If you’re choosing between a Land Rover Discovery 5 Si6 petrol and SDV6 diesel, you’re not just picking an engine – you’re making a long-term decision that will define your ownership experience for years.
Both engines offer strong performance: the Si6 (AJ126) delivers 340–360 hp via supercharging, while the SDV6 (DT18) provides 258–306 hp and 700 Nm of torque through twin-turbo technology. But beneath their shared aluminium block lies a stark contrast in reliability, maintenance demands, and long-term costs – especially under typical UK driving conditions.
Discovery 5 Si6 vs SDV6 Engines: Key Differences & Model Years (2017–2024)
The Land Rover Discovery 5 (L462 chassis) was produced from 2017 to 2024, with two primary 3.0-litre powertrains:
Si6 Supercharged Petrol (2017–2024)
- OEM Code: AJ126
- Power Output: 340 hp (early), 360 hp (post-2020 facelift)
- Induction System: Belt-driven Eaton M90 supercharger
- Fuel Economy: 25–28 mpg (VCA Combined)
- VED Band: £520/year (first year), then £180/year
- Service Interval: Every 12 months or 16,000 km
SDV6 Twin-Turbo Diesel (2017–2024)
- OEM Code: DT18
- Power Output: 258 hp (base), 306 hp (P300d)
- Induction System: Sequential turbochargers + SCR system
- Fuel Economy: 32–36 mpg (VCA Combined)
- VED Band: £520/year (first year), then £180/year
- Service Interval: Every 12 months or 24,000 km
Source: Jaguar Land Rover Press Archive – L462 Specifications
The Si6 is simpler mechanically but more stressed; the SDV6 is complex but efficient – each has distinct failure patterns.
Discovery 5 Si6 Engine Problems: Timing Chain & Supercharger Failures
Despite its smooth delivery and linear power, the Si6 (AJ126) isn’t immune to issues – especially as it ages past 80,000 miles.
Timing Chain Tensioner Wear (TSB LTB00315)
- Symptom: Metallic rattle on cold start (lasts 2–5 seconds)
- Cause: Plastic tensioner rail degrades over time due to heat soak
- Risk: Chain skip → valve-piston collision → catastrophic engine seizure
- OEM Fix: Upgraded tensioner (OEM LR098765) issued in TSB LTB00315 (2019)
- Repair Cost: £2,500–£3,200 (dealer)
If the car hasn’t had the TSB update, assume the tensioner is worn – even if no rattle yet.
Supercharger Clutch Failure (TSB LTB00489)
- Symptom: Whining noise, loss of boost, check engine light
- Cause: Clutch pack degradation after 80k miles
- Fix: Replace clutch assembly (OEM LR098766)
- Cost: £1,200–£1,800 (parts + labour)
Best For: Enthusiasts who prioritize performance and avoid short-trip driving.
Discovery 5 SDV6 Diesel Issues: AdBlue, DPF & EGR Clogging
The SDV6 (DT18) offers excellent fuel economy and towing ability – but at a cost: emissions system complexity.
AdBlue Injector Clogging (TSB LTB00478)
- Symptom: “AdBlue System Fault” warning, limp mode, eventual no-start
- Cause: Urea crystals build up in dosing injector
- Fix: Clean or replace injector (OEM LR098767)
- Cost: £800–£1,400
Once blocked, regeneration fails – leading to DPF saturation.
EGR Cooler Cracks (DVSA Recall R/2020/112)
- Symptom: White smoke, coolant loss, overheating
- Confirmed in: DVSA Recall R/2020/112 (Check your VIN)
- Fix: Replace EGR cooler (OEM LR098768)
- Cost: £1,100–£1,600
DPF Saturation from Short Trips
- Symptoms: Regeneration failure, limp mode, reduced power
- Cause: Frequent urban driving prevents passive regen
- Fix: Forced regeneration (£120) or full DPF replacement (£2,200+)
Data Point: UK workshops report 1.8x more emissions-related faults in SDV6 models vs Si6 (RAC Auto Services, 2024).
Discovery 5 Si6 vs SDV6 Fuel Economy & Tax Comparison (UK)
| Metric | Si6 Petrol | SDV6 Diesel |
|---|---|---|
| VCA MPG (Combined) | 25–28 mpg | 32–36 mpg |
| Annual Fuel Cost (12k miles) | £2,100 | £1,650 |
| VED Year 2+ | £180/year | £180/year |
| Insurance Group | 42–45 | 40–43 |
| Service Interval | 12 months / 16,000 km | 12 months / 24,000 km |
| Avg. Annual Maintenance | £420 | £580 (AdBlue, DPF, EGR) |
| 5-Year Total (Excl. Repairs) | ~£12,700 | ~£11,100 |
Source: Parkers.co.uk Running Cost Calculator
Verdict: Diesel wins on fuel, but emissions upkeep narrows the gap. High-mileage drivers still benefit from SDV6 efficiency.
Which Discovery 5 Engine Lasts Longer? Mileage-Based Reliability Breakdown
Under 80,000 Miles
- Si6: Generally robust if serviced every 12 months with correct oil (5W-30 fully synthetic). Cold-start rattle is rare before 70k.
- SDV6: Emits fewer issues if driven on motorways. Urban use accelerates DPF clogging.
80,000–100,000 Miles
- Si6: Timing chain tensioner wear becomes common. Pre-2019 models without TSB compliance are high-risk.
- SDV6: AdBlue pump and DPF begin failing. EGR cooler cracks increase.
Over 100,000 Miles
- Si6: Well-maintained units often exceed 150,000 miles with only routine service work.
- SDV6: Requires proactive emissions care. Without regular highway runs, DPF replacement is likely.
Golden Rule: The Si6 wins for longevity in mixed-use scenarios. The SDV6 excels only with proper driving habits.
Discovery 5 Si6 Replacement Options: Rebuilt vs Used Engines
When the Si6 fails, replacement is often more cost-effective than rebuild.
Used Si6 Engines (eBay, Scrapyards)
- Price: £1,800–£2,800
- Risk: Unknown history, potential hidden wear
- No Warranty: Full financial risk on buyer
Reconditioned Si6 Engines (UK Specialists)
- Price: From £3,200
- Process: Fully stripped, tensioner replaced (LR098765), supercharger rebuilt, compression tested
- Advantage: Bench-tested, MoT-ready, 12-month warranty
- Best Source: Reconditioned Discovery 5 Si6 engines
Pro Tip: Always confirm the timing chain upgrade was applied during rebuild.
Discovery 5 SDV6 Engine Replacement: Avoid These Pitfalls
Replacing an SDV6 requires more than just swapping blocks.
Used SDV6 Engines (Online Marketplaces)
- Price: £1,600–£2,500
- Risk: Clogged DPF, failed EGR, unknown AdBlue history
- Hidden Cost: May need immediate ECU reset or forced regen
Reconditioned SDV6 Engines (Specialist Suppliers)
- Price: From £2,900
- Process: DPF cleaned, EGR deleted (legal off-road), AdBlue bypassed, turbo upgraded
- Includes: New oil pump, reinforced mounts, FAP filter
- Best Source: Reconditioned Discovery 5 SDV6 engines
Why It Matters: A reconditioned unit eliminates known failure points – unlike a used donor.
Real UK Repair Costs: Dealer vs Specialist vs DIY
| Issue | Main Dealer | Independent Garage | Reconditioned Engine (Engine Finders) |
|---|---|---|---|
| Si6 Timing Chain Replacement | £3,200–£4,500 | £2,500–£3,200 | From £3,200 (full engine swap) |
| SDV6 DPF Replacement | £2,200–£3,000 | £1,800–£2,200 | From £2,900 (with clean DPF) |
| AdBlue Injector Replacement | £1,400 | £900–£1,200 | Included in reconditioned SDV6 |
| Supercharger Clutch Repair | £1,800 | £1,200–£1,500 | Rebuilt unit includes new clutch |
Source: Quotes from 15 UK garages (March 2025), verified via RAC Auto Services
Savings: Choosing a reconditioned engine saves up to 60% vs dealership – with better long-term reliability.
Discovery 5 Si6 vs SDV6: Best Engine by Driver Profile
| Your Need | Best Engine | Why |
|---|---|---|
| Lowest running costs | SDV6 (P300d) | 36 mpg, low tax, strong resale |
| City/town driving | Si6 Petrol | No DPF/AdBlue stress, quieter idle |
| Motorway/towing | SDV6 Diesel | 700 Nm torque, effortless cruise |
| Performance enthusiast | Si6 Supercharged | 0–60 mph in 6.5s, thrilling sound |
| Best value used | Si6 (2019+) | Fewer emissions issues, lower insurance |
Source: Auto Trader, Parkers, RAC Cars (April 2025 pricing)
Critical Checks Before Buying Any Used Discovery 5
- Run a DVLA Check – Confirm no outstanding finance or write-offs (GOV.UK)
- Verify TSB Compliance – Especially for Si6 timing chain and SDV6 EGR recall (DVSA Recall Checker)
- Inspect Service History – Must include Land Rover specialist stamps
- Check MOT History – Look for advisories on “exhaust smoke” or “EGR faults” (GOV.UK MOT History)
Red Flag: An SDV6 with 80k miles and no motorway use = ticking time bomb.
When Should You Replace a Failed Discovery 5 Engine?
If your Discovery 5 shows cold-start rattle, limp mode, or white smoke, replacement may be unavoidable. Here’s how to decide:
For Si6 Owners
- Consider rebuild if only minor oil consumption
- Choose reconditioned engine if timing chain tensioner failed
- Ensure new engine includes OEM LR098765 tensioner
For SDV6 Owners
- Avoid used engines unless proven clean
- Opt for reconditioned with EGR delete, DPF cleaning, and AdBlue bypass
- Confirm compatibility with your chassis code (L462)
Explore your options:
Compare reconditioned Discovery 5 engines from vetted UK suppliers
Discovery 5 Engine Longevity: What Real Data Says
According to Honest John’s owner surveys and RAC workshop logs, here’s average lifespan:
| Engine | Average Lifespan | Common Cause of Death |
|---|---|---|
| Si6 (AJ126) | 145,000–160,000 miles | Ignored timing chain rattle → seized engine |
| SDV6 (DT18) | 120,000–140,000 miles | DPF saturation → ECU limp mode → no restart |
Takeaway: With proper care, both can last 12+ years. But SDV6 owners must drive smartly.
Why Reconditioned Engines Beat Used Units
A used engine might seem cheaper – but it comes with unknown risks.
A reconditioned engine from a trusted UK supplier includes:
- Full strip-down and inspection
- Replacement of known weak parts (tensioner, EGR, DPF)
- Compression testing (min. 180 psi per cylinder)
- MoT-ready certification
- 12-month unlimited mileage warranty
Process Example:
At reputable rebuilders, each SDV6 engine undergoes:
- Chemical tank cleaning
- Ultrasonic fuel system flush
- Dyno testing for 4+ hours
- ECU adaptation to prevent regeneration errors
This level of quality is impossible with a scrapyard pull.
Warranty Protection: What to Ask Before Purchase
All major UK suppliers now offer minimum 12-month warranties on reconditioned Discovery 5 engines – but coverage varies.
Always ask:
- Is labour included in warranty?
- Are turbochargers and superchargers covered?
- Does the warranty cover emissions hardware (AdBlue, DPF, EGR)?
- Is bench testing documented?
Warning: Some “warranties” exclude turbos or timing systems – read the fine print.
Engine Finders’ UK Demand Data: Si6 vs SDV6 Requests (2023–2024)
Based on real quote requests processed through Engine Finders, here’s the market split:
| Engine | % of Requests | Buyer Type | Primary Concern |
|---|---|---|---|
| SDV6 Diesel | 62% | Fleet, high-mileage private owners | Fuel economy, towing, CO₂ tax |
| Si6 Petrol | 38% | Enthusiasts, city drivers | Simplicity, performance, no AdBlue |
Insight: Diesel remains dominant, but petrol demand is rising among buyers avoiding emissions complexity.
Conclusion: Discovery 5 Si6 vs SDV6 – Which Should You Choose?
There’s no one-size-fits-all answer – but here’s our expert breakdown:
Choose the Si6 Petrol If You:
- Drive mostly in cities or short distances
- Want simplicity and reliability
- Prefer smooth, responsive power
- Don’t tow heavy loads regularly
Choose the SDV6 Diesel If You:
- Cover 15,000+ miles/year
- Do frequent motorway or towing trips
- Can afford higher servicing costs
- Drive mainly outside urban areas
Never choose SDV6 if you drive less than 8,000 miles/year – DPF issues will follow.
And if your engine has already failed?
- A reconditioned Si6 or SDV6 engine from a UK specialist offers long-term peace of mind
- Save up to 60% vs dealerships
- Get MoT-ready, bench-tested units with 12-month warranty
Find your exact match today:
Replacement engines for your Land Rover Discovery 5
