Code Details
| Display code | SPN 157 FMI 18 |
|---|---|
| SPN | 157 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | PACCAR |
| System | Engine �?fuel system |
| Component | High-pressure fuel rail / common rail system |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The PACCAR MX-13 uses a common rail fuel system requiring very high pressure for efficient combustion. When rail pressure drops below the ECU target for the current speed and load, this code sets. Drivers typically notice reduced power, smoke under load, or hard starting. On Kenworth and Peterbilt trucks with the MX-13, fuel rail pressure is monitored by the ECM in real time. DAVIE4 and PACCAR ESA display fuel rail pressure, commanded fuel pressure, and injection timing data. An injector that leaks back excessively reduces the ability of the HP pump to maintain target rail pressure — this is detectable in DAVIE4 by observing rail pressure behavior during a specific injector-off test cycle. High-pressure pump wear shows as lower-than-expected rail pressure under load despite adequate supply pressure.
SPN 157 is Injector Metering Rail 1 Pressure in J1939. FMI 18 indicates a valid reading below the normal operating range. On the PACCAR MX-13, the ECU monitors commanded vs. actual rail pressure. PACCAR DAVIE provides commanded and actual pressure data alongside a cylinder cutout function to isolate leaking injectors from high-pressure pump issues.
Common Symptoms
- Loss of power under heavy load
- Hard starting or extended cranking
- Black or gray exhaust smoke
- Rough idle or engine misfires
- Reduced fuel economy
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- High-pressure fuel pump worn or losing efficiency
- Leaking fuel injectors depressurizing the rail
- Supply pump output too low to adequately feed the high-pressure pump
- Primary or secondary fuel filter restricted
- Air ingestion in the low-pressure supply circuit
First Checks
- Connect PACCAR DAVIE and monitor actual vs. commanded rail pressure at idle and under load
- Replace primary and secondary fuel filters if restricted or overdue
- Test supply pump output at the high-pressure pump inlet
- Inspect low-pressure fuel lines and fittings for air leaks
- Use DAVIE cylinder cutout to identify leaking injectors
- Monitor fuel rail pressure under load in DAVIE4 and compare to commanded target
- Perform an injector return flow test in DAVIE4 to identify leaking injectors
Can I Keep Driving?
Power loss is typically noticeable under load. If a leaking injector is the cause, continued operation risks cylinder damage. Address this promptly.
Related Codes
Related Lookup Pages
Sources
- PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources PACCAR Inc. · oem · accessed 2026-06-11 · confidence medium
Source: PACCAR Inc., PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - PACCAR Engine Technical Services PACCAR Inc. · oem · accessed 2026-06-10 · confidence medium
Source: PACCAR Inc., PACCAR Engine Technical Services. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
What is the normal idle fuel rail pressure on a PACCAR MX-13?
Typical idle rail pressure on the MX-13 is approximately 300 to 600 bar depending on temperature and idle calibration, rising toward 1800 to 2000 bar at rated power. PACCAR DAVIE displays both the commanded target and the measured value, allowing direct comparison to assess the gap between expected and actual performance.
Can a clogged fuel filter cause SPN 157 FMI 18 on the PACCAR MX-13?
Yes. A severely restricted filter reduces flow to the high-pressure pump, limiting its ability to maintain rail pressure targets. Replacing both filters is a recommended first step before pursuing more expensive diagnosis.
How does the PACCAR MX-13 high-pressure pump compare to the PACCAR MX-11?
The MX-13 and MX-11 are different displacement engines with different fuel system components. While the general common rail architecture is similar, the pump specifications, injector sizes, and pressure targets differ. Service procedures specific to each engine should be followed as documented in PACCAR DAVIE and the relevant service literature.