The PACCAR MX-13 is the in-house alternative to Cummins power in Kenworth and Peterbilt Class 8 trucks. It is 12.9 liters, built from a Daimler design under license, and fitted in the same chassis that can take an X15. It sells in volume. And when we went looking for documented field evidence of what actually breaks on it, we came up nearly empty.
That is the honest headline of this page. Filling the gap with plausible-sounding failure modes would be easy, and it is what most engine pages do. We are not doing it, because somebody would price a truck off it.
What we have: one electrical breakdown on a T680
A T680 came into a shop with a fuse on the aftertreatment circuit that kept blowing. Tracing it back, the harness feeding the DEF pump was the cause. New harness, new fuse, the truck went back to work. The same truck also threw an intermittent fuel-pressure warning that traced to the priming pump circuit.
Now read that for exactly what it is and no further. That is a chassis wiring fault, not an engine-internal failure. It tells you DEF-pump harness routing on a T680 can chafe or fail. It does not tell you whether that is endemic to the truck or one bad build. And the source video never confirmed which engine was in the truck, so what we actually have is a T680 data point, not an MX-13 data point.
What we don't have
Our evidence file for this engine contains no MX-13 camshaft failure. No turbo failure. No EGR valve sticking. No fuel-pump complaint. No injector or liner problem. No shop calling it a chronic trouble engine. No owner-operator naming model years to avoid. No documented repair invoice for anything that broke.
Absence of evidence is not evidence of a good engine. There are two readings of that silence and we cannot tell them apart from here. Either the MX-13 genuinely does not generate the volume of roadside failures that puts an engine on YouTube, or the failures exist and nobody with a camera has documented them in a form we could find. Both stories produce exactly the quiet we are looking at, and only one of them is good news for a buyer.
The trap: most "MX-13" numbers online are Cummins numbers
This one matters more than anything else on this page, because it is the part that will actually cost you money.
When we mined video evidence for the MX-13, half of what came back was generic Cummins ISX rebuild-kit content that had been swept up because the videos were tagged "Kenworth." Injector prices around a thousand dollars each. In-frame kit contents and pricing. All of it real, all of it accurate, and none of it describing a PACCAR engine.
The mechanism is simple once you see it. Kenworth sells both engines in the same truck, so a search for "Kenworth rebuild cost" returns Cummins figures sitting under a Kenworth badge on the thumbnail. Nobody is faking anything. The search just cannot tell the two apart, and neither can you unless you check.
So if you are budgeting an MX-13 rebuild off a number you found in a Kenworth video, confirm the engine in that video before you trust the figure. A T680 is not an engine. It is a chassis that comes two ways.
Market pricing: the listings won't tell you either
Kenworth T680 sleeper trucks across all configurations range from $12,000 at the bottom to $599,950 at the top on TruckPaper. That spread is not a price signal. It is the entire inventory pool, from a worn-out million-mile unit to a current-year truck, quoted as one range.
Individual units captured on the same pass: a 2022 at 435,976 miles, a 2025 at 66,401 miles, two 2017s past 900,000 and 1,043,000 miles respectively, and one block of twenty one-owner off-lease units averaging about 350,500 miles. Almost none of those listings carried a posted price in the page text, and none of them tagged the engine.
Which means there is no MX-13 versus X15 price delta available. Building one would mean pulling VINs and spec sheets on individual listings, one at a time. Nobody has done that at scale, us included.
What to do if you're looking at one anyway
Pull the VIN and get the build sheet before you talk price. Confirm you are actually looking at an MX-13 and not the X15 option, because the two trucks look identical from the curb and the seller may not know or may not care.
Then go find a tech. Call a Kenworth or Peterbilt dealer service department, or an independent shop that works on PACCAR iron, and ask them straight: what do you see come in on MX-13s, and at what mileage? A working diesel tech knows the pattern in their bay whether or not it ever reached the internet. That conversation is the single best source available on this engine right now, and it costs you a phone call.
Ask the seller for aftertreatment service history specifically, including any DEF pump or harness work. Run a normal pre-purchase inspection on the fundamentals that do not need engine-specific knowledge: fault code pull including derate history, blow-by check, oil analysis, compression if the shop will do it.
And be honest with yourself about which buyer you are. If you can absorb a surprise because you have cash reserves and a second truck, a thin public record might be a discount you can exploit. If this truck is the business and one five-figure shop visit ends you, buy the engine with the thick file of known problems instead. A documented failure mode you have budgeted for is worth more than a quiet one you have not.
See our T680 MX-13 record and compare it to the T680 with the Cummins X15, where the failure record is documented in detail.