Porsche 997 Oil Separator (Part 99710703800): Complete Guide

The oil separator in a Porsche 997 (2005–2012) is a critical component of the engine’s crankcase ventilation system. Also known as the AOS (air-oil separator), part number 99710703800 helps separate oil from blow-by gases before they are recirculated into the intake. Over time, this part can fail, leading to oil consumption, smoke, and performance issues. This guide covers everything you need to know about the 99710703800 oil separator, including symptoms, diagnosis, replacement options, and cost.

What Is the Porsche 997 Oil Separator (Part 99710703800)?

The oil separator sits on the top of the engine, near the intake manifold. Its job is to remove oil mist from the crankcase ventilation gases and return the oil to the sump while routing the cleaned gases back into the intake to be burned. This reduces oil consumption and keeps the intake system clean. In the 997, the separator is a plastic unit with internal baffles and a diaphragm. Part number 99710703800 is the OEM replacement for most 997 models, including the 997.1 and 997.2 Carrera, Carrera S, and Turbo (though Turbo models sometimes have a different part). It also fits the Cayman and Boxster 987.2.

How It Works

Blow-by gases (combustion gases that leak past the piston rings) carry oil vapor. The separator forces these gases through internal chambers, where oil droplets coalesce and drain back to the oil pan. The cleaned gases then pass through a diaphragm valve that regulates crankcase pressure. A failing separator can cause excessive oil consumption, smoke from the exhaust, and rough idling.

Common Symptoms of a Failing Oil Separator

A bad 99710703800 oil separator usually presents several telltale signs. If you notice any of these, it’s time to inspect the separator.

Excessive Oil Consumption

One of the first symptoms is higher than normal oil usage. If you’re adding a quart of oil every 500–1000 miles, the separator may be allowing oil to be sucked into the intake and burned.

Blue Smoke from Exhaust

Blue smoke, especially on startup or during hard acceleration, indicates oil entering the combustion chamber. A faulty separator can route oil directly into the intake, causing this smoke.

Rough Idle or Stalling

When the separator’s diaphragm fails, it can disrupt crankcase pressure, leading to a rough idle or even stalling after a cold start. The engine may hunt or surge.

Whistling Noise from Engine

A high-pitched whistle from the top of the engine is another classic symptom. This happens when the diaphragm tears, creating a vacuum leak.

Check Engine Light

A failing separator can trigger lean or rich fuel trim codes, often P0171, P0174, or P1128. A diagnostic scan can confirm these.

How to Diagnose a Bad Oil Separator

Before replacing the separator, you should confirm it’s the culprit. Here are steps a mechanic or DIYer can take:

• Visual inspection: Remove the engine cover and look at the separator for cracks or oil leaks. A wet oily film around the separator is a red flag.
• Smoke test: Using a smoke machine, inject smoke into the intake system. If smoke escapes from the separator, the diaphragm is torn.
• Check oil dipstick: Remove the dipstick while the engine is running. If there’s strong suction on the dipstick tube, the separator may be stuck open. Conversely, high crankcase pressure (oil pushed out) suggests a blocked separator.
• Monitor oil level: Track oil consumption over several hundred miles. If it’s excessive, start with the separator.

Replacement Considerations: OEM vs Aftermarket

When buying a replacement part 99710703800, you have two main options: OEM (Original Equipment Manufacturer) from Porsche or an aftermarket alternative. Here’s what to consider.

OEM 99710703800

The genuine Porsche part is made to exact specifications and ensures fit and durability. It’s the recommended choice for most owners because it has a proven track record. The cost is higher, but it’s a direct replacement with no modifications. Expect to pay $150–$250 for the OEM part.

Aftermarket Options

Several brands produce aftermarket oil separators for the 997. These may use upgraded materials, like billet aluminum or reinforced plastic, to address known failure points (such as the diaphragm). Popular aftermarket choices include those from LN Engineering, Eurospare, and others. Prices range from $80–$180. Aftermarket parts can be reliable, but quality varies. Research the specific brand and read installation feedback from 997 forums (like Rennlist) before purchasing.

Which Should You Choose?

• If you want a simple, worry-free replacement: stick with OEM.
• If you’re tracking your car or experiencing repeated failures, an aftermarket upgrade with a metal housing may reduce the chance of future issues.
• Always check compatibility with your exact model year and engine variant (3.6L vs 3.8L, S vs Turbo).

Step-by-Step Replacement Overview

Replacing the oil separator on a 997 is a moderate DIY job, but it requires care. The approximate time is 2–4 hours. Below is a general outline. For exact procedures, consult a service manual or a model-specific guide.

Tools and Parts Needed

• New oil separator (OEM or quality aftermarket)
• New gaskets and O-rings (usually included with the separator)
• Socket set, Torx bits (T20, T30), and a flathead screwdriver
• Jack and jack stands (to access the rear of the engine, optionally)
• Clean cloths and carb cleaner

General Steps

1. Disconnect the battery (negative terminal) and allow the engine to cool.
2. Remove the engine cover and intake manifold cover (if equipped).
3. Remove the intake manifold to access the separator. On some models, you may need to remove the throttle body and air intake ducts.
4. Disconnect the hoses attached to the separator. Note their routing.
5. Unbolt the separator (usually three bolts) and carefully lift it out.
6. Clean the mounting surface and install the new separator with new gaskets.
7. Reconnect all hoses securely. Ensure the hose connections are tight to prevent vacuum leaks.
8. Reinstall the intake manifold and components in reverse order.
9. Reconnect the battery and start the engine. Check for leaks and proper idle.

Note: The first start may produce a brief puff of smoke as the system clears. After a few minutes, it should settle. If smoke persists, double-check the installation.

Cost and Parts Needed

A complete oil separator replacement costs between $200 and $500, depending on parts and labor. Here’s a breakdown:

• OEM separator (99710703800): $150–$250
• Aftermarket separator: $80–$180
• Labor (if not DIY): $150–$300 (1.5–2.5 hours at a shop)
• Additional gaskets/hardware: Often included, but budget $10–$20 if not.

You may also want to replace the rubber hoses connected to the separator if they are brittle or cracked. Hose kit prices vary from $30–$60.

Final Recommendation

For most Porsche 997 owners, the best approach is to replace the oil separator with an OEM part (99710703800) whenever you suspect a failure or as preventive maintenance around 60,000–80,000 miles. The OEM part is reliable, and the replacement is straightforward if you have basic mechanical skills. If you’re tracking your car or have had repeated failures, consider an aftermarket upgrade with a metal housing for extra durability.

Before buying, confirm the part number fits your specific 997 model using your VIN. Many online retailers list the 99710703800 as a direct fit for all 997 Carrera and Carrera S models from 2005–2012. Avoid used separators; they are prone to the same failures.

Addressing a faulty oil separator promptly saves you from higher oil costs, potential catalytic converter damage, and drivability issues. If you’re not comfortable with the DIY replacement, a qualified Porsche mechanic can handle it efficiently. Either way, this is a well-known issue with a straightforward fix—don’t put it off.