Renault R18i Coolant Temperature Sensor Guide (1981-1986)
Keep your classic Renault's fuel injection system running smoothly with the right information.
- A bad sensor will most likely cause your R18i to run rich, resulting in poor gas mileage and black exhaust smoke.
- OEM parts are not available; a quality aftermarket sensor is the only practical option.
- This is a simple and inexpensive DIY repair if you are comfortable with basic tools.
- There are no known recalls or TSBs for this part on your vehicle; failures are due to age.
Understanding the Coolant Temperature Sensor on Your R18i
On your 1981-1986 Renault R18i, the coolant temperature sensor (CTS) is a small but vital component for the Bosch L-Jetronic (or similar) fuel injection system. It is a simple thermistor, which means its electrical resistance changes with temperature. When the engine is cold, the sensor's resistance is high, telling the computer to inject more fuel for a richer mixture to help the engine start and warm up. As the engine heats up, the sensor's resistance drops, signaling the computer to lean out the fuel mixture for better efficiency and lower emissions. When this sensor fails, it can cause a variety of noticeable engine problems.
Symptoms of a Failing Coolant Temperature Sensor
A bad CTS will often send a false signal to the engine's computer, typically indicating that the engine is always cold. This can lead to several performance issues:
- Poor Fuel Economy: The most common symptom. The computer continuously provides a rich fuel mixture, as if the engine is always in its warm-up phase, drastically increasing fuel consumption.
- Black Smoke from Exhaust: Unburned fuel from the excessively rich mixture exits the engine and can be seen as black smoke from the tailpipe.
- Rough Idle or Stalling: The incorrect air-fuel ratio can make the engine run poorly at idle, causing it to shake, vibrate, or even stall.
- Hard Starting: While a rich mixture helps a cold start, a faulty sensor can provide an incorrect reading that makes starting difficult, especially when the engine is already warm (a condition known as a 'hot start' problem).
- Check Engine Light: The engine computer can detect a problem with the sensor's circuit or an illogical reading, which will trigger the Check Engine Light.
No Recalls or TSBs Found
As of our latest search, there are no official recalls or Technical Service Bulletins (TSBs) from Renault or the NHTSA specifically for coolant temperature sensor issues on the 1981-1986 Renault R18i in the North American market. Any problems you encounter are likely due to the age of the component.
Choosing the Right Aftermarket Sensor
For a classic vehicle like the R18i, original equipment manufacturer (OEM) parts are typically no longer available. You will be purchasing an aftermarket part. The brand listed in our inventory is "Replacement," a term for generic aftermarket parts that meet original specifications.
Several aftermarket brands may be available through various retailers, such as Standard Motor Products (SMP), Four Seasons, SKP, and Autopart Premium. For a simple sensor like this, most quality aftermarket brands will perform well. The key is to ensure the part is listed as a direct fit for your 1981-1986 Renault R18i to guarantee the correct thread size and electrical connector.
Pro Tip: Check the Connector
Before ordering, it's a good idea to visually inspect your old sensor's electrical connector. The replacement part for the R18i should have a 2-terminal blade-style connector. Also, consider replacing the connector pigtail if the old one is cracked, brittle, or corroded, as a bad connection can mimic a failed sensor.
Cost Comparison
| Part Type | Estimated Cost |
|---|---|
| New OEM | Not Available |
| New Aftermarket | $17.00 - $25.00 |
| Used OEM | Varies, not recommended |
| Shop Labor | $75 - $150 |
Frequently Asked Questions (FAQ)
Where is the coolant temperature sensor located on the Renault R18i?
While specific Renault R18i diagrams are scarce, the coolant temperature sensor on engines from this era is almost always screwed into the engine block, cylinder head, or thermostat housing, where it can be in direct contact with the engine coolant.
Can I replace the coolant temperature sensor myself?
Yes, this is a very DIY-friendly job for most home mechanics. It requires basic hand tools, such as a wrench or a deep socket. You will need to partially drain the coolant or be prepared for some coolant to spill when you unscrew the old sensor. The most difficult part is often just reaching the sensor and unplugging its electrical connector.
Should I buy an OEM or aftermarket sensor?
For the R18i, new OEM sensors are generally not an option due to the vehicle's age. Aftermarket parts are the standard and are widely available from brands like Standard Motor Products, Four Seasons, and others. These parts are designed to meet or exceed original specifications and are a reliable choice.
Do I need to drain the coolant to replace the sensor?
You don't need to drain the entire system. However, the sensor is below the coolant level, so when you remove it, coolant will leak out. To minimize the mess, you can have the new sensor ready to quickly screw into place. Some owners prefer to drain a small amount of coolant from the radiator's petcock before starting.
Technical Specifications
OEM Part Numbers: 025906041 1338444 13621284397 13621357414 13621401945 13621709966 13621709967 6U0919501 TX18
2-Terminal Blade Connector
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.
- Renault R18i: