Truck chugs at certain speed

So every time I hit 56 mph, the truck starts chugging - vroom vroom vroom vroom like it's hiccuping. When it hits 59-60mph it stops doing this.

It also has started to do this around 42mph as well.

During the time it is chugging like this, black smoke comes out of the stack.

Is this an EGR valve issue?

My boss will be taking this truck into the shop when I come back this weekend to fix a coolant leak, and I am going to give him a heads up on this issue as well.

Any ideas would be appreciated.

 

Don't know, but it's a Freightliner, has around 670k miles. I think my boss told me it's an '07. Looks like a normal company truck, it doesn't have the big hood and the too-small windshield.

 

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I'd like to have a truck of my own some day, and learning more about how the truck works, what kind of problems crop up and what needs to be done to fix them is in the realm of what needs to be known to be a good owner op...

 

Now that you mention it, I was looking at the tachometer today when it was doing it.

It does it at abou 46mph and also 56mph, when it is about 1,000 rpms.
When I put it into 9th gear and 10th gear.

I pull the same amount of weight every week, it's around 5,000 lbs. If I'm going down a hill, it hardly does it, but going up an incline it does it.

I told my boss last week when he puts the truck in the shop he needs to have the EGR looked at, and I'm glad someone is confirming what I thought. CEL doesn't come on.

Thanks everyone.

Also, and this truck has only started doing it this week, is every so often, at random times, the display will read "Warning - Turn Signal On" and it will beep. The turn signal isn't on, and the only way to clear it is to put the 4-ways on, then hit the turn signal to turn it off.

Sometimes it does it every 10-15 minutes, then it won't do it again for an hour or so.

Maybe it's the trailer, I don't know. Pigtail isn't loose, and all the turn signals and lights work on the truck and trailer.