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McLaren 675LT Common Problems: A Specialist's Notes
Independent McLaren Service — Foreign Automotive × Thorney Motorsport — Kitchener, Ontario
MCLAREN 675LT • LONGTAIL
McLaren 675LT Common Problems: A Specialist's Notes
Rear glass delamination, titanium exhaust fatigue, storage damage — notes from the first Longtail.
The 675LT was the car that brought the Longtail name back to McLaren. Built 2015–2016 in limited production — roughly 500 coupes and 500 Spiders — it took the 650S architecture, lifted output to 666 hp through forged internals and revised turbos, shaved roughly 100 kg through carbon body panels and titanium exhaust, and tightened the chassis tune to produce what is still considered one of the most rewarding McLarens ever built. The cars have appreciated. They are now collectible, often low-mileage, and frequently stored more than they are driven. That storage pattern produces a specific failure profile.
Here is what we see on 675LTs that come through our service bay, in order of frequency. Every item below comes from real cars we have worked on, not from a parts catalogue or a forum thread.
Rear glass delamination
You asked about this specifically and it is the single most documented issue on the 675LT. The rear engine cover glass is a laminated polycarbonate panel with a heat-resistant outer layer, and on cars from the early 2015 production run, that lamination separates. The visual symptom is a milky or cloudy patch on the glass that gradually spreads, starts at the edges where heat exposure is highest, and eventually covers the panel.
Once delamination starts, it does not reverse. The fix is replacement of the rear glass assembly. The part is McLaren-supply only, and at the time of writing it is back-ordered globally for many of the model years. We have customers who have waited four to six months for the replacement panel. When you find a 675LT for sale with cloudy rear glass, the seller is either disclosing it (and the price reflects the wait and the cost), or hoping the buyer does not notice on a quick walkaround. We always check this on a PPI, including with the panel cool — heat from a recent drive can temporarily mask early delamination.
Hydraulic accumulator failure
Same architecture as the 650S, same nitrogen pre-charge that bleeds down with time and heat. The 675LT's accumulator runs the gearbox, the nose lifter, the active suspension, and the active airbrake. We are now ten to eleven years out on the earliest cars, and accumulator failure is firmly in the predictive maintenance window. We replace as a preventive item rather than a wait-and-see item, because the failure mode is a no-shift, no-lift condition that strands the car.
Active aero airbrake hesitation
The 675LT's active airbrake is the same general design as the 650S but the wing surface is larger and the deployment angles more aggressive. The hydraulic actuator at the base of the wing carries higher loads as a result, and we see weeping at the actuator inlet earlier on 675LT cars than on equivalent 650S cars. Symptom: the rear wing hesitates on deployment under braking, or deploys but does not fully retract. The fix is hydraulic line and seal replacement; we have not yet had to replace a complete actuator assembly on a 675LT.
Titanium exhaust mounting fatigue
The factory titanium exhaust system on the 675LT is light, beautifully made, and develops a specific fatigue failure at the mounting points after eight to ten years on the road. The mounting brackets that locate the exhaust to the chassis are steel; the exhaust pipe is titanium. The dissimilar metal interface, combined with the thermal expansion mismatch between the two materials, eventually cracks the bracket or elongates the mounting bolt holes.
You will hear it as a faint rattle on cold start that goes away as the system warms, or as a deeper resonance in the cabin at part throttle around 3,500 rpm. The fix is bracket replacement, sometimes with helicoil repair to the chassis mounting points. We have several customer cars on which we have performed this repair. Aftermarket bracket kits that address the dissimilar-metal issue more elegantly are available from specialist suppliers in the UK; we install them when the customer prefers a long-term solution over a return to factory spec.
Gearbox weep hole drainage
The 675LT shares the 7-speed SSG with the 650S, and the same weep hole on the bellhousing serves the same diagnostic purpose: it warns of an internal coolant-to-clutch seal failure before the contamination becomes catastrophic. Caught at the weep stage, the repair is a seal replacement, a clutch pack inspection, and a flush. Ignored, it is a clutch rebuild and a much larger bill. A drip from the bottom of the gearbox on any 675LT is never something to wait on.
Door strut failure
Same dihedral door strut design as the rest of the lineup, same five-to-seven-year pressure loss curve. We replace as a pair every time. Single-side replacements come back inside a year.
IRIS 2 quirks
The 675LT shipped with IRIS 2, which is the second-generation McLaren infotainment unit and is more reliable than the original IRIS in the 12C. The remaining failures are mostly individual components: dead pixels, SD card slot read failures, Bluetooth dropouts, and the occasional reboot. Component-level repair is almost always the right answer; full head unit replacement is rarely necessary.
Coolant connector hardening
Same quick-connect fittings as the 650S. The 675LT cars are now old enough that the original connectors are reaching end of service life. We pressure-test the cooling system at every service. A connector on its way out shows up on the test before it shows up on the road.
Storage damage on low-mileage cars
The 675LT is the McLaren most likely to have been stored more than driven. We see cars with 8,000 km from 2015 that have been started twice a year and otherwise sat in a climate-controlled collection. That kind of storage is, on balance, worse for the car than regular driving would have been. What we find on under-driven 675LTs:
Brake calipers seized at the piston bores because brake fluid is hygroscopic and the moisture-laden fluid has corroded the bore surfaces. The fix is a caliper rebuild — not a part swap, an actual rebuild — and the carbon ceramic discs need careful inspection because the seized piston can have galled the disc face.
Clutch packs with flat spots on the friction faces from sitting with the same clutch surfaces in contact under spring pressure for months at a time. Symptom is judder on first engagement after the car warms up. Sometimes a careful run-in cycle resolves it. Sometimes the clutch pack needs replacement.
Hydraulic system pressure that has bled down completely and which the pump struggles to re-establish on first start. We bring stored cars back into service by running the hydraulic pump through a controlled re-pressurization cycle before any driving, to avoid the air-pocket damage that can come from trying to drive a 675LT straight off the trailer.
Tires that look new but are seven or eight years old and have flat-spotted from sitting on a single contact patch. We always check tire date codes on a 675LT PPI. Date codes more than five years out are an automatic recommendation for replacement regardless of tread depth.
Why a PPI on a 675LT is non-negotiable
The 675LT is now a $300,000–$500,000 car in Canadian pricing depending on spec and condition. The cost spread between a car that has been properly serviced and a car that has been stored badly is easily six figures of corrective work. We have inspected 675LTs that needed almost nothing and we have inspected 675LTs that needed a full hydraulic system service, a brake caliper rebuild on all four corners, rear glass replacement, exhaust bracket repair, and a clutch service before they were roadworthy. The car on the listing photo tells you almost nothing.
A proper PPI on a 675LT takes most of a day. Cold-start observation is critical. A full diagnostic scan of every module, not just the engine. A hydraulic pressure test on the accumulator. A function test on the active aero and the nose lifter. A close inspection of the rear glass under bright light. A pressure test of the cooling system. A close look at the exhaust mounting brackets. A storage data pull from the chassis modules to see how the car has actually been used.
Service intervals on a 675LT
McLaren's annual service framework, 1st through 15th services. The headline items:
- 1st Service: Annual or first 10,000 km. Oil and filter, basic consumables, systems check.
- 2nd Service: Year two. Oil, filter, brake fluid, cabin filter, engine air filters.
- 3rd Service: Minor — oil and filter only.
- 4th Service: Major — oil, filter, brake fluid, cabin filter, air filters.
- 5th Service: Year five. Oil, filter, full coolant flush.
- 6th Service: Major plus fuel filter and charcoal canister replacement.
- 10th and 12th Services: Major plus fuel and charcoal filter, plus brake system bleed.
On a low-mileage stored 675LT, the calendar-based intervals matter more than mileage. A car at the 5th service mark with 4,000 km on the clock still needs the coolant flush. The fluid has aged regardless of how many kilometers it has covered, and a coolant flush is cheap insurance against the corrosion problems that come from old coolant sitting in an aluminum cooling system.
Where we fit in
The 675LT deserves specialist attention. The cars are rare, the parts are specific, and the failure modes — particularly on stored, low-mileage examples — are not what most general European specialists are equipped to find or fix. We are the only authorized McLaren independent specialist in Canada outside the dealer network. Our partnership with Thorney Motorsport in the UK — who have serviced more 675LTs than almost any shop in Europe — gives us their accumulated knowledge, the factory-spec parts pipeline, and the diagnostic tools required to service these cars correctly.
If you own a 675LT in Ontario, or you are evaluating one anywhere in Canada, book a McLaren service or pre-purchase inspection. The car deserves it. The investment deserves it.
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