Applicable Systems
LS Series
Symptoms
Minimum pressure valve (MPV) is not maintaining the required 4.5 bar (65 psi) sump pressure during loaded operation. The compressor cannot build line pressure efficiently.
Possible Causes
- MPV spring broken or weakened, allowing blow-back to the intake
- MPV seat damaged by debris or carbon buildup preventing proper closure
- MPV piston stuck in the open position due to oil varnish or corrosion
Troubleshooting Steps
- Step 1: Isolate the compressor from the air system and measure sump pressure during a load test
If sump pressure cannot reach 4.5 bar within 30 seconds, the MPV is likely leaking - Step 2: Remove the MPV from the separator tank and inspect the poppet, seat, and spring
Clean carbon deposits with non-flammable solvent; replace the spring if free length is <5% below spec - Step 3: Inspect the MPV bore for scoring or corrosion that could bind the piston
Hone the bore or replace the complete MPV cartridge if wear exceeds 0.1 mm clearance
How to Reset
Reset: Repair or replace the MPV, reassemble, and run a load test. The alarm clears automatically when the compressor maintains sump pressure above 4.5 bar for 60 seconds.
Common Replacement Parts
- 2500-3950-00: Minimum pressure valve kit, LS 16-25 (Includes spring, poppet, seat, and O-rings)
- 2500-3951-00: MPV cartridge assembly (Pre-assembled, drop-in replacement)
Frequently Asked Questions
- Q: What pressure should the MPV hold?
The MPV should remain closed until sump pressure reaches 3.5-4.5 bar. Below that, it stays closed to prevent blow-back. Above 6 bar, it opens fully to the receiver.
- Q: Can a bad MPV cause high oil consumption?
Yes. A leaking MPV allows compressed air to blow back through the separator, carrying oil aerosol into the intake and out the breather.
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Source: Sullair LS Series Operation and Maintenance Manual, 02250141-841
Category: Compressor
Published: 2026-10-29
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