If you run EPA21 MX-13 or MX-11 engines in Kenworth or Peterbilt trucks, there is a specific piece of knowledge that will save you money: PACCAR’s first-line answer to a lot of MX injector complaints is not replacement. It’s a documented cleaning procedure.

There’s also a much more serious piece of knowledge, on EPA21 MX engines, injectors and engine software are paired, and PACCAR warns that mismatching them can cause severe engine and aftertreatment damage. Getting this platform wrong is not like getting an older platform wrong.

This guide walks through what PACCAR has published, what the diagnostic thresholds are, and how to decide between cleaning and replacing.

The MX Platform in Brief

PACCAR MX engines use high-pressure common rail with ECM-managed injector control, and the platform’s diagnostics run through DAVIE4, PACCAR’s proprietary service tool. Unlike Cummins INSITE or Detroit DiagnosticLink, a great deal of MX fuel-system diagnosis is only accessible through DAVIE4, which shapes how independent shops can and can’t work on these engines.

Part numbers vary by emissions family, and this platform supersedes aggressively:

Start with the E290 Cleaning Procedure

This is the part most people don’t know, and it can be the difference between a three-and-a-half-hour job and a full set of injectors.

PACCAR service bulletin E290 documents an EPA2021 MX fuel injector cleaning procedure applicable to all chassis with an EPA21 MX-13 or MX-11 engine with more than 10,000 miles. It exists because EPA21 MX engines past 10,000 miles were presenting a recognized set of DTCs and drivability symptoms that respond to cleaning rather than replacement.

Key facts from the bulletin:

• Applicability: EPA21 MX-13 or MX-11, greater than 10,000 miles.
• Standard Repair Time: 6 labor hours, SRT number B44-FSC.
• A cleaning procedure claim can be filed under any applicable warranty for the affected chassis. If your truck is in warranty, this may cost you nothing.
• The procedure also provides criteria for determining whether injector replacement is necessary, so it functions as a diagnostic gate, not just a repair.

The published procedure detail is worth reading in full if you’re doing the work. With this procedure, cleaning solution is circulated through the fuel filtration module using a cleaning tool, the engine is run at idle for one minute, then at 1,200 rpm for 20 minutes, then three consecutive DAVIE4 Check Fuel System Health tests are run back-to-back, chassis fuel lines are reconnected, and the module flushed, and results are verified.

The threshold that matters. On the DAVIE4 Check Fuel System Health test, average unbalance for each individual cylinder should be within ±6 rpm during the stable phase of the test, at 18 to 37 seconds. That number is your objective clean-or-replace criterion. It’s far better than a subjective judgement about how the engine sounds.

One procedural caution from the bulletin; if the vehicle has NA31 RC1 software, do not perform the injector calibration procedure. And if the calibration procedure was run and DAVIE4 reports “Service is not run for the specific cylinder” across all six cylinders, disregard those results and continue.

The E318 Pack 2 Injector Campaign and the Warning that Comes with It

PACCAR bulletin E318 covers a redesigned injector for EPA2021-compliant MX-series engines. The published bulletin sets out terms that are unusual and important:

• Injectors must be replaced as a complete set of six. Not one, not three.
• Associated software must be updated at the same time.
• Applies to selected EPA2021 MX-11 and MX-13 engines identified by PACCAR, with affected chassis flagged in SIR. PACCAR contacts dealers and customers directly.
• Recommended part numbers: 2426856PEX for MX-13 (package of 1), 2434417PEX or 2461236PEX for MX-11 (package of 6).
• The associated parts list includes six flange bolts (1313120PE), valve cover gasket, valve train casing gasket, fuel filter element set with O-rings, and fuel filtration module O-rings and sealing ring; i.e. this is a substantial teardown, not a swap.

Read that carefully, because it has a strategic implication beyond this single repair. On EPA21 and later MX engines, the injector is no longer a standalone mechanical part, it is half of a hardware/software pair. That is the direction the entire Class 8 industry is moving, and it changes how you should think about injector sourcing on the newest platforms. On EPA10 through EPA17 engines, a correctly specified, correctly tested injector is a correctly specified injector. On EPA21+, you also need to know what software the engine is running.

Practical guidance: If you have an EPA21 MX engine with injector symptoms, check whether the chassis is flagged in SIR before doing anything else. If it is, this is a dealer-coordinated campaign repair, and you should let it be one. If it isn’t, work the E290 cleaning procedure first.

E326 — The Connector Shell Nobody Expects

A smaller but genuinely useful one. Bulletin E326 covers EPA17–EPA21 MX-13 engines and documents that the locking tabs on the injector connector shell, part number 1439888PE, can break when injectors are disconnected or the valve train casing is removed.

The history here is worth knowing; connector shells were not previously available separately, which meant a broken locking tab required replacing the entire valve train casing assembly. They’re now stocked individually, and PACCAR advises dealers to keep them on the shelf to prevent downtime.

If you’re going into an MX-13 valve train casing for any reason, injectors, rocker work, anything, have connector shells on hand before you start. This is the kind of detail that turns a one-day job into a three-day job when you don’t know it.

High-Pressure Fuel Lines: Do Not Assume Reuse

PACCAR’s MX-13 service documentation is unusually explicit on this point, and it’s worth quoting the principle because it applies well beyond this platform. The service manual states that high-pressure fuel injection pipes are not intended to be reused indefinitely, that pipes and connections must be properly checked before any reuse, and that damaged or incorrectly installed injection pipes can cause hot fuel leakage and flammable gases or vapors.

Additional handling requirements from the same documentation:

• Wait at least one minute after switching off the ignition before working on high-pressure fuel components. Pressure in the common rail and injection pipes bleeds down slowly and is present even with the engine off.
• If reusing a pipe, install the labelled pipe in its original position, pipes are not interchangeable between positions.
• When all injection pipes are replaced, loosen all rail attachment bolts first and hand-tighten them before final torque, to avoid bending the pipes.
• On some models the cab must be raised to install the pipes without bending them.
• Always check the high-pressure system for leaks after loosening or replacing pipes and perform the DAVIE4 “Evaluate injector back leak” test after the repair. If that test cannot be completed successfully, do not reuse the pipe.

Clean or Replace — The Decision

Where Remanufactured Injectors Fit on this Platform

Honestly and specifically:

• EPA10 through EPA17 MX engines are a good fit for quality remanufactured injectors. The hardware is mature, cores are available, part numbers are stable enough to verify against an ESN, and there’s no software-pairing constraint. This is where the value is.
• EPA21 and later requires more care. If the chassis is in an active PACCAR campaign, use the campaign. If it isn’t, and the engine needs injectors outside the campaign, the software-pairing question has to be answered before parts are ordered, which means knowing the current engine software level, not just the part number.

We’d rather tell you to go to the dealer for a campaign repair than sell you a set of injectors that shouldn’t go in your truck. Send us your VIN or engine serial number through the Make Sure It Fits form and we’ll tell you which situation you’re in before you spend anything.

We supply remanufactured injectors across the PACCAR MX-13 and MX-11 range built on genuine OEM cores, flow-tested and leak-tested to OE specification, with a 12-month unlimited-mileage warranty, shipped nationwide from Elk Grove Village, Illinois.