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Old 06-18-2021, 04:23 PM   #1
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Cat C7 rattling/knocking at specific throttle level

Caterpillar C7 engine owners: do any of you have issues with a knocking/rattling sound at certain levels of throttle? This only seemed to happen when going up a slight incline at 65mph, the top speed of the governor.

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Old 06-18-2021, 05:15 PM   #2
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The idler pulley would be the first thing I would check. Or the fan bearings since it's when the fan should fully engage.
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Old 06-19-2021, 10:20 AM   #3
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The idler pulley would be the first thing I would check. Or the fan bearings since it's when the fan should fully engage.
I had the tensioner pulley do something similar only under heavy load pulling hills. It looked fine visually when the truck was idling, when the belt was removed you could really tell it was dying.
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Old 06-19-2021, 11:54 PM   #4
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Once it has started to knock, what do you do to make it stop?

What speed does it start to happen?

While it could be a pully, I'm wondering, since you said it starts knocking at it's governed speed, if it's trying to "govern" the speed and it's "knocking" against that governor limit??
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Old 06-20-2021, 09:21 AM   #5
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Once it has started to knock, what do you do to make it stop?

What speed does it start to happen?

While it could be a pully, I'm wondering, since you said it starts knocking at it's governed speed, if it's trying to "govern" the speed and it's "knocking" against that governor limit??
I think it’s more of an RPM/torque level combination. It will happen briefly when accelerating from a stop to say 35-45 mph. It happens consistently when climbing small inclines at the governor limit I think just because I hit that magic RPM level. My tach doesn’t work but I have a BlueFire so I plan to monitor it next time I go for a drive.

Basically, the way to get it to stop is to take my foot off the gas, or get the transmission to downshift.
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Old 06-20-2021, 01:42 PM   #6
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I had the tensioner pulley do something similar only under heavy load pulling hills. It looked fine visually when the truck was idling, when the belt was removed you could really tell it was dying.
You're right. The tensioner and/or it's pulley is more likely.
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Old 06-21-2021, 07:24 AM   #7
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sounds like a pulley or engine accessory to me.
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Old 10-18-2023, 02:58 PM   #8
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I just took a long drive from Toledo OH to Townsend TN and back. Especially in the mountains on a grade I could reproduce the knocking sound. I was having trouble with my BlueFire but for the 5 or so minutes at a time I could get it to work I was able to observe the reported engine load and torque while going uphill and hearing the knocking: 100% load and about 75% torque. This matches up with the situations where I see intermittent power loss or strange behavior while accelerating when the engine is cold: in that scenario, the ECM is likely telling the injectors to push extra hard which would put me on the higher end of load/torque.

The fun part will be figuring out if I can reach 100% load while parked. I'm thinking I'll want to get the bus somewhere a little more remote so I don't drive the neighbors nuts revving up the engine to max RPMs repeatedly while I run cutout tests.

To complete the thought: I can't find any other components that are loose and after going through this long mountain drive I'm pretty sure I AM experiencing some power loss, it's just hard to tell when you're almost at max vehicle speed- momentum kind of makes up for the injector missing at 65 MPH. As others have mentioned, HEUIs usually either work or they don't, so I'm back to believing I have a faulty (albeit not totally failed) injector.
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