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!