Engine Coolant Thermostat Guide for 1975-1980 Dodge D100
Keep your classic Dodge D100's engine running at the right temperature by understanding, diagnosing, and replacing a faulty thermostat.
- A failing thermostat can cause your D100 to overheat or run too cool.
- Overheating is a critical issue; pull over immediately if the temperature gauge enters the red zone.
- High-quality aftermarket is the only option; Gates is a reliable choice, while brands like Motorad have mixed reviews.
- Always replace the gasket with the thermostat to prevent leaks.
Is Your D100's Thermostat Failing?
A thermostat is a wear item. After decades of service in your Dodge D100, it can fail in two ways: stuck open or stuck closed. A stuck-open thermostat is less dangerous but will cause your engine to take a very long time to warm up, leading to poor fuel economy and performance. A stuck-closed thermostat is a serious problem that will cause the engine to overheat quickly, potentially leading to severe engine damage like a blown head gasket or warped cylinder heads.
Choosing the Right Aftermarket Thermostat
For a vehicle from the 1975-1980 era, finding a genuine Mopar "New Old Stock" (NOS) thermostat is nearly impossible. Aftermarket parts are your only realistic option. The good news is that a quality aftermarket part from a reputable brand will meet or exceed the original 1970s specifications. However, quality varies between brands.
Forum discussions and mechanic reviews show a wide range of experiences with different brands. Here’s a general breakdown:
- Tier 1 (Good Quality): Gates is a consistently recommended brand for cooling system parts and is a safe bet for reliability. Historically, Stant was a top choice, but they are now owned by Motorad, and quality concerns have been raised.
- Tier 2 (Use With Caution): Motorad is one of the largest manufacturers, even making parts for other brands. However, they have very mixed reviews, with many owners reporting premature failures or inconsistent temperatures.
- Tier 3 (Performance/Specialty): Brands like Mr. Gasket and Edelbrock offer high-flow thermostats. These are generally not necessary for a stock or lightly modified engine and can sometimes cause issues if not part of a larger cooling system upgrade.
A Note on "Fail-Safe" Thermostats
Some brands offer "fail-safe" thermostats designed to lock in the open position if the engine overheats once. While this prevents catastrophic overheating, it means the thermostat is then permanently stuck open and must be replaced again. Many mechanics prefer standard thermostats for their simplicity and predictability.
Cost of a New Thermostat for 1975-1980 Dodge D100
| Part Type | Price Range |
|---|---|
| New Aftermarket Thermostat | $7.33 - $56.13 |
| Shop Labor for Installation | $75 - $150 |
Prices are estimates and can vary based on brand, location, and vehicle condition.
Pro Tip: Easy Installation
Replacing the thermostat on these trucks is a simple job for a DIY mechanic. The thermostat is usually located where the upper radiator hose connects to the engine. Just drain some coolant, unbolt the two bolts on the housing, swap the thermostat (don't forget a new gasket!), and bolt it back together. Always make sure the new thermostat is installed in the correct orientation, with the spring side facing the engine.
Frequently Asked Questions
What temperature thermostat should I use for my D100?
For a stock engine, a 180°F or 195°F thermostat is typically recommended. A 195°F thermostat is often preferred for better engine efficiency and heater performance. Using a lower temperature thermostat (like 160°F) is generally not recommended unless you have specific cooling issues in a hot climate or a modified engine.
Do I need to replace the gasket too?
Yes. Always use a new gasket when replacing the thermostat. Reusing the old gasket will almost certainly cause a coolant leak. Some thermostat kits come with a gasket, but be sure to check.
My truck still overheats with a new thermostat. What's wrong?
If you've replaced the thermostat and the engine still overheats, you could have air trapped in the cooling system. Try parking the truck on an incline with the radiator cap off and letting it run to "burp" the air out. Other causes could be a clogged radiator, a failing water pump, or a bad fan clutch.
Are there any recalls for the thermostat on my Dodge D100?
No. Searches of the National Highway Traffic Safety Administration (NHTSA) database show no recalls or technical service bulletins (TSBs) related to the engine coolant thermostat for the 1975-1980 Dodge D100.
Technical Specifications
Standard flange diameter is approximately 2.125 inches. Available temperature ratings include 160°F, 180°F, and 195°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.