why wont my propane fork forklift start

Electric trucks simply refuse to travel when sick; diesel trucks cough black smoke but usually limp along. A propane (LPG) forklift, however, will silently lay down and die—no smoke, no drama, just a crank-crank-crank that melts the starter and leaves 2 tonnes of steel blocking the only bay door. The following text is a systematic map to move from “nothing happens” to “root cause found” in ≤30 min, using only the tools found in an average maintenance cage (digital multimeter, propane pressure gauge, basic hand tools) plus the OEM service manual you already lost. Follow the flow exactly; LPG systems punish “parts darts” more than any other industrial power-train.

Safety and lock-out (every time, no exceptions)

1.1 Park on level ground, mast fully lowered, wheels chocked, parking brake applied.
1.2 Close the service valve on the LP tank slowly; allow engine to run until it starves (clears vapor line).
1.3 Disconnect negative battery terminal; remove ignition key to pocket.
1.4 Wear Class-0 gloves and safety glasses when cracking any fuel fitting—liquid propane at –42 °C causes instant frost-bite and will find the only gap in your glove.
1.5 Keep a dry-chemical extinguisher within 3 m; LPG is heavier than air and pools under the truck.

The four energy domains we must prove good

A propane engine needs exactly the same four things as gasoline—air, fuel, compression, spark—but the form of “fuel” is more complex. Divide the problem into domains:

A. Electrical domain (12 V supply, ignition, sensors)
B. Vapor-fuel domain (tank → regulator → vaporizer → mixer)
C. Air/mechanical domain (filter, valves, timing, compression)
D. Control domain (safety interlocks, micro-switches, ECU on EFI engines)

Diagnose in that order; 72 % of “no-starts” are found in domain A, 18 % in B, 7 % in C, 3 % in D.

Domain A: Electrical—30-minute flow chart

3.1 Battery ≥12.6 V static, ≥9.8 V while cranking (<9.8 V → charge or replace; LPG starters draw 20 % more current than gasoline because compression ratios are higher).
3.2 Terminal voltage drop test: positive cable <0.3 V, ground cable <0.2 V during cranking. Excessive drop usually hides inside the ½” eyelet that looks perfect from the outside.
3.3 Ignition switch continuity: measure on the back of the switch; expect <0.1 Ω between BATT and IGN in START. Many Toyota 7-Series trucks eat the switch at 6,000 h.
3.4 Starter current: 90–130 A for 4-cylinder 2.2 L, 150–190 A for 6-cylinder 4.3 L. Higher current with sluggish crank indicates either weak battery or hydro-lock from a flooded vaporizer (see §4.6).
3.5 Spark test: remove #1 plug, ground base, connect adjustable gap tester. Minimum 25 kV blue snap must jump 10 mm gap while cranking. Yellow 6 mm spark = failing coil or low supply voltage.
3.6 Distributor cap: look for hairline cracks under the towers; propane burns 3× drier than gasoline and creates static tracking. Nexen/NGK caps show carbon trails that wipe off with finger—replace cap.
3.7 Plug condition: propane plugs must be one heat-range colder than gasoline equivalents. Chalky white insulator = lean burn from vacuum leak or regulator icing. Oily black = flooding vaporizer.
3.8 Engine shut-off solenoid (ESS): on the carburettor or mixer body. Key-ON should produce 11.5 V and audible click. No click = open coil (∞ Ω). Temporary fix: remove plunger and plug hole with 5 mm allen to limp truck back to shop—then replace solenoid before returning to floor.
3.9 Neutral-start and seat-belt interlocks: jump them only for test; 70 % of “it cranks but won’t start” on Hyster S80 is a mis-adjusted seat switch that opens when operator bounces.

Domain B: Vapor Fuel—Measure, Don’t Guess

4.1 Tank service valve: open slowly ½ turn; listen for hiss. No hiss + frost on outside = tank empty or excess-flow valve tripped. Remove tank, stand upright 30 s, reinstall.
4.2 Tank dip tube: modern 33.5 lb forklift cylinders have internal eductor tube to deliver liquid. If tube is cracked the valve delivers mostly vapor → hard start, no power. Weigh tank full vs. tare; 15 kg liquid content expected. <14 kg with gauge reading ¾ = dip tube fault → replace cylinder.
4.3 Hose leak test: brush 60 % water / 40 % dish-soap on every joint. Bubbles form at >0.5 psi differential—propane systems operate at 0–2 psi so even tiny leaks show.
4.4 Lock-up (primary) regulator: screw-on 0–30 psi gauge at vaporizer inlet. Key-OFF, open tank, static pressure should stabilise at 90–125 psi (temperature dependent). <70 psi on 20 °C day = nearly empty tank or plugged POL adapter.
4.5 Cranking pressure test: crank engine 5 s with ESS disconnected. Vaporizer outlet must read 4–7 psi for Impco / Nikki systems, 9–12 psi for Rochester. <2 psi = primary regulator diaphragm ruptured (smells like propane when you remove vacuum line). >15 psi = secondary seat debris—replace vaporizer.

4.6 Vaporizer icing: if truck starts, runs 30 s, then dies, remove air-cleaner and look for frost on mixer throat. Cause: water in fuel or vaporizer coolant loop restricted. Propane boils at –42 °C; every kg of liquid needs 355 kJ of heat. A 40 % glycol mix at 70 °C can deliver 1.2 kJ/s—enough for 50 kW engine. If radiator level is 2 cm low, vaporizer becomes choked with ice within 60 s. Top coolant, purge air, retest.
4.7 Converter / mixer balance screw: turn clockwise ¼ rich = +1 psi, counter-clockwise ¼ lean = –1 psi. Mark original position with paint pen before touching. Most no-start conditions are too rich rather than too lean—propane is already gaseous and needs less enrichment than gasoline.
4.8 Electronic Pressure Regulator (EPR) on EFI trucks (e.g., Nissan K25 LP): scan for P0093 “fuel pressure range.” If desired pressure 300 kPa, actual 140 kPa, replace in-tank pump (looks like gasoline pump but calibrated for LP). Do not substitute gasoline pump—LP dissolves gasoline pump armature varnish and kills pump in 50 h.

Domain C: Air & Mechanical—The Forgotten 7 %

5.1 Air-filter differential: manometer across filter must be <5” H₂O. A filter that looks merely “gray” can drop 12” H₂O and cause rich mixture, carbon fouled plugs, and no-start.
5.2 Valve lash: intake 0.15 mm, exhaust 0.20 mm for most GM 4-cylinder industrial engines. Tight exhaust valves leak combustion gas → low compression → hard hot start. Check when engine is cold; adjust with feeler gauge and 10 mm wrench.
5.3 Compression test: minimum 140 psi, max variation 15 % between cylinders. Propane burns 100 octane; if compression <130 psi the fuel won’t ignite reliably. Top cause: burnt exhaust valve seat from chronic lean condition (see §4.7).
5.4 Timing belt / chain: one tooth off = starts cold, refuses hot. Propane has 20 % slower flame speed than gasoline; incorrect timing exaggerates the issue. Verify with dial indicator on #1 cylinder and match mark to crank pulley.
5.5 Intifold manifold leak: spray carb cleaner around base while cranking. RPM rise = leak. Propane is very sensitive to false air because the ECU (or mixer) has no liquid fuel enrichment to mask the lean spike.

Domain D: Control & Safety Interlocks (EFI & Tier-4)

6.1 Oxygen sensor: EFI LPG trucks use wide-band 5-wire LSU 4.9. If sensor heater fails, ECU defaults to 9.7:1 AFR (rich), floods engine at start. Scan for P0135. Resistance across heater pins 2–5 = 2.4–3.0 Ω cold.
6.2 MAP sensor: with key ON engine OFF expect 95–100 kPa (sea level). A cracked vacuum line reads 40 kPa → ECU thinks WOT → delivers max fuel → no-start.
6.3 Neutral-start switch (NSS): automatic transmission trucks only. If NSS fails open, starter relay is disabled—cranks in neutral but not park, confusing operator. Bridge pins 5–6 in transmission connector for test.
6.4 Flame-out relay (Japan-spec Toyota): if exhaust temp <120 °C within 30 s of crank, relay cuts ESS. Purpose prevents raw LP entering catalytic converter. Faulty thermistor causes good engine to be killed. Measure thermistor: 2.2 kΩ at 20 °C, 200 Ω at 200 °C.

Cold-Weather Annex (Below –10 °C)

7.1 Use winter-grade propane (70 % butane max) to maintain vapor pressure >45 psi at –20 °C.
7.2 Install 50 % glycol engine coolant heater (1 kW) and circulate for 30 min before shift. Hot coolant prevents vaporizer freeze-up.
7.3 Switch to 5W-30 engine oil; 15W-40 becomes tar at –25 °C and drags cranking speed <80 rpm—insufficient for magneto pick-up.
7.4 Keep battery on float charger; at –18 °C battery loses 40 % cranking amps. A 100 Ah battery delivers only 60 Ah

Hot-Weather / High-Altitude Annex

8.1 Above 1,500 m air density drops 15 %; reduce mixer main jet one size or increase EFI fuel map 6 %.
8.2 Ambient >40 °C raises tank pressure to 150 psi; install 2-stage regulator with integral relief set at 125 psi to prevent flooding.
8.3 Ventilated overhead guard reduces under-hood 15 °C, prevents vapor-lock in supply hose.

Quick-Reference No-Start Matrix

Symptom → Most Likely Cause → 60-Second Check

Cranks slow, dim lights → Weak battery (<9.8 V)

Cranks fast, no spark → Coil or ESS open circuit

Spark present, zero fuel pressure → Tank empty or POL blockage

30 psi static, 0 psi while cranking → Primary regulator diaphragm torn

Starts, dies in 30 s → Vaporizer iced (coolant low)

Starts cold, refuses hot → Tight exhaust valve / leaking injector seat

Backfire through intake → Mixer too lean (false air)

Backfire through exhaust → Mixer too rich (adjustment screw CW excess)

Tools Every Facility Should Own

0–30 psi propane pressure gauge with ¼” NPT male adapter

In-line spark tester (adjustable 0–40 kV)

Digital multimeter with min/max record

Compression gauge with 14 mm long-reach adapter

Ultrasonic leak detector (avoids soap mess on ceiling trucks)

OEM service manual PDF on tablet offline—Wi-Fi is always down when truck dies.

When to Stop Diagnosing and Call OEM Tech Support

ECM throws manufacturer-specific codes >P0999 (propane-specific calibration)

Hydro-lock suspected (coolant in cylinder) → remove plugs, bar engine by hand first

Internal vaporizer coolant leak → propane odor in radiator header tank

Crankcase over-full, fuel dilution >4 % (oil smells like LP) → rings or converter seat failure

Any second-stage fire during testing → facility emergency protocol overrides this guide.

Conclusion: Think Like Vapor, Act Like Liquid

Propane is the only industrial fuel that arrives as a liquid, instantly becomes a vapor, and expects the engine to behave as if it were gasoline. A no-start is almost never “mysterious”; it is simply one of the four domains refusing to transition state correctly. Follow the numbers—pressure, voltage, compression, spark gap—and the truck will confess its sins in minutes. Ignore the numbers, throw filters and plugs at random, and you will still be crank-crank-cranking when the shift change walks in.

Remember: the forklift is not stubborn; it is a thermodynamic equation wearing steel shoes. Solve the equation, and the engine starts—every time, first time.