Hard Start or Long Crank — LLY / LBZ / LMM Duramax
Diagnostic guide for hard start or long crank on LLY/LBZ/LMM Duramax 2004.5–2010.
What’s going on
On LLY/LBZ/LMM Duramax, a long crank/hard start usually means the engine is not building adequate fuel rail pressure for start, or cold-start aid is weak. Typical causes are low fuel rail pressure from supply/air ingress or high-pressure leak, a rail pressure sensor fault, or weak glow plugs/glow plug module in cold conditions.1
Most likely causes
- Low fuel rail pressure from supply restriction, air ingress, or high-pressure leak — CP3 common rail needs sufficient rail pressure to start. Supply restriction, air in fuel, or a high-pressure leak can cause extended cranking and low rail pressure codes P0087, P0093, P0191.1
- Rail pressure sensor fault or wiring — Erroneous rail pressure feedback can prevent proper start fueling. This can set P0191 and contribute to hard start/long crank.1
- Weak glow plugs or glow plug control module — Cold-start aid is glow plugs + control module. Weak plugs or module cause hard start/rough cold start and can set P0380, P0670.1
How to narrow it down
- Read stored trouble codes first; note any of P0087, P0093, P0191, P0380, P0670.
- Check fuel supply for restriction, leaks, and air ingress; verify lift pump delivery and filter condition.
- Monitor rail pressure during cranking with a scan tool; compare actual vs desired pressure.
- Inspect glow plug system: check control module output and individual glow plug resistance; verify operation in cold conditions.
- Inspect rail pressure sensor and wiring for damage, corrosion, or connector issues.
Specs and test values
| Check | Normal reading | When to measure | Source |
|---|---|---|---|
| Battery and charging voltage | 12.6–12.7 V at rest when fully charged; about 13.5 V running | Engine off, then running with loads on | DieselHub |
| Glow plug resistance | Low — usually no more than about 6 Ω — and similar on every plug; an open reading means a failed plug | Engine off, meter lead to engine ground | DENSO |
Common mistake to avoid
Replacing glow plugs or the glow module before checking the batteries. The glow system draws heavily, and low battery voltage causes circuit faults that no part swap will fix. Source: FleetRabbit — P0670 glow plug control module fault.
Parts and tools you may need
- Scan tool with live data for rail pressure and glow plug control
- Fuel pressure test gauge
- Multimeter
- Glow plug test light
Tools you’ll need
- OBD-II scan tool — Reads stored codes and monitors actual vs desired rail pressure during cranking to confirm low fuel pressure or sensor fault.
- Digital multimeter — Checks glow plug resistance and control module output voltage to verify cold-start aid integrity.
- Fuel pressure test kit — Measures lift pump delivery and supply-side pressure to detect restriction, air ingress, or leaks affecting rail pressure.
- Borescope — Inspects rail pressure sensor connector and wiring for corrosion or damage without major disassembly.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Fuel filter service | $30–$150 | 0.3–1 hrs | Easy DIY |
| Glow plugs (full set) | $80–$250 | 2–6 hrs | DIY with tools and a manual |
| Fuel rail pressure sensor | $100–$400 | 1–3 hrs | DIY with tools and a manual |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If rail pressure remains low after supply checks, or internal high-pressure leak is suspected, have a diesel specialist perform leak testing and injector return flow evaluation. Do not disassemble high-pressure fuel components without proper training and safety equipment.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Related trouble codes
- P0087 — Fuel Rail/System Pressure Too Low
- P0093 — Fuel System Leak Detected - Large Leak
- P0191 — Fuel Rail Pressure Sensor A Circuit Range/Performance
- P0380 — Glow Plug/Heater Circuit Malfunction
- P0670 — Glow Plug Control Module Circuit
Sources
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