Looking at trucks....

I met a fellow that had a 2013 Mack, he had the same experience. Only in the shop once in three years just for a sensor. In and out the same day.
Cummins is a great motor, anyone can work on it. You just have to be diligent about doing oil changes like we use to do, don't try running extended oil changes or it will take out the cam and rockers. See rawze dot com for everything about cummins.

 

The only real manufacturing issue with the D13 MP8's was the injectors and cups. Mine were replaced under warranty from he fleet I bought from (I would strongly advise making sure those have been changed). I average 6.8MPG, and run flatbed. I think the after treatment system of the Volvo Mack product is superior to the cummins system (everyone uses the cummins system except Volvo Mack.) Volvo, Mack has fixed the injectors in the new engines and gone common rail.

Also something else about engine size. I've heard 13 liter engines have less after treatment issues than 15 liter engines because the smaller engine works more efficient with fuel burn than the larger 15 liter engines.

 

I agree with above posts. I put a reman Cummins isx back in my 2001 international 9200. Before I spent the money I looked hard for another truck. Only choice for me was a NEW Volvo or Mack. No way I was going to touch a truck from 2008 to to 2016 used. The price difference between a new truck and one with 500k miles was not enough to get me to even look at used with any seriousness.

 

Unless that MX-13 has been serviced by Paccar, you'll regret it, badly. Those 2012's last about 350,000 miles before they pretty much need a full overhaul. My company wisely leased about a dozen of them when they came out, because of the untested new technology, and dumped them all back off at the dealer when they all started going down.

They aren't Maxxforce bad, but close. New Paccar's are great if properly operated and maintained (proper idling procedure and hitting every single maintenance interval).

With $50,000, you could get a 2000-2007 13 litre CAT or Detroit and get 6-7 mpg no problem out of a conventional truck.

I can pull 6 mpg out of a 2006 NXS 550 hp C-15 grossing between 50,000-75,000 on flat ground on a no wind day.

 

587 of course is the model that came after the 387. It's a sharp looking truck. 2013 ISX Cummins have hi-pressure fuel pump that have a bad reputation for failing. I would really think twice and hard before getting mixed up with one.

The motor is full of potential and can be great on fuel. 7mpg+

Ceramic plunger pieces would break off from the fuel pump, get into the oil, and take out whatever in its path.

First would be to check if the plunger update was done. Then plan on overhauling the fuel pump every 300k-400k to stay ahead of it.

Or... why not just keep what you got and rebuild the motor from scratch... new head, new cylinders, counterbore to the right height, etc from a shop who hand builds and takes pride in a finished product... my thoughts at least

FYI I owned a 387, ISX 871,

 

The 870 ISX that you have has a good reputation in some circles when built and tuned right

 

I can understand SOME of what's being said about the Paccar engine...but not the drive train.
I'm in a 2014 T-800 with less than 75,000 miles on it. This thing has power, pull, and tork (more so after about the 50,000 mile break-in point).
I have 7 axles (cheater on tractor, and cheater on trailer), and can legally pull 96,000lbs. all day long.
There are times during harvest, I pull 100,000 to 108,000 pounds out of a field (on soft ground), and can hit 3rd. or 4th. gear without problem (or wrecking anything) before I reach the driveway.
We have (aside from Volvo) every major engine in our various trucks, and this Paccar will out pull all of them except for a jacked-up Cummins in one of them.

Now the down side:
In under 75,000 miles, it has been in the shop (Kenworth) to have "Software Updates", and faults cleared TWICE. And also a Humidity Sensor replaced once. Even though the first time the truck was still under warranty...total cost of the two visits was just under $1,000.00 plus my wages to take the truck to and from a certified Kenworth shop, plus the cost of fuel.
Needless to say; the Boss is not impressed with the DEF bologna (and the issues it brings), and vows to never buy another one---Gliders only from this point on.

I CAN NOT complain about the power this Paccar is putting out though.

 

An B model Cat is capable of 7+ MPG if you drive it right.