996 World / Know-how / DIY

Air-oil separator replacement

The AOS keeps oil mist out of the intake. When it fails, the engine can smoke, run rough and pull oil into the intake. Here's how to confirm it, what to buy for your year, and how owners and specialists do the job.

AdvancedDifficulty
About 3 hoursGuide estimate1. Often longer at home
1999–2005Carrera · C4 · C4S · Targa
M96 engineNot Turbo, GT2 or GT3

Which cars this covers

Covered

1999–2001 Carrera and Carrera 4 (3.4)

Covered

2002–2005 Carrera, Carrera 4, Carrera 4S and Targa (3.6)

Not covered

Turbo, Turbo S, GT2 and GT3. These use the Mezger engine with a different breather system.

The removal sequence is the same across 1999–2005 Carreras1, but the separator and several hoses changed in 2002, so the parts are different. Owners of 3.6 cars describe the job as very hard, even with extra access4.

Order parts by VIN. Fitment changed mid-production, in some cases by engine number. Give your VIN to the parts seller and match the numbers below before you buy.

Symptoms

  • Heavy smoke from the exhaust, often white or blue, sometimes with poor running1
  • Check engine light, lean running or misfires16
  • The oil filler cap is hard to remove with the engine running, because of high crankcase vacuum1
  • A high-pitched squeal or honk as air is pulled past the crankshaft seal16
  • Oil in the intake, which can foul plugs and damage the catalytic converters1
  • Oil leaking from the rubber bellows under the separator57

Don't ignore it

A badly failed separator can let enough oil into the intake to hydrolock the engine, and that failure doesn't always produce much smoke first6. A single puff of smoke at startup on a flat-six can be normal; continuous smoke isn't7.

Confirm it first

Smoke alone doesn't prove the AOS has failed. The reliable check is to measure crankcase vacuum at the oil filler with a slack-tube manometer, engine fully warm and idling.

Reading (inches of water)MeaningSource
about 4–7NormalPelican Parts1
−4.0 to −6.0 (target −5.0)Normal for M96/M97, at sea level and operating temperature. Expect lower readings at altitude.Article quoted on Carpokes6
9–12 or moreFailed separatorPelican Parts1

Before blaming the AOS, rule out a missing oil cap or cap O-ring, a loose dipstick and cracked breather tubes, which all change the reading6. Overfilling the oil can also cause smoke, so check the level is correct6.

Parts list

996 World may earn a commission when you buy through the links below, at no extra cost to you. Part numbers are from the retailer catalogs credited in Sources; confirm against your VIN.

PartPorsche part no.QtyNotesBuy
Air-oil separator
Required		996 107 023 551Carrera / C4, 1999–200128. Genuine and aftermarket versions exist; one owner had an aftermarket unit fail after about 10,000 km7.
Pelican
Hose clamp for separator
Required		999 512 630 001Listed by Pelican as "also use" with this separator2. Owners advise not reusing old clamps7.
Pelican
Separator O-rings, 12 × 2 mm
Required		999 701 789 402Two per car, fits 1999–20052.
Pelican
Lower separator seal
Required		996 107 122 001Carrera / C4, 1999–20012.
Pelican
Upper vent line
If cracked		996 107 145 551Carrera, 1999–20012. Replace if hard or cracked.
Pelican
Upper hose clamp, 25–40 mm
Recommended		999 512 551 011Carrera / C4, 1999–20012.
Pelican
Vent line O-ring, 18 × 2.5 mm
Recommended		999 707 446 401Fits 1999–20052.
Pelican

Pelican also sells a complete AOS kit for 1999–2001 Carreras (PEL-AOSKIT-996-B)2.

Worth doing at the same time

  • Coolant expansion tank, hoses and clamps if the tank comes out and they aren't new3
  • Intake O-rings and throttle body O-ring, since the intake comes apart10
  • An oil change1

Tools

Pelican's guide simply lists "all of them." These are the tools owners and the guide mention by name1347.

Jack and four jack standsShop
1/4" ratchet, 8 mm and 10 mm socketsShop
13 mm socketShop
Extensions and universal jointsShop
Torx bit setShop
Long ball-end 5 mm Allen, 1/4" drive (3.6 cars)Shop
Short open-end wrenchesShop
Hose clamp pliersShop
Telescoping magnet pickupShop
Slack-tube manometer (for the vacuum test)Shop
Clean rags, gloves, pressure washerShop

Step by step

Where the AOS is: on 1999–2005 Carreras it sits at the top left rear of the engine, under the left intake manifold1. Most of the work is from the engine bay above; the bellows and the separator's two bolts are reached from underneath13.

Prepare

  1. Disconnect the battery

    The starter circuit is always live, and you'll be working near it1.

  2. Clean the area

    Pressure wash around the separator first. Owners find it caked in oily dirt, which hides fasteners and falls into the engine7.

  3. Raise the rear of the car

    Support it securely on jack stands. You'll need access from below later1.

Remove the intake

  1. Remove the airbox

    Loosen the boot clamp, unplug the mass airflow sensor connector, remove the 13 mm front bolt and lift the airbox out1.

  2. Remove the throttle body

    Remove the four 10 mm bolts and the 10 mm nut on the bracket. Disconnect the throttle cable on early cars or the electrical connector on later cars. Early cars also have a rear hose clamp and an O-ring in the plenum1.

  3. Remove the intake plenums

    Squeeze the black connector to free the AOS air hose. Loosen the clamps, inner first, then outer, and work the plenums free, front first, then rear1.

  4. Remove the left intake manifold

    This is the hardest part of the job1. Remove the two 8 mm screws at the left manifold hose connection, which also hold a vacuum solenoid plate. The manifold bolts are hard to reach; loosen each until it can be pulled out with the manifold. Extensions, universal joints and short open-end wrenches used a little at a time all help13. Owners describe the front manifold bolt as the most awkward3.

    Coolant tank: Pelican recommends removing it for room1. Some owners did the job without removing it, and one found reinstalling the tank harder than the AOS itself37.

  5. Seal the engine

    Stuff clean rags into the open intake ports. Use the magnet for any dropped fastener before you go further1.

Remove the separator

  1. Release the top line

    Squeeze the connector on top of the AOS and pull off the vacuum line1.

  2. Work from below

    Remove the bellows hose clamp and pull the bellows off. Squeeze and release the vacuum connector. Remove the separator's two 13 mm bolts1. The rear bolt is the hardest fastener on the job and has to be reached from underneath3.

  3. Pry the separator off

    Ease it off the case. Note the nipple that extends into the engine so you seat the new one the same way1. One owner found it easier to lift the unbolted AOS and reach the lower clip from the engine bay7.

Install

  1. Fit new O-rings and clamps

    Use the new O-rings from the parts list and new clamps rather than the old ones, which owners report leaking27.

  2. Bolt the separator in, then attach the bellows

    Secure the AOS bracket to the engine before pushing the bellows onto the bottom pipe. The top and bottom snap clips may be different sizes5.

  3. Reassemble in reverse

    Reconnect the lines and route the breather tube exactly as originally fitted; owners report failures from misrouted lines1. Remove every rag from the intake ports before the plenums go back on1.

Dropping the engine instead

Some owners and shops lower or drop the engine for this job, especially on 3.6 cars or when the engine is coming out for other work, such as an IMS or clutch job134. Pelican notes the AOS adds about an hour when the engine is already out1. If you're planning a clutch or IMS service soon, combining the jobs saves labor.

After the job

  • Check the coolant level and top up if the expansion tank came out or hoses were disconnected1.
  • Clear any fault codes and start the engine.
  • Expect some smoke for a few days while oil left in the intake burns off7.
  • Check the bellows from underneath for leaks after the first drives. One owner's bellows still leaked after a new AOS was fitted5.
  • Re-test crankcase vacuum to confirm the reading is back in the normal range16.

Torque specs

We couldn't confirm torque values for the separator bolts, intake manifold or throttle body from a documented source, so we haven't listed any. Pelican's own guide doesn't give them and refers readers to the factory repair manual1. Use the Porsche workshop manual for your car, or ask your specialist.

Upgrade options

Porsche Motorsport AOS (996 107 926 00). Listed for 1999–2005 Carrera, C4 and C4S at about $840, used with two 900 067 316 01 bolts2. It's taller than the standard unit and requires rerouting a vent line4. One owner reports it doesn't fit early M96-01 engines without major modification5. LN Engineering sells an adapter for fitting it to earlier engines2.

Aftermarket designs such as the "Ultimate AOS" move the wear-prone diaphragm up near the throttle body, so it can be serviced in minutes instead of hours6. Those claims come from the maker and haven't been independently tested.

Service interval. One widely shared article suggests replacing the AOS every 6 years or 75,000 miles; some owners on track-driven cars replace it far sooner6.

Sources and further reading

This guide was written by 996 World from the sources below. Each is worth reading in full. Last checked October 16, 2026.

  1. Porsche 911 Air Oil Separator Replacement, 996 (1998–2005) / 997 (2005–2012)
    Best step-by-stepPhotos

    Pelican Parts technical article. The most complete published procedure, with the vacuum test, the 3-hour estimate and the early versus late car differences.

  2. Pelican Parts catalog: 996 engine oil separator parts
    Best fitment data

    Part numbers by model, year and engine number, including the vent line change at engine 66309950/51 and complete AOS kits.

  3. Rennlist: Who's done an AOS without a motor drop?
    Without dropping the engine

    Owners who did the job from above, which bolts are hardest, and the coolant tank question.

  4. Rennlist: MkII AOS DIY
    3.6 carsMotorsport AOS

    What makes the 3.6 harder, tool tricks for the front manifold screw, and fitting the Motorsport unit.

  5. Rennlist: My AOS adventure
    Bellows tips

    Bellows and clip installation order, and checking the bellows afterward.

  6. Carpokes: 996 Air Oil Separator replacement
    Testing & symptoms

    Crankcase vacuum targets, causes of false readings, hydrolock risk and aftermarket separator designs.

  7. 911UK: 996 Air Oil Separator
    3.4 TiptronicReal-world time

    A 3.4 owner's experience without lowering the engine, cleaning tips, clamp advice and genuine versus aftermarket parts.

  8. Design911: Air-oil separator 996 107 023 55
    3.4 part listing

    Retailer listing for the 1998–2001 3.4 separator.

  9. Heritage Parts Centre: Air-oil separator 996 107 026 51
    3.6 fitment dates

    Fitment for 3.6 cars by production date, with related hoses.

  10. Rennlist: Which AOS do I need?
    Parts to add

    Owner-reported part numbers and the extra seals worth replacing while the intake is apart.

General information only. Work on your car at your own risk, and use the factory workshop manual for specifications. If in doubt, have a specialist do it.