The BMW M20 B25 (256K1) is a 2,494 cc, inline — six, naturally aspirated petrol engine produced between 1985 and 1993. It belongs to BMW's M20 engine family, known for smoothness and mechanical durability, featuring a single overhead camshaft (SOHC) and Bosch Motronic fuel injection. Delivering 110 kW (150 PS) and 220 Nm of torque, it offered refined performance ideal for executive sedans and coupes of the era.
Fitted to key models including the E28 5 Series, E30 3 Series, an…
Production years 1985–1987 meet Euro 0 standards; 1988–1993 models comply with Euro 1 depending on market .
The BMW M20 B25 (256K1) is a 2,494 cc inline-six naturally aspirated petrol engine engineered for executive sedans and coupes (1985–1993). It combines SOHC valvetrain architecture with Bosch Motronic fuel injection to deliver smooth power delivery and mechanical refinement. Designed to meet early European emissions standards (Euro 0–1), it balances responsive performance with long-term durability.
| Parameter | Value | Source |
|---|---|---|
Displacement | 2,494 cc | |
Fuel type | Petrol | |
Configuration | Inline-6, SOHC, 12-valve | |
Aspiration | Naturally aspirated | |
Bore × stroke | 80.0 mm × 72.6 mm | |
Power output | 110 kW (150 PS) @ 5,800 rpm | |
Torque | 220 Nm @ 4,200 rpm | |
Fuel system | Bosch Motronic DME | |
Emissions standard | Euro 0 (pre-1988); Euro 1 (1988–1993) | |
Compression ratio | 9.8:1 | |
Cooling system | Water-cooled | |
Turbocharger | Not applicable | |
Timing system | Timing chain (single-row) | |
Oil type | Refer to vehicle handbook / manufacturer specification | |
Dry weight | 168 kg |
The BMW M20 B25 (256K1) was used across BMW's E28/E30/E34 platforms with longitudinal mounting and shared core architecture. This engine received platform-specific adaptations-revised intake manifolds in the E34 and different accessory drives in the E28-and from 1988 the facelifted E30 and E34 models adopted updated Motronic DME systems, creating interchange limits. All adaptations are documented in OEM technical bulletins.
Engine Finders UK Ltd, which operates EngineCode.uk, can pass your enquiry to engine suppliers in the UK. The service is for vehicles in the United Kingdom, with UK delivery or fitting. This is a commercially connected service, and sending an enquiry does not confirm diagnosis or compatibility.
Diagnostic reminder: If a technician has recommended replacing the engine, you can send an enquiry to engine suppliers in the UK. Confirm the diagnosis first: an engine-code record does not diagnose a fault.
Field reports and technical service bulletins highlight component wear patterns under high-load operation. Adherence to manufacturer service intervals and approved fluid specifications is critical for long-term reliability.
Analysis derived from manufacturer technical service bulletins and industry workshop records. Repair procedures should follow manufacturer guidelines.
The most common questions about engine codes, what they mean, how to find them and how this database works
Yes, the M20 B25 is known for mechanical robustness and longevity when properly maintained. Early models (pre-1988) are more prone to cylinder head cracking, especially with poor cooling system upkeep. Later Motronic-equipped versions show improved reliability. Regular oil changes, coolant service, and valve adjustments every 30,000 km are essential for long-term durability.
The most documented issues are cylinder head cracking around exhaust ports, timing chain wear, and fuel injection faults (Motronic sensors, relays). Oil leaks from the valve cover and rear main seal are also common with age. These are referenced in BMW service documentation and owner reports from the period.
The M20 B25 powered several key BMW models: the E28 5 Series (525i, 1985–1988), E30 3 Series (325i, 1987–1992), and E34 5 Series (525i, 1988–1993). It was phased out in favour of the M50 in the early 1990s. All variants used longitudinal mounting and shared core architecture with model-specific tuning.
Aftermarket ECU tuning is common on the M20B25-256K1 platform, but increasing boost or altering engine parameters accelerates component wear, impacts emissions compliance, and may void vehicle warranties. Any modifications should be evaluated carefully by a qualified specialist.
Fuel consumption is moderate by modern standards. In an E30 325i, expect approximately 10.0–12.5 L/100km (22–28 mpg UK) in mixed driving. Highway efficiency improves to ~8.5 L/100km (~33 mpg UK). Carburetted variants are less efficient; fuel-injected models with Motronic offer better metering and consistency.
Yes, the M20 series is an interference engine. If the timing chain fails or jumps, the pistons can contact the open valves, resulting in bent valves or piston damage. This makes regular inspection and timely replacement of the timing chain and guides essential to prevent catastrophic engine failure.
BMW specifies SAE 10W-40 mineral or semi-synthetic oil meeting API SF/CC standards. Use of high-quality oil is critical for timing chain lubrication and camshaft protection. Change intervals should not exceed 15,000 km or annually, with more frequent changes recommended for high-mileage or performance use.
Comprehensive technical documentation and regulatory references
EngineCode.uk is operated by Engine Finders UK Ltd. It provides engine-reference information and an optional commercial enquiry service. It is not an official manufacturer website. Check the record's sources and confirm vehicle-specific details before relying on it for parts, repairs or replacement.
Our editorial standard prioritises relevant manufacturer documentation and official regulatory sources. The sources and verification status for a record should be checked alongside the data.
A statement is not verified merely because a source is listed. Where supporting evidence is missing or conflicting, the record should identify that limitation. Please report unsupported or inconsistent information.
Transparency in gaps
A missing or unverified value does not mean the manufacturer has withheld it. We distinguish information not yet verified, information not available to us, and information explicitly not disclosed by the source.
Regulatory Stability
EU regulations are referenced using CELEX identifiers for long-term stability.
EUR-Lex
EU emissions and type-approval regulations (e.g., CELEX:32007R0715, CELEX:32017R1151).
GOV.UK: Vehicle Approval & V5C
UK vehicle approval processes, import rules, and MoT guidance.
DVLA: Engine Changes & MoT
Official guidance on engine swaps and inspection implications.
Vehicle Certification Agency (VCA)
UK type-approval authority for automotive products.
Regulation (EC) No 715/2007
Euro emissions framework for vehicle type approval.
Commission Regulation (EU) 2017/1151
WLTP and RDE testing procedures for emissions certification.
GOV.UK: Vehicle Approval
UK compliance and certification requirements for imported and modified vehicles.
VCA Certification Portal
Type-approval guidance and documentation.
Records are subject to review and correction. Source links, dates and verification notes should be read with the technical information. Our Editorial and Sourcing Policy explains our standards and review process.
Found an error or missing source? Report a problem with this page. Include the field or statement concerned and a supporting source if available. You can also email corrections@enginecode.uk.
Copyright and permissions
Images and other material may be protected by copyright. Attribution does not by itself grant permission to reuse material. If you believe material on this site infringes your rights, contact copyrights@enginecode.uk with the page URL and details of your claim. We will review the material and take appropriate action.
Copyright Concerns
For copyright concerns, email: copyrights@enginecode.uk
Our Privacy Policy explains how Engine Finders UK Ltd uses information from visits, vehicle lookups, enquiries and messages. Our Cookie Policy explains cookies and similar technologies and how to change your choices.
For access, correction, or deletion requests, email: gdpr@enginecode.uk
Trademark Notice
All trademarks, logos, and engine codes are the property of their respective owners. Use on this site is strictly for reference and identification.
Ownership and commercial services
EngineCode.uk is owned and operated by Engine Finders UK Ltd. Alongside our reference content, we offer an optional replacement-engine enquiry service through Engine Finders and may identify relevant services provided by businesses connected with us. These commercial connections are disclosed. Using an enquiry service is not required to read our reference content.
Engine Finders UK Ltd operates this website. Our optional enquiry service has a commercial purpose. Our Ownership and Commercial Disclosure explains the relationship between the reference website and that service. Participating suppliers pay a fee or subscription to participate on the network and receive quote requests.
Use this record as a starting point for identification. Specifications and vehicle applications may contain errors or omissions. Confirm the exact engine variant and compatibility using vehicle-specific documentation and a qualified supplier or technician.
All external links open in new tabs. Please verify current availability of resources.