect the engine from potential damage caused by pre-ignition or detonation, which can lead to piston damage, ring land breakage, or even hole formation in the piston.

The knock sensor essentially acts like a microphone for the engine, tuned to listen specifically for the frequencies of knock. This allows for optimal engine performance while preventing harmful conditions that could lead to engine wear or failure.

1. Check for Mechanical Noise: Other mechanical issues might be mistaken for knocking. This includes loose components, like timing chains, or issues with the valvetrain. Ensure there are no other parts vibrating or making noise at frequencies similar to knock.
2. Examine Electrical Interference:
   • Inspect the wiring leading to the knock sensor for any signs of damage, wear, or poor connections. Electromagnetic interference (EMI) from nearby electrical components or poor grounding could be interpreted as knock.
3. Sensor Calibration and Sensitivity:
   • Sometimes, the knock sensor or the ECU's interpretation of its signals might be overly sensitive. Check if there's a possibility to recalibrate or update the ECU software to adjust the sensitivity or to better filter out false signals.
4. Environmental Factors:
   • As hinted by a post on X, components can behave differently at various temperatures. If the sensor or its circuitry is sensitive to temperature changes, this could lead to false detections. Investigate if temperature changes correlate with the false knock reports.
5. Sensor Placement and Mounting:
   • Ensure the knock sensor is mounted correctly. A sensor that's not properly torqued or is mounted on a part of the engine that vibrates more than expected might give false readings.
6. Use of Diagnostic Tools:
   • Employ an oscilloscope or a high-quality engine analyzer to capture the vibration or noise pattern. This can help in distinguishing between actual knock and other noises or electrical signals.
7. Testing with Known Good Sensor:
   • If possible, replace the sensor temporarily with a known good one to see if the problem persists. This can help rule out sensor failure.
8. Check for Software Issues:
   • Look into whether there have been any complaints or recalls related to the ECU software for your vehicle model causing false knock detection. Manufacturers sometimes release software updates to fix such issues.
9. Acoustic Confirmation:
   • Use mechanical listening tools like a stethoscope to listen for actual knocking sounds in the vicinity of the sensor. This can confirm if there's an acoustic reason for the sensor's behavior.
10. Review Engine Modifications:
    • If the vehicle has been modified, consider if any changes could affect the engine's vibration characteristics or the knock sensor's operation.
11. False Signal Analysis:
    • If accessible, analyze the signal waveform from the knock sensor. False positives might show a different pattern compared to genuine knock signals.

 

[SALEEN961]

Have you tried giving it an exorcism? Chasing gremlins is no fun

I'm completely willing to drive it into a lake or swimming pool if you can find one filled with holy water. This truck has been the most problematic vehicle I've ever owned by a large margin.

Maybe try a Mechanic stethoscope if you haven't? You might need some noise cancelling headphones too. Could help pin point the rattling/grinding noise that might be related to the false knock. I'd try and isolate the exhaust somehow, like a wedge or strap to preload it so it can't resonate as much just for testing. The exhaust on my truck is so noisy cooling down, there's got to be a mess of thermal expansion oddities.

I have all my skid plates removed and a stethoscope sitting on the passenger seat just waiting for the noise to get particularly loud, right now the noise is annoying inconsistent.

The loudest resonate frequencies I have with my exhaust are in the range of 53-63Hz. I have some tuned mass dampers on order from Germany that I'll be clamp to different parts of the exhaust system in an effort to minimize the resonate vibration. I'm interested to see how they will wind up working and how much the placement of each damper impacts the outcome.

Given it's on the same bank likely from the same sensor, perhaps its faulty? FOD is also a likely culprit as you've touched on, possibly even a loose fastener somewhere in the neighborhood allowing something to rattle.

I couldn't see tearing into the motor down to the sensor and not replacing it, but putting a wrench on every fastener near it seems to be a likely-required diagnostics approach. Tapping with a hammer is a given as you've done, another option is to try using the side of a variable speed sander to impart varying frequencies of vibration into components. That combined with a mech's stethoscope may help. Granted if the sensor itself is faulty you're not chasing anything.

I completely agree with the idea of going back together will all new parts. I was considering the possibility of a faulty knock sensor right up until the noise became audible, at that point I had to admit that the knock sensor has better hearing than I do.

Starting in 2021 the 3.5EB uses 4 knock sensors that are part of a single assembly, so if any single knock sensor fails, you get 4 new ones. I should probably check to see if any of my scan tools can identify if the issue is being reported by one specific knock sensor or multiple sensors. I still find it odd that only cylinders #1 and #3 and showing knock retard.

Using a variable speed sander on the exhaust is an excellent idea that never occurred to me. I may even have one already if I can find it, it's been at least 15 years and multiple houses since I last used it. It would be great if I could find the noise without repeatedly burning myself on the hot exhaust.

 

[SALEEN961]

Are those turbine blades and a bypass valve?

Yes with the catalytic converters removed, the turbine blades and wastegates on the turbochargers are easily viewed. Turbochargers don't always have internal wastegates, but most OEM turbochargers seem to use internal wastegates and not the easily replaced external wastegates that are popular in custom builds.

 

[HammaMan]

I've used RO sanders for all sorts of purposes. Makes a great ad hoc concrete vibratory tool for small pours especially to remove voids at form edges. Just let that sucker bounce off the form edge for void free edges

Use with a metal pipe to seat around rebar, etc...

 

[Samson16]

Yes with the catalytic converters removed, the turbine blades and wastegates on the turbochargers are easily viewed. Turbochargers don't always have internal wastegates, but most OEM turbochargers seem to use internal wastegates and not the easily replaced external wastegates that are popular in custom builds.

The little fan is so small! ?

 

[HammaMan]

The little fan is so small! ?

Turbine -- but yes, crazy to think these little things move enough air to generate up to 350hp each (turbine is exhaust side, compressor / impeller is the intake side)

Learn how to rebuild one here

 

[Babbage]

Sorry you have issues. How about a new knock sensor(s) I know in the old ford mod motors they are under the upper intake. That would be cheap/easy to try first.

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