118001

What the Code Means

118001 — lean mixture on cold start, logged by a positive long-term fuel correction (long fuel trim). The code is read from the DME together with 11CF30.

In the case described, the failure mechanism is this: the high-pressure fuel pump (HPFP) leaks at the shaft, and gasoline gets into the engine oil. At high operating temperature the fuel evaporates out of the oil and enters the intake through the crankcase ventilation system — when hot, the mixture goes rich and the DME builds a negative correction. That correction stays in memory, so on the next cold start, when the oxygen sensor joins mixture formation, the mixture turns out lean: long fuel trim swings strongly positive. If the add-on exceeds 25%, the code sets or the check engine light comes on.

The characteristic paradox of this picture: when hot, the corrections are small, under load they approach ideal, and the car drives fine with no hesitation. The deviations show up only off load, at a small amount of working mixture.

Other candidate causes for this symptom pattern include the mass air flow sensor, the oxygen sensor, the tank vent valve, intake leaks and exhaust cracks, the injectors, carbon buildup on the valves, and the HPFP itself.

Symptoms

• Code 118001 appears on cold start, after startup, when the oxygen sensor begins to participate in mixture formation.
• Long fuel trim swings strongly positive — up to 25%; above that threshold the code sets or the check engine light comes on.
• When hot, the corrections are small, under load they tend toward ideal — deviations only off load, at a small amount of working mixture.
• The smell of gasoline from the oil filler neck — the key sign of an HPFP shaft leak; the cap itself may not smell, the smell comes from the oil in the neck.
• A companion code for valve position: the valve opens about 0.3 mm (adaptation limit) — a consequence of the DME trying to normalize the mixture.
• No obvious hesitation or loss of power: the car runs and drives normally.

After flashing the stock tune and warming up to stock operating temperature (100+ °C), the picture flips: short fuel trim swings deeply negative (mixture too rich), and codes 118002, 118402, and 133710 also appear.

Diagnosis and Repair

Diagnosis runs from simple to complex, in order of likelihood:

1. Check the intake connections, tighten the clamps — rule out an air leak.
2. Check the tank vent valve. In the case described, replacement made no difference — a version 06 valve was already fitted.
3. Check the mass air flow sensor against the logs. In the case described, replacing it with a Bosch 0280218324 did not fix the problem.
4. Flash the stock tune and warm the engine to stock operating temperature (100+ °C). At the reduced temperature only one lean-mixture code showed up; after switching to stock, the opposite picture appeared — rich mixture when hot (short fuel trim deeply negative) and codes 118002, 118402, 133710. This step helps you see the real state of mixture formation without the influence of custom calibrations.
5. Smell the oil, for example at the next oil change: there should be no strong gasoline smell. In the case described, the smell was noticeable right at the oil filler neck — the main sign of an HPFP shaft leak.
6. Check the HPFP for a shaft leak. If confirmed, replace the pump, OEM part number 13517642466. In the case described, a used HPFP with 44,000 km (27,000 miles) was installed, with a warranty when replaced at the shop.
7. After replacing the HPFP, always change the engine oil — gasoline has accumulated in it.

No adaptation is needed when replacing the HPFP: the pump is mechanical, unmanaged, with two contacts. Resetting the mixture corrections is optional — the corrections build up quickly.

After the new HPFP went in, the corrections normalized almost immediately and the codes no longer appear.

Secondary Problems

• Gasoline entering the engine oil from the HPFP shaft leak — the oil thins out and loses its lubricating properties. After the fault is fixed, the oil must be changed.
• A companion code for valve position (opening about 0.3 mm, adaptation limit) — a consequence of the DME trying to normalize the mixture with corrections.