A Guide to the Engine Coolant Thermostat for the 1988-1995 Audi 90
Keep your classic Audi 90's engine running at the perfect temperature with the right replacement thermostat.
- For V6 models, always replace the thermostat during a timing belt and water pump service to save on massive labor costs.
- A stuck-closed thermostat can cause critical engine damage from overheating; pull over immediately if the temperature gauge enters the red.
- Use a thermostat with the factory 87°C (190°F) temperature rating for proper engine operation.
- Always replace the thermostat gasket or O-ring to ensure a leak-free seal after installation.
Is Your Audi 90 Thermostat Failing?
A thermostat failure can happen in two ways: stuck open or stuck closed. Each has distinct symptoms.
Stuck Closed: Risk of Overheating
This is the most dangerous failure. If the thermostat is stuck closed, it traps coolant in the engine and prevents it from reaching the radiator. Heat builds up quickly, causing the engine to overheat. You will see the temperature gauge climb into the red zone. Pull over immediately and shut off the engine to prevent catastrophic damage.
Stuck Open: Running Too Cool
If the thermostat is stuck open, coolant flows to the radiator constantly. This prevents the engine from reaching its ideal operating temperature. Symptoms include the temperature gauge staying low, poor heater performance in the cabin, and decreased fuel economy. While less immediately dangerous, it causes increased engine wear over time.
Choosing the Right Aftermarket Thermostat
For a critical part like the thermostat, quality matters—especially on the V6 models where replacement is labor-intensive. Aftermarket brands offer a range of quality and price points.
Tier 1: OEM-Equivalent Brands
For owners who prioritize reliability and performance, choosing a brand known to be an Original Equipment (OE) supplier is the safest bet. Brands like Calorstat (by Vernet), Mahle, and Wahler have a strong reputation in the European car community. While they may cost more, they are engineered to match the original part's performance, such as opening at the precise temperature and to the correct degree. This is the recommended choice for V6 owners.
Tier 2: Reputable Aftermarket Brands
Brands like Gates and CRP (Rein) are well-regarded in the aftermarket for producing quality parts. They offer a good balance of price and reliability and are a solid choice for all Audi 90 engines.
Tier 3: Standard & Budget Brands
Brands like Motorad and other private labels are the most affordable. Many, like Motorad, offer a "fail-safe" design engineered to lock in the open position if they fail, which can prevent overheating. However, some mechanics report these can fail in the open position prematurely. These can be a viable option for a quick repair on a 4 or 5-cylinder engine, but for the V6, the higher labor cost makes using a top-tier part a better investment.
Installation: A Tale of Two Engines
The difficulty of replacing the thermostat on an Audi 90 depends entirely on which engine you have.
- 5-Cylinder Engines: On the 5-cylinder models, the thermostat is located on the lower part of the engine and is relatively easy to access. A competent DIYer can typically complete the job in an hour or two. 🎬 Watch: Troubleshooting cooling system problems on a 5-cylinder Audi 90.
- V6 Engines: On the 2.8L V6, the thermostat is located behind the timing belt cover. This makes replacement a much more involved and difficult job. While some experts say it can be done by just loosening the timing belt, it is extremely tight work. Most owners and shops replace the thermostat, water pump, and timing belt at the same time. If you are paying a shop, expect a significant labor charge for a standalone thermostat replacement on a V6.
Estimated Replacement Costs
| Part/Service | Estimated Cost |
|---|---|
| New Aftermarket Thermostat | $12 - $75 |
| New OEM Thermostat | $50 - $100 |
| Shop Labor (5-Cylinder) | $120 - $250 |
| Shop Labor (V6) | $350 - $600+ (often part of a larger timing belt service) |
Frequently Asked Questions
What temperature thermostat does my Audi 90 need?
Your Audi 90 was designed to use a thermostat with an opening temperature of 87°C (190°F). It is critical to use a part with the correct OEM temperature rating to ensure proper engine performance and efficiency.
My temperature gauge is acting weird. Is it the thermostat?
It could be, but it could also be the Multi-Function Temperature Sensor (MFTS). This sensor sends the temperature reading to your gauge. A faulty MFTS can mimic the symptoms of a bad thermostat, so it's worth testing before starting a difficult repair, especially on a V6.
🎬 Learn how to test your Audi coolant temperature sensor.Should I replace the thermostat with the timing belt on my V6?
Absolutely. On the V6 engine, the thermostat is located behind the timing belt. Since the majority of the cost is the labor to access this area, it is standard practice and highly recommended to replace the timing belt, water pump, and thermostat all at the same time.
🎬 See this walkthrough of an Audi V6 timing belt and thermostat service.Do I need to replace the thermostat housing or gasket?
You should always replace the O-ring or gasket when you replace the thermostat to prevent leaks. The plastic or metal housings can also become brittle and crack over time. Inspect the housing carefully and replace it if you see any signs of damage or heavy corrosion.
Helpful Videos
Technical Specifications
OEM Part Numbers: 078121113F
Standard Opening Temperature: 87°C (190°F)
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- Audi 90:
- 🎬 Helpful Videos
- 🛍️ Shop This Part
- Is Your Audi 90 Thermostat Failing?
- Stuck Closed: Risk of Overheating
- Stuck Open: Running Too Cool
- Choosing the Right Aftermarket Thermostat
- Tier 1: OEM-Equivalent Brands
- Tier 2: Reputable Aftermarket Brands
- Tier 3: Standard & Budget Brands
- Installation: A Tale of Two Engines
- Estimated Replacement Costs
- Frequently Asked Questions
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