2006 Duramax LBZ hard-starts after sitting overnight but fires right up if it's been shut off for less than an hour.

Start with the fuel system—this is the most common cause of overnight hard-starts on the LBZ.

Check the fuel filter head (the lift pump housing on the driver's side valve cover). The internal O-rings and seals deteriorate over time, allowing fuel to drain back to the tank when the engine sits. This leaves the filter housing and lines empty, requiring the lift pump to re-prime the system on startup.

Quick test: Turn the key to "ON" (do not crank) for 5-10 seconds, then off. Repeat 2-3 times before cranking. If it fires immediately, you've confirmed a loss of prime issue.

Replace the fuel filter head O-rings (GM part # 97328718 or equivalent kit). This is a common fix and costs under $50. Also inspect the fuel lines from the tank to the engine for cracks or loose clamps.

If that doesn't resolve it, check the injector return lines and CP3 injection pump inlet fitting for leaks. A failing lift pump can also cause this, but the filter head seals are the usual culprit.

Safety note: Relieve fuel system pressure before removing any lines. Diesel under high pressure can cause serious injury.

This is a straightforward DIY repair if you're comfortable working on fuel systems. If you're not, or if replacing the O-rings doesn't fix it, consult a diesel specialist.

Start with a fuel pressure bleed-down test. This is the classic LBZ symptom of losing prime overnight.

Test Procedure:

1. Install a gauge on the fuel filter housing Schrader valve (or tee into the supply line at the filter head).
2. Key on, engine off (KOEO) to prime. Note peak pressure (should hit ~15-20 psi).
3. Shut off key. Watch gauge for 10 minutes, then overnight. Spec:* Pressure should hold >5 psi for 20+ minutes. Overnight drop to 0 psi confirms a leak.

Top 3 Culprits (Most to Least Likely):

1. Fuel Filter Head / Water Separator Assembly (WIF sensor/O-rings): The #1 failure point. The internal check valve or housing O-rings leak air in/fuel back to tank. Replace the entire head assembly (ACDelco TP3018 or equivalent) – don't just swap O-rings on a 18-year-old plastic housing.
2. Injector Return Flow (Excessive): If the head holds pressure, the CP3 is bleeding down through worn injector return check valves. Requires injector return rate test (special tooling, measure ml/min per cylinder at idle). Spec is ~25-35 ml/min total; >60 ml/min total indicates bad injectors.
3. Supply Line / Quick Connects: Check the line from tank to filter head for wetness or air bubbles in clear line during prime.

Quick Diagnostic: Cycle the key 3-4 times (wait 5 sec between cycles) before cranking tomorrow. If it fires instantly, you confirmed a bleed-down issue.

Don't guess injectors first. Filter head is a $120/1-hr job. Injectors are $3k+/8-hr job. Test pressure first.