LINK TO THE VIDEO

Hey everyone, sorry for the long read but please God read it and help me!!

I've been dealing with a persistent and frustrating issue on my truck for years, and it's seriously driving me crazy. As a mechanic, it really eats at me that I haven’t been able to solve it. I'm absolutely racking my brain here, and I'm hoping someone can offer some insight I haven’t considered.

The Problem:

My truck has a terrible knock at idle that’s so harsh it literally shakes the inside of the cab. It’s loud and clearly audible, especially in person. The knock doesn’t seem to increase with RPM—it’s primarily noticeable at idle—but it does affect the exhaust note, making it sound choppy (you can hear this clearly in the video I’ll link).

What Makes It Go Away (Temporarily):

Here’s the strange part:

• If I unplug the VP44 connector and supply 12V power directly to the pump, bypassing the ECU entirely, the knock disappears completely. This made me suspect it’s related to ECU inputs or sensor feedback.

• Using my scan tool, I can disable ANY injector and then re-enable it. The knock then goes away for about 10 seconds before coming back. During those 10 seconds, the engine runs smooth as butter—no shake, clean exhaust note.

Watch the video and you’ll see what I mean. When I cycle an injector, the knock vanishes for a moment and returns sharply—you’ll especially hear it in the exhaust.

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Parts I've Replaced / Things I've Checked:

Here’s what I’ve done so far (this issue predates many of these replacements):

• Replaced VP44 with a known good used unit from a running truck

• Replaced crank sensor and verified wiring from ECU to sensor

• Replaced MAP sensor and verified wiring to ECU

• Installed brand-new injectors (again, issue existed before this)

• New alternator (unrelated, but noting for completeness)

• Installed AirDog Raptor and Beans Diesel sump – fuel pressure is spot on

• Swapped ECUs from a known good truck – no change

• Verified APPS wiring and voltage – originally low (~0.34V), adjusted to 0.5V as per MoparMan's advice

• Checked IAT sensor and verified voltage/wiring to ECU

• Physically checked the timing marks on the timing gears - all good there.

None of these changes have made a dent in the issue.

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Final Thoughts:

This truck is driving me insane. I know the signs are pointing to some sort of input/communication issue, but I’ve tested and replaced so much already that I’m at a loss.

If anyone out there has run into something similar or has suggestions for what to check next, please let me know. I’m open to trying anything at this point.

Thanks in advance!

Note:

I did find a guy on the 2nd gen Facebook page that had the exact same knocking sound and his issue was resolved by replacing the crank sensor. I've done this already, but he did say he replaced his with a cheap amazon one and the issue wasn't fixed, but he tried a name brand one from the part store and it went away.. I'm hesitant to try a new sensor again, but I will buy the Cummins one if y'all think this would fix it!

35 minutes ago, Sufarry said:

I did find a guy on the 2nd gen Facebook page that had the exact same knocking sound and his issue was resolved by replacing the crank sensor. I've done this already, but he did say he replaced his with a cheap amazon one and the issue wasn't fixed, but he tried a name brand one from the part store and it went away.. I'm hesitant to try a new sensor again, but I will buy the Cummins one if y'all think this would fix it!

First, I must commend you for a well written description of the symptoms of a problem that you are trying to resolve. People rarely give that attention to detail when describing a symptom or communicating the order of what they have done to resolve the issue. Well done!

So, it is interesting that the noise goes away completely with the VP44 running on its own (no ECM communication) and then when the ECM is back in operation, the delayed return of the sound after you inhibit any single cylinder.

I don't have the answer, but the fellow you referred to in the above quote may be on to something here. The VP44 has an internal encoder that replicates and compares the crank / cam sensor signal, which could explain why the noise is gone when the ECM is out of the loop (the VP44 no longer needs the crank / cam sensor). And, maybe, when you inhibit a cylinder there is a delay in the knock returning possibly because the timing shifted from the internal encoder to the crank / cam sensor? Just speculating here.

Another very good possibility is that if your truck uses a crank sensor, the ring on the flywheel that triggers the sensor may be loose, cracked, or partly missing.

• John

8 hours ago, Tractorman said:

First, I must commend you for a well written description of the symptoms of a problem that you are trying to resolve. People rarely give that attention to detail when describing a symptom or communicating the order of what they have done to resolve the issue. Well done!

So, it is interesting that the noise goes away completely with the VP44 running on its own (no ECM communication) and then when the ECM is back in operation, the delayed return of the sound after you inhibit any single cylinder.

I don't have the answer, but the fellow you referred to in the above quote may be on to something here. The VP44 has an internal encoder that replicates and compares the crank / cam sensor signal, which could explain why the noise is gone when the ECM is out of the loop (the VP44 no longer needs the crank / cam sensor). And, maybe, when you inhibit a cylinder there is a delay in the knock returning possibly because the timing shifted from the internal encoder to the crank / cam sensor? Just speculating here.

Another very good possibility is that if your truck uses a crank sensor, the ring on the flywheel that triggers the sensor may be loose, cracked, or partly missing.

• John

Hey! Thank you for the kind words and thank you for replying! I believe you're right. I'm sure disabling the injectors causes it to temporarily not read from sensor data, therefore causing the knocking to go away.

I did forget to add that I did check the tone ring - partially. I removed the crank sensor 2 days ago to inspect it again. I took a photo inside of the crank sensor hole and tried to budge it with a flat head. No movement. What I DIDN'T do is rotate the crank around to verify each tooth. I suppose I could do that this evening and see. I would like to think if it was missing a tooth that there would be a CEL for CAM/Crank sync, but who knows!

When I get home I'll whip out the inspection camera and see what I see!

Thank you again!

@Sufarry It was a great conversation on the phone. Sorry, I've been missing in action for a while between the landslide, clearing @Tweety Bird place out, and moving here, and then my bladder cancer.

My notes on this after this hour-long phone call. It is not mechanical that we can find it's a bad signal being created by the ECM and feeding the VP44 the bad information, which makes it knock. I know he is waiting for a Cummins crank sensor to arrive. I've got a few ideas, but I just have to wait for the sensor to arrive for the owner to reinstall the starter and give it a try.

46 minutes ago, Mopar1973Man said:

@Sufarry It was a great conversation on the phone. Sorry, I've been missing in action for a while between the landslide, clearing @Tweety Bird place out, and moving here, and then my bladder cancer.

My notes on this after this hour-long phone call. It is not mechanical that we can find it's a bad signal being created by the ECM and feeding the VP44 the bad information, which makes it knock. I know he is waiting for a Cummins crank sensor to arrive. I've got a few ideas, but I just have to wait for the sensor to arrive for the owner to reinstall the starter and give it a try.

I appreciate the phone call as well. Good to chat with someone that actually knows what they're talking about! I hope to update this next week with a solution! This has been an ongoing thing for years now!

UPDATE: I ordered a Cummins OEM Crankshaft Position Sensor. I got it installed today. Unfortunately no change. The truck is still behaving the exact same way. So I guess my journey to figure this out continues. I am starting to lose hope again.

I think my next step is to buy the Quadzilla tuner again (I had it on the truck before, but it died on me and was out of warranty so I never bought another one) at the recommendation of MoparMan. He says the quadzilla has the ability to data log actual timing and compare it to commanded timing. I'd like to see what changes when I end the kill test (causing the knock to go away). Maybe that will help point me in a new direction.

Correction. It only reports commanded timing only.

15 hours ago, Sufarry said:

So I guess my journey to figure this out continues. I am starting to lose hope again.

This is definitely a most unusual problem. I have listened to your video many times. The sound seems mechanical to me, but I can't think of any mechanical issue that would behave the way it does when you inhibit a cylinder and then reactivate the cylinder, or that the sound would go away when the ECM is removed from the loop. Also, the sound is rythmic, like it is only affecting one cylinder. I don't know if any of this is true, but this is what I keep coming back to.

Have you partially blocked the exhaust to see if there is a steady miss when the sound is occurring?

Have you removed the valve cover to see if anything unusual is happening - with or without the engine running?

Can you detect if there is a cylinder miss when the sound is occurring?

Gotta figure this one out!

• John

10 hours ago, Tractorman said:

This is definitely a most unusual problem. I have listened to your video many times. The sound seems mechanical to me, but I can't think of any mechanical issue that would behave the way it does when you inhibit a cylinder and then reactivate the cylinder, or that the sound would go away when the ECM is removed from the loop. Also, the sound is rythmic, like it is only affecting one cylinder. I don't know if any of this is true, but this is what I keep coming back to.

Have you partially blocked the exhaust to see if there is a steady miss when the sound is occurring?

Have you removed the valve cover to see if anything unusual is happening - with or without the engine running?

Can you detect if there is a cylinder miss when the sound is occurring?

Gotta figure this one out!

• John

Thanks for the reply! It sounds mechanical, but the fact that I can make it go away makes me think its not. If it were, it would be there under all conditions I would believe.

"Have you partially blocked the exhaust to see if there is a steady miss when the sound is occurring?" - I'm not sure what you mean by this? Can you explain?

"Have you removed the valve cover to see if anything unusual is happening - with or without the engine running?" - I have. Many times. It all seems fine. Nothing appears to change valvetrain wise if I do those procedures with the valve cover off. Not that I can tell. All looks normal. I should add that I have adjusted the valve lash and checked it many times. All within spec.

"Can you detect if there is a cylinder miss when the sound is occurring?" - Doesn't appear to be missing. It sounds like over advanced/retarded timing to me. Like the fuel injection is happening too soon or too late causing this sound. It can be heard through the lower end of the rev range, but seemingly smooths out under load, but it's hard to tell.

Like I said above, I found ONE other person ( I've been searching for years to find someone that had this issue ) just last week that had the same exact sound as my truck. Even the exhaust note was the same "choppy/stutter" sound. He fixed his by replacing the crankshaft position sensor. I had already done that, but I replaced it again just for giggles. I've Ohm'd out every sensor and sensor wiring on the truck and have yet to find a fault.

Having said all of that, the only thing I haven't changed on this truck is the batteries. They are from 2018. I'm going to buy a set of batteries and possibly do the W-T mod as well. I don't suspect any of that will help, but it can't hurt! Well.. It does hurt my wallet!

1 hour ago, Sufarry said:

"Have you partially blocked the exhaust to see if there is a steady miss when the sound is occurring?" - I'm not sure what you mean by this? Can you explain?

Set up your inhibiting tool again and cover the end of the tail pipe with your gloved hand (or block of wood) to see if you can feel a cylinder miss. Do the test with the inhibitor on and with it off. Actually, do three tests - inhibitor on, inhibitor off (within the first 10 seconds), and then with the inhibitor off after the noise returns.

If there is a miss, you should be able to tell if it is random, multiple cylinders, or just one cylinder. I am just trying to think of ways to get some useful information.

1 hour ago, Sufarry said:

It sounds mechanical, but the fact that I can make it go away makes me think its not. If it were, it would be there under all conditions I would believe.

I agree that even though it sounds mechanical, why does the sound go away when certain conditions are met? I wouldn't rule out either a timing knock or something mechanical at this point.

What about that tone ring? Were you able to perform a complete inspection of the whole tone ring?

• John

Edited Thursday at 01:41 AM5 days by Tractorman

38 minutes ago, Tractorman said:

Set up your inhibiting tool again and cover the end of the tail pipe with your gloved hand (or block of wood) to see if you can feel a cylinder miss. Do the test with the inhibitor on and with it off. Actually, do three tests - inhibitor on, inhibitor off (within the first 10 seconds), and then with the inhibitor off after the noise returns.

If there is a miss, you should be able to tell if it is random, multiple cylinders, or just one cylinder. I am just trying to think of ways to get some useful information.

I agree that even though it sounds mechanical, why does the sound go away when certain conditions are met? I wouldn't rule out either a timing knock or something mechanical at this point.

What about that tone ring? Were you able to perform a complete inspection of the whole tone ring?

• John

Thank you for the clarification. I'll give that a shot.

I'm on my phone now, so this will be a little less wordy reply lol. I checked the tone ring. No missing teeth (except the one thats supposed to be 😅) and it wasn't loose or movable in anyway. I had had my brother in law rotate the engine over while used my inspection camera to verify it's condition. All good from what I could tell.

It definitely sounds like a ignition timing issue to me as well.

Do you know if the ECM still has an existing tune - Smarty, Quadzilla, etc., that might have introduced timing mods other than the stock ignition timing? If the ECM wasn't returned to the stock tune, it might have too aggressive of ignition timing programmed into it, causing the knock issue.

7 hours ago, 01_Cummins_4x4 said:

It definitely sounds like a ignition timing issue to me as well.

Do you know if the ECM still has an existing tune - Smarty, Quadzilla, etc., that might have introduced timing mods other than the stock ignition timing? If the ECM wasn't returned to the stock tune, it might have too aggressive of ignition timing programmed into it, causing the knock issue.

I've actually swapped in a know good ECM. It seemingly helped for a few miles and then returned to knocking. I might have a video of before and after the new ECM somewhere come to think of it!

It's hard to tell in this video but the knock appeared maybe a mile or two after the "used but pulled from a running truck" ECM. Almost like it relearned this behavior after a few miles.

VIDEO