Chapter 21 · Fuel System & Injector Mathematics

Cold Start Enrichment

The four stages of cold start and retuning them for big injectors or E85.

Calibration symbols

KFZWTVS KRKTE TEMIN DFPM FKST FR_W J299 N112 TMOT FAW FST SLS Z19

ECUs covered

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13 symbols · 0 diagrams · ~839 words

The four stages of cold start and retuning them for big injectors or E85.

Starting a cold internal combustion engine presents a severe thermodynamic challenge. At low ambient temperatures (T_{\text{ambient}} < 20\text{°C}), fuel injected into the intake manifold does not vaporize readily; instead, heavy fuel droplets adhere to the cold intake runner walls and intake valves, forming a static liquid fuel puddle (Wandfilm).

To ensure that sufficient vaporized fuel reaches the spark plug gap to initiate combustion, Bosch ME7.5 executes a four-stage cold-operation sequence: Starter Cranking Enrichment (FST), After-Start Flare Enrichment (FAW), Catalyst Heating Ignition Retard (KATHEIZUNG), and Warm-Up Closed-Loop Transition (KFFWL).

BOSCH ME7.5 COLD START & WARM-UP FUELING TIMELINE

Phase 1: Cranking Starter Engaged Nmot < 450 rpm	Phase 2: Flare Engine Flare-up Nmot > 600 rpm	Phase 3: Cat Heating Active SLS Active	Phase 4: Decay Warm-Up Ramp 20°C to 80°C	Phase 5: Normal Closed-Loop Lambda Tmot > 80°C
• Heavy enrichment	• Peak enrichment	• Retarded spark	• Exponential	• Stoichiometric
• FST / FKST maps	• FAW multiplier	• KFZWTVS (-15°)	decay curve	feedback (λ=1.0)
• Injection per rev	• High idle RPM	• Secondary air	• TFAW timer	• Adaptive LTFT

21.1. The Four Stages of Cold Operation#

1. Stage 1: Cranking Enrichment (`FST` & `FKST`)#

Activation Boundary: N_{\text{mot}} < 450\text{ rpm} during starter motor engagement.
- At -20\text{°C}, FST commands up to 400\% to 600\% extra fuel to compensate for fuel wall condensation.
- At +80\text{°C} (hot restart), FST drops to 1.00 (zero cold enrichment).
Physical Operation: The ECU triggers all fuel injectors synchronously to wet the intake runners.
Calibration Multiplier: Base injection duration is multiplied by coolant-dependent factor FST: $t_{\text{start}} = t_{\text{base}} \times \mathbf{FST}(T_{\text{mot}}) \times \mathbf{FKST}(T_{\text{mot}})$

2. Stage 2: After-Start Flare Enrichment (`FAW` & `TFAW`)#

Activation Boundary: Engine catches and flares above 600\text{ rpm}.
Physical Operation: The engine transitions from synchronous cranking pulses to sequential injection timed to the intake stroke.
Duration Timer (TFAW): Maintains enriched fueling for 10 to 45 seconds depending on starting coolant temperature (T_{\text{mot}}).
Decay Dynamics: Fueling decays exponentially toward normal idle targets to prevent spark plug fouling while combustion chamber temperatures rise.

3. Stage 3: Catalyst Heating (`SLS` & `KATHEIZUNG`)#

Emissions Intent: Modern Euro 4 and US ULEV standards require the catalytic converter to reach its light-off temperature (350\text{°C}) within 30 seconds of starting.
- Ignition angle is retarded to -12.0\text{°} to -18.0\text{° ATDC} (firing late after Top Dead Center).
- Burning combustion gases exit through the open exhaust valves into the exhaust manifold at over 800\text{°C}.
- The unburned hydrocarbons in the rich exhaust gas mix with the secondary air, igniting an exothermic oxidation reaction directly upstream of the catalytic converter, rapidly heating the ceramic core.
Secondary Air Injection (SLS):
Severe Ignition Retard (KFZWTVS):

4. Stage 4: Closed-Loop Lambda Transition#

Sensor Heating: The wideband LSU 4.2 sensor heater (Z19) is driven at 100\% PWM duty until internal ceramic impedance drops below 80\ \Omega (indicating 750\text{°C} operating temperature).
Control Handshake: Once CJ125 status flag B_lsu_rdy = 1 is asserted and coolant exceeds 40\text{°C}, closed-loop PI feedback (fr_w) activates, trimming fueling to \lambda = 1.00.

21.2. Tuning Cold Start for Large Injectors & E85 Ethanol Conversions#

COLD START RECALIBRATION: GASOLINE VS. E85 ETHANOL

Parameter / Map	Stock Gasoline (AWP)	E85 Ethanol Conversion	Large Injectors (1000cc)
FST (Crank Fuel)	Base (100%)	+80% to +120% (<15°C)	-60% (Scaled for KRKTE)
TFAW (Decay Time)	15–30 Seconds	45–60 Seconds	10–20 Seconds
ZWKAL (Crank Spark)	+5.0° BTDC	+10.0° to +12.0° BTDC	+5.0° BTDC
NMIN (Cold Idle)	1100–1200 rpm	1250–1350 rpm	950–1050 rpm

1. The E85 Cold-Start Crisis#

Pure ethanol has a substantially higher latent heat of vaporization (840\text{ kJ/kg} vs. 350\text{ kJ/kg} for gasoline) and virtually zero vapor pressure below 15\text{°C} (59\text{°F}). If an E85-converted engine is started on factory cranking maps in near-freezing weather, the fuel cannot vaporize, resulting in prolonged cranking, battery depletion, and fuel washing down the cylinder bores into the engine oil.

The E85 Calibration Fix:
1. In FST (Cranking Enrichment Table at 0x0182C0), double the fuel values below 15\text{°C}.
2. Advance cranking ignition angle ZWKAL from +5\text{°} to +11\text{° BTDC} to provide earlier flame initiation for slower-burning ethanol.
3. Increase TFAW decay timer constants by 50\% to maintain warm-up stability.

2. Large Injector Rescaling Trap#

When installing 1000cc injectors, KRKTE is reduced to approximately one-third of stock (0.03119 vs 0.09845). However, because TEMIN (Minimum Pulse Width) clamps injection duration, if FST is left at factory scale, cranking pulse widths can saturate against hardware delivery minimums, causing the engine to flood instantly on hot restarts.

The Large Injector Fix: Ensure TEMIN is lowered to 0.38\text{ ms} and reduce FST hot-temperature multipliers (> 60\text{°C}) to prevent hot-cranking flooding.

Cross-referenced on shared calibration symbols, not on subject matter — these are the chapters that touch the same maps.