Code Details
| Display code | SPN 157 FMI 18 |
|---|---|
| SPN | 157 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | Mack |
| 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 Mack MP8 shares its fuel system with the Volvo D13 and requires very high fuel rail pressure �?over 1800 bar at full load. When rail pressure drops below target, the ECM logs this code and drivers typically notice reduced power under load, smoke, or hard starting. On Mack Anthem, Pinnacle, and Granite trucks with the MP8, SPN 157 FMI 18 represents a moderately below-normal fuel rail pressure condition. VCADS Pro monitors rail pressure in real time and can distinguish between a low-supply-pressure cause (confirm with SPN 94 companion code) and a high-pressure pump or injector return flow issue. Regular fuel filter service is the key preventive measure for this fault on the MP8.
SPN 157 is Injector Metering Rail 1 Pressure in J1939. FMI 18 indicates a valid reading below the normal range on the Mack MP8. The same high-pressure fuel system design is used on both the D13 and MP8 platforms, and the same diagnostic approach applies: compare commanded vs. actual rail pressure, check the supply circuit, and use cylinder cutout to identify leaking injectors.
Common Symptoms
- Loss of power under 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 failing
- Leaking fuel injectors depressurizing the rail
- Supply pump output too low
- Primary or secondary fuel filter restricted
- Air leak in low-pressure supply circuit
First Checks
- Connect Mack diagnostic software and monitor actual vs. commanded rail pressure
- Replace primary and secondary fuel filters if restricted
- Test supply pump output at the high-pressure pump inlet
- Inspect low-pressure fuel lines for air leaks
- Use Mack service tool cylinder cutout to identify leaking injectors
- Connect VCADS Pro and monitor fuel rail pressure and delivery pressure simultaneously
- Check for companion SPN 94 FMI 1 — if present, address fuel supply first
Can I Keep Driving?
Power loss is typically noticeable under load. Continued operation with a leaking injector risks cylinder damage. Address this promptly rather than waiting for scheduled service.
Related Codes
Related Lookup Pages
Sources
- Mack Trucks Service Support Public Resources Mack Trucks Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Mack Trucks Inc., Mack Trucks Service Support Public Resources. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - Volvo Trucks / Mack Trucks Technical Support Volvo Trucks North America / Mack Trucks Inc. · oem · accessed 2026-06-10 · confidence medium
Source: Volvo Trucks North America / Mack Trucks Inc., Volvo Trucks / Mack Trucks Technical Support. 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
Is the Mack MP8 fuel rail pressure diagnosis the same as the Volvo D13?
Yes �?the MP8 and D13 share the same high-pressure fuel system. The diagnostic procedure using Mack service tools mirrors the VCADS Pro procedure for the D13: monitor commanded vs. actual pressure, check the supply circuit, and perform a cylinder cutout test. The main difference is the diagnostic software used.
Can a clogged fuel filter cause SPN 157 FMI 18 on the Mack MP8?
Yes. A severely restricted primary or secondary fuel filter starves the high-pressure pump, limiting its ability to build and maintain rail pressure. Replacing both filters is a low-cost first step before pursuing more expensive diagnoses.
How much does it cost to replace the high-pressure fuel pump on a Mack MP8?
This question is best answered by current dealer or shop pricing, which varies by region and time. Generally, high-pressure fuel pump replacement on Class 8 engines is a significant service event involving several hours of labor and a costly pump assembly. A thorough diagnosis to confirm the pump is the actual fault before replacement is important to avoid unnecessary expenditure.