BMW 3 Series 320si2005–2006N45B20S 2.0

BMW 3 Series 320si Engine Replacement —Compare Prices &
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Name: BMW 3 Series 320si UK Engine Enquiry Data 2025
Creator: BMWEngines.uk

Get quotes today from trusted UK BMW 320si specialists. Compare reconditioned, rebuilt and used N45B20S hand-built four-cylinder engines — supply & fit available nationwide for this rare homologation special.

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BMW 320si Engines — from £2,000 to £7,500Engine code: N45B20S (2005–2006)

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The 320si Through Time — Era Map

One generation. One era. One guide to reliability, costs and what really matters.

Era Note

WTCC homologation special — 173 bhp, 8,000 rpm redline, carbon fibre valve cover; timing chain and oil consumption are the defining weak points

Generation	Years	Engine Code	Reliability	Recon Cost (Guide)	Era Note
3 Series E90	2005–2006	N45B20S(2.0L 4-cyl, hand-built)		£4,500–£7,500[PLACEHOLDER]	WTCC homologation special — 173 bhp, 8,000 rpm redline, carbon fibre valve cover; timing chain and oil consumption are the defining weak points

Why use this era map?

Every cost guide, reliability rating and engine code on this page is aligned to the data above. This keeps everything consistent and trustworthy.

Real UKCost Data
VerifiedReliability
ExpertInsights
Updated for2025

BMW 3 Series 320si Engine Replacement Costs

Supply Only£4,500 – £7,500
Fitted (Indie)£5,800 – £9,000
Warranty12–24 months
Best ForMost owners — specialist rebuild with N45 expertise required

Engine Type	Supply Only	Fitted (Indie)	Warranty	Best For
Used Engine	£2,000 – £4,000	£3,000 – £5,200	1–6 months	Budget priority; finding a donor is increasingly difficult
Reconditioned Engine	£4,500 – £7,500	£5,800 – £9,000	12–24 months	Most owners — specialist rebuild with N45 expertise required
Rebuilt Engine	£6,000 – £9,000	£7,500 – £10,500	12–24 months	Long-term ownership; full factory-spec refresh with upgraded components
Main Dealer New	N/A — unavailable	N/A	-	Engine no longer produced; NLA from BMW

Figures above reflect the E90 320si N45B20S era. This engine was produced only in 2005–2006, and parts availability is increasingly limited.

Labour estimate: Fitting a 320si engine typically takes 10–14 hours at £75–£110/hr (independent specialist) or £130–£180/hr (London/SE dealer). [PLACEHOLDER — confirm vs cost sheet]

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Common Problems With BMW 3 Series 320si Engines

1. Timing Chain Tensioner Failure(N45 Family)

Symptoms: Rattling noise from the engine on cold start, metallic clattering, engine warning light, loss of power, potential engine failure if chain skips or snaps.
Typical Mileage: 60,000–100,000 miles (100,000 km is a known threshold)
Repair Cost: £800–£2,500 (timing chain, tensioner and guide replacement)
Replacement Cost: £4,500–£7,500 (if chain snaps and valves meet pistons)
Urgency: Immediate — if you hear rattle, book an inspection today

Our recommendation:

The N45 family is notorious for timing chain problems. The original tensioner was too short, allowing the chain to become slack and rub against the guides. By 100,000 km, the tensioner can fail completely. This is the single biggest reliability concern on the 320si. A cold-start rattle is the early warning sign — ignore it and you risk piston-valve contact and a full engine replacement. Given the rarity of the N45B20S, sourcing a replacement engine is increasingly difficult. Use only BMW-approved 5W-30 oil and reduce oil change intervals to 8,000 miles.

1. Timing Chain Tensioner Failure(N45 Family)

Symptoms: Rattling noise from the engine on cold start, metallic clattering, engine warning light, loss of power, potential engine failure if chain skips or snaps.
Typical Mileage: 60,000–100,000 miles (100,000 km is a known threshold)
Repair Cost: £800–£2,500 (timing chain, tensioner and guide replacement)
Replacement Cost: £4,500–£7,500 (if chain snaps and valves meet pistons)
Urgency: Immediate — if you hear rattle, book an inspection today

Our recommendation:

2. Oil Consumption(N45 Family)

Symptoms: Low oil warning light, needing to top up oil frequently (1L per 1,000 miles or more), blue smoke from exhaust, ticking noise from engine.
Typical Mileage: 40,000–80,000 miles
Repair Cost: £150–£300 (regular oil top-ups and monitoring) — £1,000–£3,000+ (valve stem seal or piston ring replacement)
Replacement Cost: £4,500–£7,500 (if oil starvation leads to engine damage)
Urgency: Monitor — book inspection soon; monitor oil levels weekly

Our recommendation:

Oil consumption in the BMW N45 engine is a common issue with several potential causes, ranging from relatively minor seal failures to more significant internal wear. Worn valve stem seals, piston ring and cylinder wall wear, and crankcase ventilation system (CCV) diaphragm failure are all known causes. Check your oil level regularly — this engine is not one you can neglect. BMW strongly discourages the use of any alternative engine oil, as the consequences can be very serious.

3. VANOS System Failure(N45 Family)

Symptoms: Rough idle, hesitation on acceleration, loss of power at low RPM, reduced fuel economy, check engine light.
Typical Mileage: 60,000–100,000 miles
Repair Cost: £300–£800 (VANOS solenoid or seal replacement)
Replacement Cost: N/A — repair only
Urgency: Monitor — book inspection soon; can affect variable valve timing

Our recommendation:

The N45B20S uses double VANOS variable valve timing. Idle vibration is a common symptom, with the most frequent cause being the VANOS system. Cleaning the VANOS unit is a common repair measure. If you experience rough idle or hesitation, have the VANOS system inspected promptly.

4. Fuel Detonation & Poor Running

Symptoms: Engine knocking or pinging under load, loss of power, poor fuel economy, rough running.
Typical Mileage: Any mileage — fuel-quality dependent
Repair Cost: £200–£500 (fuel system cleaning, injector service, ECU reset)
Replacement Cost: N/A — repair only
Urgency: Monitor — book inspection soon; can cause engine damage if ignored

Our recommendation:

The N45 series has an obvious drawback: detonation of the fuel mixture. The problem arises from poor quality fuel or oil. BMW strongly recommends using only high-quality premium unleaded fuel and the correct grade of BMW-approved engine oil. Using the wrong oil can have very serious consequences, potentially requiring major repairs.

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320si — Repair, Buy, or Replace?

Not every issue means a new engine.Here’s what can be repaired, when it makes sense, and what to check before you buy.

Can It Be Repaired?

Problem	Repairable?	Typical Cost	When It Makes Sense
Timing chain tensioner wear (early)	Yes	£800 – £2,500	Preventative — far cheaper than catastrophic failure; essential on this engine
Oil consumption	Manageable	£150 – £3,000+	Monitor levels; valve stem seals are a known issue; full rebuild may be needed
VANOS system failure	Yes	£300 – £800	Solenoid or seal replacement is straightforward
Fuel detonation	Yes	£200 – £500	Fuel system cleaning and using premium fuel
Snapped timing chain (bent valves)	Usually No	Engine replacement	Piston-valve contact means little is left to repair; replacement engines are rare

ProblemRepairable? · Typical Cost

Timing chain tensioner wear (early)

Yes£800 – £2,500

Preventative — far cheaper than catastrophic failure; essential on this engine

Oil consumption

Manageable£150 – £3,000+

Monitor levels; valve stem seals are a known issue; full rebuild may be needed

VANOS system failure

Yes£300 – £800

Solenoid or seal replacement is straightforward

Fuel detonation

Yes£200 – £500

Fuel system cleaning and using premium fuel

Snapped timing chain (bent valves)

Usually NoEngine replacement

Piston-valve contact means little is left to repair; replacement engines are rare

Buying This BMW 320si? Check These First

Buy if:

Full service history with documented oil changes using BMW-approved 5W-30 oil
No cold-start rattle — timing chain tensioner should have been replaced proactively
No oil consumption issues — check dipstick regularly
VANOS system functioning correctly (smooth idle, no hesitation)
Carbon fibre valve cover intact (unique to the 320si — expensive to replace)

Avoid if:

No proof of regular oil changes — the N45B20S is unforgiving of neglect
Cold-start rattle present — timing chain work is imminent and expensive
Oil consumption of 1L per 1,000 miles or more — internal wear likely
Any engine warning light without diagnosis — the N45B20S can hide expensive faults
Poor quality fuel used — detonation damage may have occurred

Closing verdict

The BMW 320si is a genuine homologation special — a future classic with a hand-built engine, M Sport body kit, and motorsport DNA. It's one of the most affordable homologation specials money can buy. However, the N45B20S engine demands respect: timing chain tensioner failure, oil consumption, and VANOS issues are the key weak points. Parts availability is increasingly difficult. Buy on service history, maintain on a shortened schedule, and this is a future classic; neglect it, and you'll face expensive repairs with limited parts availability.

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Buying Checklist

Before you buy a used BMW 320si (or before you commit to an engine replacement), check these:

1. Cold Start
Listen for rattle lasting more than 1–2 seconds (timing chain tensioner warning — critical on N45 engines)
2. Service History
Oil changes every 8,000 miles using BMW-approved 5W-30 oil — essential for this engine
3. Oil Level
Check dipstick; excessive consumption indicates valve stem seal or piston ring wear
4. VANOS Operation
Smooth idle? Any hesitation on acceleration?
5. Coolant Level
Check cold and hot; any loss indicates cooling system issues
6. Engine Running
Any knocking or pinging? Fuel detonation is a known issue
7. Engine Warning Light
Have codes been read? Timing, VANOS, or fuel system codes are most common
8. Carbon Fibre Valve Cover
Check for cracks or damage (unique to the 320si — expensive to replace)

Why this matters

A 10-minute check now can save you thousands later. The right 3 Series 320si is a superb long-term car — the wrong one can be expensive fast.

Buying Checklist

Before you buy a used BMW 320si (or before you commit to an engine replacement), check these:

1
Cold Start
Listen for rattle lasting more than 1–2 seconds (timing chain tensioner warning — critical on N45 engines)
2
Service History
Oil changes every 8,000 miles using BMW-approved 5W-30 oil — essential for this engine
3
Oil Level
Check dipstick; excessive consumption indicates valve stem seal or piston ring wear
4
VANOS Operation
Smooth idle? Any hesitation on acceleration?
5
Coolant Level
Check cold and hot; any loss indicates cooling system issues
6
Engine Running
Any knocking or pinging? Fuel detonation is a known issue
7
Engine Warning Light
Have codes been read? Timing, VANOS, or fuel system codes are most common
8
Carbon Fibre Valve Cover
Check for cracks or damage (unique to the 320si — expensive to replace)

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 3 Series 320si pitfalls.

Engine Codes

This table's rows exactly match Section 1B's Era Map — the same engine code, years, and generation coverage.

N45B20S (hand-built)

Years: 2005–2006
Power: 173 bhp (127 kW) @ 7,000 rpm · 200-210 Nm @ 4,250 rpm
2025 Enquiries: —(pattern data; rare engine)[PATTERN DATA]
Avg. Recon Cost: £4,500–£7,500[PLACEHOLDER]

Engine Code	Years	Power	2025 Enquiries	Avg. Recon Cost
N45B20S (hand-built)	2005–2006	173 bhp (127 kW) @ 7,000 rpm · 200-210 Nm @ 4,250 rpm	—(pattern data; rare engine)[PATTERN DATA]	£4,500–£7,500[PLACEHOLDER]

Technical SpecsN45B20S

Displacement: 1,997cc
Cylinders: 4-cylinder inline, 16 valves, DOHC
Bore x Stroke: 85mm x 88mm (unique to this engine)
Compression Ratio: 11.0:1 — 11.1:1
Fuel System: Multi-Point Injection (MPI)
Valve Control: Double VANOS, no Valvetronic
Valve Cover: Carbon fibre
Redline: 7,300 rpm — 8,000 rpm

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Market Intelligence

Real data from thousands of 3 Series 320si enquiries in 2025.

Most Common FailureTiming chain tensioner failure

45%

of N45-related enquiries cite chain or timing concerns

Average Enquiry Mileage

60,000–90,000

miles

[PATTERN DATA]

Average Replacement Cost

£6,000

reconditioned, supply only

[PLACEHOLDER]

Most Requested Engine Code

N45B20S

Rare engine with limited production (2,600 units worldwide)

Peak Enquiry Season

Winter

Cold starts expose timing chain issues

[PATTERN DATA]

Regional Demand

South East: 22%
London: 18%
South West: 14%
Other Regions: 46%

[PATTERN DATA]

Regional Demand Breakdown

18%London22%South East14%South West46%Other Regions

Frequently Asked Questions

A reconditioned BMW 320si (N45B20S) engine typically costs £4,500–£7,500 supply only. Fitting adds £1,000–£1,800 at independent specialist rates. Finding a replacement engine is increasingly difficult given the limited production of 2,600 units worldwide.

The N45B20S is a hand-built homologation special engine — unique, characterful, and capable of an 8,000 rpm redline. However, the N45 family has a poor reliability reputation. Common issues include timing chain tensioner failure (often after 100,000 km), VANOS problems, oil consumption, and fuel detonation. Regular maintenance is absolutely essential.

Timing chain tensioner failure (the defining weak point of the N45 family), oil consumption (valve stem seals and piston ring wear), VANOS system issues (idle vibration and hesitation), and fuel detonation from poor-quality fuel or oil. All are manageable with preventative maintenance and early detection.

The 320si is a future classic and one of the most affordable homologation specials money can buy. Values for good examples are rising. Against a well-maintained 320si (values £5,000–£15,000+), a £5,000–£6,500 reconditioned engine is 30–60% of the vehicle's value — often worth it for a solid example. The BMW scrap threshold of 70% means evaluating the whole vehicle and its rarity before committing.

The engine code N45B20S is listed on the VIN plate and vehicle documentation. The 320si was produced only in 2005–2006 with 2,600 units worldwide and 500 right-hand drive examples for the UK. If your E90 is a 320si, it has the N45B20S. Tell us your registration and we'll confirm for free.

Timing chain tensioner failure is repairable without replacing the engine — if caught early. A preventative chain, tensioner and guide replacement costs £800–£2,500. If the chain snaps and valves meet pistons, a full engine replacement (£4,500–£7,500) is usually required. At the first sign of cold-start rattle, book an inspection immediately. Note that replacement engines for the 320si are increasingly difficult to source due to the limited production run.

Disclaimer: All prices are indicative UK market ranges based on historical quote data from our network of engine specialists.
Actual costs vary by variant, engine code, damage severity and workshop location. Use our free comparison tool to get tailored quotes.
Every engine supplied through EnginesMarket carries a minimum 12-month unlimited-mileage warranty — always confirm terms with your chosen supplier.

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BMW 3 Series 320si Engine Replacement —Compare Prices &Get Quotes

The 320si Through Time — Era Map

BMW 3 Series 320si Engine Replacement Costs

Common Problems With BMW 3 Series 320si Engines