Priming Procedure Failure / Air Lock After Filter Service
After filter service, follow the OEM priming procedure exactly and check for newly introduced air leaks before condemning pumps or injectors.
Fault Overview
Symptom page for failure to prime or persistent air lock after fuel filter service, leading to extended cranking or no-start.
Engine & Platform Coverage
Symptoms
- •Will not prime after filter change
- •Air lock fuel system
- •Hard start after filter service
- •Primer pump failure
- •Fuel system will not bleed
- •Extended cranking after filter change
Possible Causes
- 1.Incorrect priming procedure
- 2.Failed manual primer pump
- 3.Lift pump not commanding during prime
- 4.Large air leak still present
- 5.Clogged primer circuit
- 6.Operator stopped cranking too soon
Wiring & Circuit Notes
Confirm lift pump enable circuit operates during key-on prime cycle if electric priming is used.
DVOM Specs & Sensor Tests
Supply pressure during prime and crank. Aeration sight glass. Lift pump voltage during key-on. Number of primer strokes or prime cycle time per OEM.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Step 1 — Verify OEM Priming Procedure: Verify the exact OEM priming procedure for the platform. Most "won't start after filter change" complaints are incomplete priming or a newly introduced suction leak.
- 2Step 2 — Confirm Lift Pump Runs During Prime: Confirm lift pump runs during prime if electric. Verify enable circuit is active during the key-on prime cycle.
- 3Step 3 — Check for New Suction Leaks: Check for new suction leaks introduced during service — seals, fittings, drain valve. These are the most common cause of post-service air locks.
- 4Step 4 — Perform Aeration Confirmation: Perform aeration confirmation with a clear sight glass. Confirm air is present before extended cranking attempts.
- 5Step 5 — Measure Supply Pressure While Priming and Cranking: Measure supply pressure while priming and cranking. Pressure should build as air is purged.
- 6Step 6 — Extend Cranking in Controlled Intervals: Extend cranking in controlled intervals while monitoring pressure build. Allow the system to purge air gradually.
- 7Step 7 — Address Remaining Causes: Only after priming and leaks are addressed, consider other no-start causes such as lift pump failure or primer pump failure.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Complete proper priming — no parts cost; repair primer pump $50–150; correct new leaks $30–150; replace failed lift pump if it does not run during prime $150–400.
Start Diagnosing This Fault Now
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AI Diesel Tech — The Technician's Diagnostic Engine
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.