Engine Coolant Thermostat Guide for 1981-1989 Dodge D100
Keep your classic D100's engine temperature in check by learning how to spot a bad thermostat and choose the right replacement.
- A thermostat fails by either sticking open (engine runs too cold) or sticking closed (engine overheats). Overheating is an emergency.
- For a stock D100, a 195-degree thermostat from a reputable brand like Gates or Stant is a reliable choice.
- Replacement is an easy DIY job that requires basic tools and should take less than an hour.
- Always replace the gasket and test the new thermostat in hot water before installing it to ensure it works correctly.
Is Your D100's Thermostat Failing?
A thermostat is a common wear item. For a truck from the 1980s, it's a part you can expect to replace. The two main ways a thermostat fails are sticking open or sticking closed. Both can cause problems, but one is much more dangerous for your engine.
Symptoms of a Bad Thermostat
- Engine Overheating: If the thermostat sticks closed, coolant can't get to the radiator to cool down. Your temperature gauge will climb into the red, especially when driving. This is a serious issue that can cause major engine damage.
- Engine Slow to Warm Up: If the thermostat sticks open, coolant constantly flows to the radiator. This makes it hard for the engine to reach its proper operating temperature. You might notice poor heater performance and decreased fuel mileage.
- Temperature Gauge Fluctuating: A gauge that swings wildly between hot and cold can sometimes point to a thermostat that is opening and closing erratically. However, it can also indicate air in the cooling system or a faulty temperature sending unit.
- Coolant Leaks: Coolant might leak from around the thermostat housing. This is often due to a failed gasket, which should always be replaced when you replace the thermostat.
Overheating is an Emergency
If you see the temperature gauge on your D100 climbing into the red, pull over and shut off the engine as soon as it is safe. Driving an overheating engine can lead to a blown head gasket, warped cylinder heads, or complete engine seizure.
Choosing the Right Aftermarket Thermostat
For a 1981-1989 Dodge D100, finding an original Mopar thermostat is difficult and not necessary. Today's aftermarket offers a wide range of quality options. The key is choosing a reputable brand.
Aftermarket Brand Tiers for Your D100
- Standard Replacement (Good): Brands like Gates and Stant are well-known and generally provide reliable, OE-equivalent performance. They are a solid choice for a daily driver or stock restoration. While quality control can be an issue with any brand, these are trusted names in the industry.
- Performance (Better): Brands such as Edelbrock, Mr. Gasket, and Mishimoto offer high-flow thermostats. These are designed to allow more coolant to pass through when open, which can improve cooling efficiency in high-performance or heavy-duty applications. They may also offer different temperature ratings (e.g., 160°F or 180°F) for owners who want to run their engine slightly cooler.
- Budget (Use with Caution): Motorad is a very common brand, but forum discussions show mixed to negative reviews, with frequent complaints of premature failure or incorrect opening temperatures. Some reports indicate that other brands may sometimes re-box Motorad parts, so inspect the part upon arrival.
Test Before You Install
No matter which brand you choose, it's a great idea to test your new thermostat before installing it. Suspend the thermostat in a pot of water on the stove with a thermometer. Heat the water and watch to see that the thermostat opens near its rated temperature (e.g., 195°F). This simple step can save you from having to do the job twice.
Cost of a New Engine Coolant Thermostat
| Part Type | Typical Cost |
|---|---|
| New Aftermarket Thermostat | $8 - $56 |
| Shop Labor for Installation | $75 - $150 |
Frequently Asked Questions
What temperature thermostat should I use for my D100?
For a stock Slant-6 or V8 engine, a 195°F thermostat is the standard original equipment temperature. This allows the engine to run efficiently and produce good heat. Using a 180°F thermostat is a common modification, but going lower on a stock, computer-controlled engine (on later models) isn't recommended as it can affect performance.
Is it hard to replace a D100 thermostat myself?
No, this is a great DIY job for beginners. The thermostat is usually located at the front of the engine where the upper radiator hose connects. The process involves draining some coolant, removing two bolts, cleaning the housing surface, and installing the new thermostat and gasket.
Do I need to replace the gasket too?
Yes. Always install a new gasket with a new thermostat to prevent leaks. Make sure both the engine and housing surfaces are clean and free of old gasket material before installation.
My temperature gauge is acting weird, is it the thermostat?
It could be. But on these older trucks, you should also check for other causes. A fluctuating gauge could be from air trapped in the cooling system, a bad radiator cap, or a faulty temperature sending unit or gauge. Don't assume it's the thermostat without checking the basics first.
Were there any recalls on the D100 thermostat?
No. Searches of the NHTSA database show no recalls or technical service bulletins (TSBs) related to the engine coolant thermostat for the 1981-1989 Dodge D100.
Technical Specifications
Standard Temperature: 195°F (89°C) Optional Temperatures: 180°F (82°C), 160°F (71°C) Required component: Gasket
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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.
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