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overshot · 08-09-2016, **(Subject: Knocking sound )**

Ok fellas,
I have a 2012 peterbilt 386. With a cm 2250.
It was de mandated in Ga. Since it was de mandated in may it has a hard tapping or knock if you want to call it that. The knock starts at 1300 rpm and from 20 psi boost and up. Loaded or empty it does it. It doesn't knock when using engine brake.

Any ideas to what it could be.

I haven't been able to get back to GA yet, to get it checked out. But, I have scheduled an appt. for the 23rd to get it in.

Am I destined to drop a lot of cash soon.

overshot · 08-09-2016, **(Subject: Knocking sound )**

Let me clarify.
It knocks from 1300 rpm and up.
When the boost is at 20 psi and higher.
Below 20 psi it doesn't do it or not that I can hear.
You can't hear it with windows up. But with windows cracked open you can.

biscuits and gravy · 08-09-2016, **(Subject: Knocking sound )**

I would check the air compressor. Those compressors can fool drivers sometimes, sometimes it's them knocking and not a rod in your motor.

Waterloo · 08-09-2016, **(Subject: Knocking sound )**

Air compressor here too... They can get ugly, might be time to pull the head on it and clean it out... Cummins has a rebuild kit for them when they get sooted up and start knocking like a rod... But, have you done an overhead lately, and how many miles on the truck? The damper needs to be replaced every 500,000 miles as they wear out around then, per Cummins. Oil analysis done lately? How often is the oil changed? All just off of the top of my head...

hhow55 · 08-09-2016, **(Subject: Knocking sound )**

It may be a good idea to disconnect the discharge air line from the compressor, thereby removing the load from the compressor, and see if the knocking sound stops.

overshot · 08-09-2016, **(Subject: Knocking sound )**

(08-09-2016 )Waterloo Wrote: Air compressor here too... They can get ugly, might be time to pull the head on it and clean it out... Cummins has a rebuild kit for them when they get sooted up and start knocking like a rod... But, have you done an overhead lately, and how many miles on the truck? The damper needs to be replaced every 500,000 miles as they wear out around then, per Cummins. Oil analysis done lately? How often is the oil changed? All just off of the top of my head...

Overhead was ran in may.
Last sample looked good.
15-18000 on oil changes.
Dampner on the schedule to be changed on the 23rd.
531000 mi/ 11000 hrs on the truck.
I hope it the air compressor

in2trux · 08-09-2016, **(Subject: Knocking sound )**

Maybe try timing change and see if it goes away after you eliminate the air compressor. One bad injector can ruin your thought process, been there,done that.

overshot · 08-09-2016, **(Subject: Knocking sound )**

(08-09-2016 )in2trux Wrote: Maybe try timing change and see if it goes away after you eliminate the air compressor. One bad injector can ruin your thought process, been there,done that.

I don't have the software to change timing.
I have done the leak down test and Injector cutout. All Seemed good.
I did attempted a injector performance test. But Inite said it could complete due to pressure being to erratic.

I will be at Mr. Hagg's on the 23rd. So we'll see what's happening. Maybe rawze will come by tell what's wrong.

in2trux · 08-09-2016, **(Subject: Knocking sound )**

Insite showed nothing, leak down was "bubblitious" according to Mr. Hagg but here was still that slight hiccup in the timing and he heard it without the aid of a computer from 20 feet away. Number 3 was spraying a bad pattern and burnt a hole in a damaged piston,PDQ.

You're in good hands with a real engine guy that wants to make it right!

Blessings to Jerry and Family!