Porsche · Known Issues

1965-2019 Porsche 911 Problems: 12 Issues Every Owner Should Know

1965-2019 model years · 20,800+ owner reports · Updated June 2026

According to Au7o's analysis of 20,800+ owner reports, the 1965-2019 Porsche 911 has 12 documented known issues, with 6 rated critical. The most serious are Intermediate Shaft (IMS) Bearing Failure ($2,000-$4,500 repair), Cylinder Bore Scoring (Nikasil Wear) ($8,000-$25,000 repair), Timing Chain Tensioner and Guide Failure ($3,000-$7,000 repair), Coolant Pipe and Fitting Leaks ($800-$2,500 repair), Structural rust in kidney bowls, longitudinals, floor pan and battery box ($3,000-$30,000 repair) and Timing chain tensioner collapse on pre-1984 cars destroys the engine ($700-$1,800 repair). The most commonly reported issue is Intermediate Shaft (IMS) Bearing Failure with 5,500 owner reports. Across all issues, repair costs range from $60 to $30,000. DIY maintenance guides at au7o.io.

All 12 Known Issues

When Issues Typically Appear

Cylinder Bore Scoring (Nikasil Wear)

20K-80K

Coolant Pipe and Fitting Leaks

30K-80K

Direct Injection Intake Valve Carbon Buildup

30K-70K

Intermediate Shaft (IMS) Bearing Failure

40K-120K

Timing Chain Tensioner and Guide Failure

40K-100K

Rear Main Seal (RMS) Oil Leak

50K-120K

Air-Oil Separator (AOS) Failure

50K-100K

038K75K113K150K mi

Community Reported

5,500 owners

On the 1999-2008 Porsche 911, the IMS bearing on 996 and early 997 flat-six engines is a single-row or dual-row sealed bearing that loses lubrication over time, leading to catastrophic bearing failure and engine destruction. The dual-row bearing (1999-2005) fails less frequently than the single-row (2006-2008), but both are a major concern. This is the most feared and well-documented Porsche defect of the modern era.

Common Symptoms

Metallic debris in oil filter at oil change
Rumbling or grinding noise from rear of engine
Sudden catastrophic engine seizure
Oil analysis showing elevated copper and lead
Check engine light with oil pressure codes

How to Fix

Replace the IMS bearing with an upgraded ceramic hybrid bearing (LN Engineering IMS Solution or IMS Retrofit). This can be done with the engine in the car during a clutch replacement. On 2006-2008 single-row cars, replacement is strongly recommended as a preventive measure.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (5,500+ fixed this)

UpgradeLN Engineering IMS Solution — ceramic hybrid bearing replacement that eliminates the OEM failure-prone sealed bearing. Install during any clutch replacement. (LN Engineering #IMS-SOLUTION)
View on Amazon
NoteCut open the oil filter at every oil change and inspect for metallic debris — this is the earliest warning sign of IMS bearing deterioration.

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$2,000 - $4,500

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Community Reported

High Confidence5,500 reportsLast reported by owners Jan 2026Reviewed Mar 2026

Community Reported

3,200 owners

On the 2009-2019 Porsche 911, the 991-generation 911 suffers from cylinder bore scoring where the Lokasil cylinder liners develop deep vertical scores, typically on cylinders 1 and 6. This is caused by inadequate oil supply to the cylinder walls during cold starts and short-trip driving. The issue results in excessive oil consumption and eventually requires engine rebuild or replacement.

This issue also affects: Audi Q7, Audi S4, Audi A4

Common Symptoms

Excessive oil consumption (more than 1 quart per 1,000 miles)
Blue or white smoke on cold start
Rough idle when engine is cold
Metallic rattling from engine
Compression test showing low values on cylinders 1 or 6

How to Fix

Engine must be removed and disassembled. Scored cylinders are repaired by installing iron sleeves (LN Engineering Nickies) or replacing the cylinder block. Some owners opt for a used or rebuilt engine. Prevention: always warm the engine fully before spirited driving and avoid short trips.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (3,200+ fixed this)

NoteWhen buying a used 991, always perform a leak-down and compression test before purchase. Bore scoring is extremely expensive to repair and often not covered under Porsche warranty after the initial period.

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$8,000 - $25,000

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Community Reported

High Confidence3,200 reportsLast reported by owners Feb 2026Reviewed Mar 2026

Community Reported

1,800 owners

On the 2009-2019 Porsche 911, the 991-generation timing chain tensioners can lose hydraulic pressure, particularly after the car has been sitting for extended periods. This allows chain slack on startup, which can jump timing and cause catastrophic valve-to-piston contact. The plastic chain guides also wear prematurely on high-mileage engines.

This issue also affects: Audi A4, Audi A1, Audi A5

Common Symptoms

Rattling noise on cold start for 1-3 seconds
Chain slap noise from engine
Check engine light with timing codes
Rough running after sitting for weeks
Metallic debris in oil

How to Fix

Replace timing chain tensioners with updated Porsche parts. On early 991.1 cars, the tensioner design was revised (updated part number). Replace chain guides at the same time. Some shops recommend replacing chains as well if mileage is above 80,000.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (1,800+ fixed this)

NoteIf your 991 has been sitting for more than 2 weeks, do not rev above 3,000 RPM for the first 5 minutes — allow oil pressure to fully pressurize the tensioners.

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$3,000 - $7,000

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High Confidence1,800 reportsLast reported by owners Jan 2026Reviewed Mar 2026

On the 1965-1983 Porsche 911, every 911 built through 1983 uses spring-loaded mechanical cam chain tensioners that lose their hydraulic damping oil and collapse with age and heat cycling. When a tensioner fails the slack timing chain jumps a tooth (or off the sprocket entirely), the cams lose sync with the crank, and the valves slam into the pistons. The onset is sudden and hard to detect — a 'bad tensioner isn't easy for even the most astute to notice' — and the result is a bent-valve, possibly cracked-piston engine that needs a full teardown. This is the single most feared mechanical failure on an air-cooled 911 and the first thing any restorer addresses on an unmodified early car.

Common Symptoms

rattle or chatter from the timing chain area on startup or under load
sudden loss of power / engine stops running
valve-to-piston contact and bent valves after a chain jump
low compression on one bank after a tensioner failure

How to Fix

Retrofit the 1984-89 Carrera 3.2 pressure-fed (oil-fed) chain tensioners. These are continuously lubricated and damped by engine oil pressure instead of a self-contained spring/oil cartridge, so they cannot dry out and collapse. The bolt-on kit includes both housing covers, two external oil feed lines tapped off the oil galleries, the two Carrera tensioners and fittings (~$700-1,000 in parts). It is the universally recommended bulletproofing upgrade for any pre-'84 911. A cheaper interim fix some owners use is a bronze-bushing internal stop that prevents full collapse, but the Carrera oil-fed conversion is the proper permanent cure.

Typical repair cost:$700 - $1,800

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High ConfidenceVerified0 reportsLast reported by owners Invalid DateReviewed Jun 2026

Community Reported

3,800 owners

On the 1999-2019 Porsche 911, the rear main seal on the M96, M97, and MA1 flat-six engines is prone to leaking oil onto the clutch and flywheel. The seal hardens over time and the flange design allows oil to migrate past the seal. This is often discovered during clutch replacement or when oil drips appear under the rear of the car.

Common Symptoms

Oil drip from bellhousing area
Burning oil smell from rear of car
Clutch slipping due to oil contamination
Oil spots on garage floor near rear axle
Visible oil on flywheel during inspection

How to Fix

Replace the rear main seal. This requires transmission removal and is commonly done alongside clutch replacement and IMS bearing upgrade to save labor costs. Use the updated Porsche OEM seal design.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (3,800+ fixed this)

TipAlways replace the RMS when doing a clutch job — the labor overlap saves $1,000+. Bundle with IMS bearing replacement on 996/997 cars.

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$1,500 - $3,500

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High Confidence3,800 reportsLast reported by owners Dec 2025Reviewed Mar 2026

Community Reported

2,800 owners

On the 1999-2008 Porsche 911, the AOS diaphragm on M96/M97 engines ruptures over time, causing unmetered air leaks into the intake manifold and excessive oil consumption. The failed AOS allows oil vapor to be drawn directly into the intake system rather than being separated and returned to the crankcase. This leads to rich running, fouled spark plugs, and smoke from the exhaust.

This issue also affects: Audi Q7, Audi A4, Audi A5

Common Symptoms

Excessive oil consumption
White or blue smoke from exhaust
Rough idle and vacuum leak symptoms
Oil fouled spark plugs
Strong oil smell in cabin with heat on

How to Fix

Replace the AOS unit. Use an updated OEM Porsche unit or aftermarket upgrade from LN Engineering. The AOS is located in the engine V between the cylinder banks and requires moderate disassembly to access.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (2,800+ fixed this)

UpgradeLN Engineering upgraded AOS with improved diaphragm material — lasts significantly longer than the OEM unit. Direct bolt-in replacement. (LN Engineering #LN-AOS)
View on Amazon

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$500 - $1,500

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High Confidence2,800 reportsLast reported by owners Nov 2025Reviewed Mar 2026

Community Reported

2,200 owners

On the 2009-2019 Porsche 911, the direct injection system on 991 engines does not spray fuel over the intake valves, allowing carbon deposits to accumulate on the valve stems and ports. Over time this restricts airflow, causing rough idle, misfires, and reduced power. The flat-six layout makes walnut blasting more labor-intensive than on inline engines.

This issue also affects: Audi Q7, Audi S4, Audi A4

Common Symptoms

Rough idle that worsens over time
Hesitation on acceleration
Misfires at low RPM
Reduced fuel economy
Check engine light with misfire codes

How to Fix

Perform intake valve walnut blasting to remove carbon deposits. This typically needs to be done every 40,000-60,000 miles on DFI engines. Some shops use chemical soak methods as well. Consider installing a catch can to reduce oil vapor entering the intake.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (2,200+ fixed this)

TipInstall an oil catch can (JHM or similar) to significantly reduce carbon buildup on intake valves. This extends the interval between walnut blasting services.

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$800 - $1,800

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High Confidence2,200 reportsLast reported by owners Jan 2026Reviewed Mar 2026

On the 1965-1989 Porsche 911, air-cooled 911s carry oil all around the car through an external dry-sump system, and after decades the seals and hoses leak everywhere. The classic offenders are the two short rubber crimp hoses joining the hard lines to the front-fender oil cooler (they crack and weep), the oil thermostat's internal O-ring (it sees high pressure and seeps around the housing), the thin paper timing-chain-housing cover gaskets (often re-glued without gaskets and then leak), and the cam tower / valve cover seams. A leaky air-cooled 911 also runs hotter because oil IS its cooling system, so leaks are more than a nuisance.

Common Symptoms

oil drips under the front of the car (cooler line area)
oil film around the oil thermostat housing
seepage at the timing chain covers and cam tower joints
burning-oil smell and higher-than-normal oil temps
low oil level between changes

How to Fix

During restoration, reseal the whole oil system as a system rather than chasing one drip: replace the crimped front-cooler rubber hoses with new (ideally braided/AN-style) lines, renew the thermostat O-ring, and properly re-gasket the chain housing covers and cam towers with correct gaskets and sealant. Many restorers upgrade to a larger front cooler and braided cooler lines while everything is apart, improving both sealing and cooling margin. Doing all the seals at once — and torquing covers to spec — is the proven way to end the chronic seep for good.

Typical repair cost:$300 - $2,500

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High ConfidenceVerified0 reportsLast reported by owners Invalid DateReviewed Jun 2026

Community Reported

1,500 owners

On the 2012-2019 Porsche 911, the plastic coolant pipes and quick-connect fittings on the 991 generation are prone to cracking and leaking, especially at the connections near the front radiators. The plastic becomes brittle with heat cycling and age. Coolant loss can be gradual and difficult to detect until the engine overheats.

Common Symptoms

Coolant level dropping with no visible external leak
Sweet smell from front of car near radiators
Overheating warning on dashboard
Coolant pooling under front bumper area
Steam from front wheel wells

How to Fix

Replace cracked plastic coolant pipes with updated Porsche parts or aftermarket aluminum replacements. Inspect all quick-connect fittings and replace any that show signs of degradation. Pressure test the cooling system annually.

What you need to fix it

The exact parts — OEM, plus what owners actually use. Skip the internet hunt.

From owners — upgrades & tips (1,500+ fixed this)

TipReplace plastic coolant pipes with aluminum versions from Numeric Racing or similar — they eliminate the cracking issue permanently.

Part links may earn au7o a commission. Confirm fitment by VIN before buying.

Typical repair cost:$800 - $2,500

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High Confidence1,500 reportsLast reported by owners Feb 2026Reviewed Mar 2026

On the 1965-1975 Porsche 911, pre-1976 911 bodies were NOT galvanized, and the unibody is full of closed box sections that trap moisture. The classic rot spots are the kidney bowls (where the rear torsion-bar tube/longitudinal meets the inner sill — a critical structural junction), the longitudinals (rocker box sections), the front and rear sills, the inner front fender tops, the front and rear floor pans, the suspension/torsion pans, and the battery box, which also gets eaten by acid and blocked drains. Because the kidney bowl and longitudinals are load-bearing, this isn't cosmetic — it compromises chassis rigidity and is the make-or-break factor in any air-cooled restoration's cost.

Common Symptoms

bubbling/flaking paint at sills, fender tops and around the battery box
perforated or soft metal at the kidney bowls and longitudinals
doors that no longer line up (chassis flex from rotted longitudinals)
wet carpets / water in the floor pans
crusty, acid-eaten battery tray

How to Fix

Cut out the rot and weld in correct repair panels — Porsche/OEM-pattern kidney bowls, longitudinal/inner-sill sections, floor and suspension pans and battery box are all reproduced (Stoddard, Heritage, Restoration Design). The bulletproofing step that beats the factory: after metal repair, fully seam-seal and then inject cavity wax (Wurth/Dinitrol) into every box section the factory left bare on pre-'76 cars, and re-apply a flexible underbody coating. Replicating the post-1976 hot-dip galvanizing logic with modern cavity wax is what stops the rust from ever coming back. Address blocked battery-box and sunroof/windshield drains so water can't pool again.

Typical repair cost:$3,000 - $30,000

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On the 1973-1983 Porsche 911, 1973-83 normally-aspirated 911s with Bosch CIS (K-Jetronic / 'Continuous Injection System') use a large molded plastic airbox between the throttle body and intake runners. A lean or ill-timed condition causes an intake backfire, and that small explosion pressurizes the sealed airbox — with no relief path, the airbox literally cracks or blows apart. It is a common, well-known failure, and an original undamaged CIS airbox is now expensive and hard to find, so protecting it is a priority on any CIS-car restoration.

Common Symptoms

loud pop/bang from the intake on start-up or hard throttle
cracked or split airbox
rough running / vacuum leak after the box has cracked
hesitation or lean stumble that precedes a backfire

How to Fix

Install a backfire pop-off (pressure-relief) valve in the airbox — a spring-loaded flapper that vents backfire pressure harmlessly into the intake instead of letting it split the box. It's a cheap, well-documented modification (hole-saw the box, epoxy in the valve). The more durable bulletproofing route is to replace the plastic box altogether with an aftermarket aluminum CIS airbox, which simply cannot be blown apart, while still running a pop-off valve. Also fix the root cause (set CIS mixture/timing correctly and cure any lean stumble) so backfires stop happening in the first place.

Typical repair cost:$60 - $1,200

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On the 1972-1986 Porsche 911, the 915 transmission (1972-86, replacing the earlier 901) is the air-cooled 911's known shifting weak point. Its Porsche-style synchros wear, and second gear in particular suffers — the synchro ring loses its bite and the engagement 'dog teeth' get rounded off (or snapped if someone missed a power shift). The result is the classic notchy, baulky 1st-to-2nd shift and a grind/crunch into 2nd, especially when cold. Owners abuse this further by speed-shifting, which bypasses the synchro and accelerates dog-tooth wear, leaving a short-lived gearbox.

Common Symptoms

notchy or baulky 1st-to-2nd shift
grinding/crunching going into 2nd gear, worse when cold
gear lever pops out or won't engage under power
improves slightly as the transmission warms up

How to Fix

Rebuild the 915 with new/updated synchro rings and replace the worn 2nd-gear (and often 1st) gear sets with good dog teeth, set the correct shift-rod and shifter adjustment, and fill with a quality GL-4/GL-5 gear oil (many specialists add Swepco or a friction modifier that noticeably improves synchro feel). Restorers commonly fit updated late-style synchro components and a short-shift kit, and rebush the worn shift linkage and coupler. A properly rebuilt and correctly adjusted 915 with fresh synchros shifts cleanly — the bulletproofing is doing the rebuild right plus disciplined, non-speed-shifting driving.

Typical repair cost:$1,500 - $4,500

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Frequently Asked Questions

What are the most common Porsche 911 problems?

According to Au7o's analysis of 20,800+ owner reports, the 1965-2019 Porsche 911 has 12 documented issues. The most frequently reported are: Intermediate Shaft (IMS) Bearing Failure, Cylinder Bore Scoring (Nikasil Wear), Timing Chain Tensioner and Guide Failure. Of these, 6 are rated critical and should be addressed promptly.

Is the Porsche 911 reliable?

The 1965-2019 Porsche 911 has 12 known issues documented across 20,800+ owner reports. 6 issues are rated critical: Intermediate Shaft (IMS) Bearing Failure and Cylinder Bore Scoring (Nikasil Wear) and Timing Chain Tensioner and Guide Failure and Coolant Pipe and Fitting Leaks and Structural rust in kidney bowls, longitudinals, floor pan and battery box and Timing chain tensioner collapse on pre-1984 cars destroys the engine. Prospective buyers should inspect for these issues and factor potential repair costs into their purchase decision. Regular maintenance following the manufacturer's schedule helps prevent many common problems.

How much does it cost to fix common Porsche 911 problems?

Repair costs for known Porsche 911 issues range from $60 to $30,000, depending on the specific problem and whether you choose DIY or professional repair. The most critical issue, Intermediate Shaft (IMS) Bearing Failure, typically costs $2,000-$4,500 to repair. Au7o provides step-by-step DIY maintenance guides that can help reduce repair costs.

What year Porsche 911 is the most reliable?

Reliability varies across model years of the Porsche 911. Based on documented issues, problems are most commonly reported in earlier model years. Au7o recommends checking the specific known issues for your target year before purchasing, and having a pre-purchase inspection performed by a qualified mechanic. Our known issues database covers the 1965-2019 Porsche 911 with 12 documented issues documented across 20,800+ owner reports.

What is the 1999-2008 Porsche 911 Intermediate Shaft (IMS) Bearing Failure?

The IMS bearing on 996 and early 997 flat-six engines is a single-row or dual-row sealed bearing that loses lubrication over time, leading to catastrophic bearing failure and engine destruction. The dual-row bearing (1999-2005) fails less frequently than the single-row (2006-2008… Repairs typically run $2,000-$4,500. Severity: high.

All 12 Known Issues

When Issues Typically Appear

1999-2008 Porsche 911: Intermediate Shaft (IMS) Bearing Failure#

Common Symptoms

How to Fix

What you need to fix it

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2009-2019 Porsche 911: Cylinder Bore Scoring (Nikasil Wear)#

Common Symptoms

How to Fix

What you need to fix it

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2009-2019 Porsche 911: Timing Chain Tensioner and Guide Failure#

Common Symptoms

How to Fix

What you need to fix it

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1965-1983 Porsche 911: Timing chain tensioner collapse on pre-1984 cars destroys the engine#

Common Symptoms

How to Fix

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1999-2019 Porsche 911: Rear Main Seal (RMS) Oil Leak#

Common Symptoms

How to Fix

What you need to fix it

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1999-2008 Porsche 911: Air-Oil Separator (AOS) Failure#

Common Symptoms

How to Fix

What you need to fix it

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2009-2019 Porsche 911: Direct Injection Intake Valve Carbon Buildup#

Common Symptoms

How to Fix

What you need to fix it

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1965-1989 Porsche 911: Chronic oil leaks from cooler lines, thermostat seal and chain housings#

Common Symptoms

How to Fix

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2012-2019 Porsche 911: Coolant Pipe and Fitting Leaks#

Common Symptoms

How to Fix

What you need to fix it

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1965-1975 Porsche 911: Structural rust in kidney bowls, longitudinals, floor pan and battery box#

Common Symptoms

How to Fix

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1973-1983 Porsche 911: CIS airbox blows apart from an intake backfire#

Common Symptoms

How to Fix

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1972-1986 Porsche 911: 915 gearbox worn 2nd-gear synchros and rounded dog teeth#

Common Symptoms

How to Fix

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Other Porsche Models

Frequently Asked Questions

What are the most common Porsche 911 problems?

Is the Porsche 911 reliable?

How much does it cost to fix common Porsche 911 problems?

What year Porsche 911 is the most reliable?

What is the 1999-2008 Porsche 911 Intermediate Shaft (IMS) Bearing Failure?

What is the 2009-2019 Porsche 911 Cylinder Bore Scoring (Nikasil Wear)?

What is the 2009-2019 Porsche 911 Timing Chain Tensioner and Guide Failure?

What is the 2012-2019 Porsche 911 Coolant Pipe and Fitting Leaks?

What is the 1965-1975 Porsche 911 Structural rust in kidney bowls, longitudinals, floor pan and battery box?

What is the 1965-1983 Porsche 911 Timing chain tensioner collapse on pre-1984 cars destroys the engine?

What is the 1999-2019 Porsche 911 Rear Main Seal (RMS) Oil Leak?

What is the 1999-2008 Porsche 911 Air-Oil Separator (AOS) Failure?

What is the 2009-2019 Porsche 911 Direct Injection Intake Valve Carbon Buildup?

What is the 1973-1983 Porsche 911 CIS airbox blows apart from an intake backfire?

What is the 1972-1986 Porsche 911 915 gearbox worn 2nd-gear synchros and rounded dog teeth?

What is the 1965-1989 Porsche 911 Chronic oil leaks from cooler lines, thermostat seal and chain housings?