GMC S15 Coolant Temperature Sensor Guide for 1982-1990 Models
If your S15 is running rough, getting poor gas mileage, or blowing black smoke, a bad coolant sensor is a likely and inexpensive culprit.
- A bad ECT sensor is a very common cause of a rich-running condition, leading to poor gas mileage and black exhaust smoke.
- Ensure you are replacing the two-wire ECT sensor for the computer, not the single-wire sender for the dashboard gauge.
- Replacement is a simple, low-cost DIY job that most owners can perform in under an hour with basic tools.
- Ignoring the problem can lead to fouled spark plugs and damage to the catalytic converter over time.
Is Your S15 Running Rich? Check the Coolant Sensor
A faulty Engine Coolant Temperature (ECT) sensor is a common reason for a range of engine problems on the 1982-1990 GMC S15. When this sensor fails, it typically gets stuck in a “cold” reading, tricking the engine’s computer into thinking the engine never warms up. This causes the computer to constantly dump extra fuel into the engine, a condition known as “running rich.” The symptoms are often mistaken for more complex fuel system issues, but checking this simple sensor first can save you time and money.
Symptoms of a Bad Coolant Temperature Sensor
A failing ECT sensor will cause noticeable issues. Here are the most common signs to watch for:
- Poor Fuel Economy: This is one of the most frequent complaints. The engine wastes gas by running on a rich fuel mixture all the time.
- Black Smoke from Exhaust: The excess, unburnt fuel is pushed out the tailpipe, creating visible black smoke.
- Rough Idle or Stalling: The incorrect fuel-to-air ratio can make your S15's engine idle poorly, hesitate, or even stall, especially after it should be warmed up.
- Hard Starting: The computer receives incorrect temperature data, leading to the wrong fuel amount being delivered for starting, which can make the engine crank longer before it fires up.
- Check Engine Light: On later models in this range with basic diagnostics, a bad sensor can trigger a trouble code. The most common codes for this issue are Code 14 (indicating high temperature or an open circuit) and Code 15 (indicating low temperature or a shorted circuit).
Don't Confuse the Sensor with the Sender!
Your GMC S15 likely has two temperature units. The ECT sensor has two wires and sends information to the engine computer; this is the part that causes running problems. The temperature sender has one wire and only operates the temperature gauge on your dashboard. If your truck runs poorly but the gauge seems fine, the ECT sensor is the likely problem. Make sure you are ordering the correct two-wire part.
Choosing the Right Aftermarket Part
For a vehicle from this era, new original GM parts are often unavailable or very expensive. A quality aftermarket sensor provides a reliable and affordable solution. While there are many brands, the key is to ensure you get the correct part for your engine and year. Aftermarket brands like Standard Motor Products and Delphi are well-regarded choices. Even if opting for a value-focused brand, ensure it has a brass body and a connector that matches your truck's harness perfectly.
| Part Type | Estimated Cost |
|---|---|
| New Aftermarket | $18.79 |
| Shop Labor | $50 - $100 |
| New OEM | $45 - $80+ (if available) |
| Used OEM | $10 - $25 |
Pro Tip: Installation
This is a great DIY job for beginners. The sensor is usually located on the intake manifold near the thermostat housing. You only need to drain a small amount of coolant from the radiator petcock until the level is below the sensor. Most new sensors come with thread sealant already applied. If not, use a small amount of high-temperature thread sealant, not Teflon tape. Be careful not to overtighten the new sensor, as the brass can break.
Frequently Asked Questions (FAQ)
Where is the coolant temperature sensor on my 1982-1990 GMC S15?
The ECT sensor is typically screwed into the front of the intake manifold, close to the thermostat housing. Look for the small sensor with a two-wire electrical connector.
Is this a difficult part to replace myself?
No, this is considered a very easy repair suitable for a beginner. It typically takes less than an hour with basic hand tools like a wrench or a deep socket.
Do I need to drain all the coolant to change the sensor?
No, you do not need to drain the entire system. Just drain enough coolant from the radiator so that the coolant level is below the sensor's position on the engine. This will prevent a large spill when you remove the old part.
What happens if I don't replace a bad coolant sensor?
Continuing to drive with a bad sensor will cause consistently poor fuel mileage and performance. Over time, the rich fuel mixture can foul your spark plugs and potentially damage the oxygen sensor and catalytic converter, leading to more expensive repairs.
Technical Specifications
OEM Part Numbers: 25037345 8992016 8993105 8993106 8993107 8993372
Typically features a brass body, a 2-pin connector, and pre-applied thread sealant.
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.
- GMC S15: