A Deep Dive into the 1975-1983 Jeep CJ5 Engine Coolant Thermostat
Don't let a simple, inexpensive part cause major engine trouble in your classic CJ5.
- A thermostat fails either stuck closed (causing overheating) or stuck open (causing the engine to run too cold).
- The standard temperature is 195°F for stock CJ5 engines; use this unless your engine is heavily modified.
- For reliable stock replacements, choose Jeep-specific brands like Crown or Omix-Ada, or a reputable brand like Gates.
- Drilling a small 'bleed' hole in the thermostat flange is a common and effective trick to prevent air pockets in the cooling system.
Is Your CJ5 Thermostat Failing?
A thermostat failure in your CJ5 will almost always happen in one of two ways: stuck closed or stuck open. The symptoms are usually very clear.
A stuck-closed thermostat is the most dangerous failure. It traps coolant inside the engine block, preventing it from reaching the radiator. This will cause your engine to overheat very quickly, often within minutes of starting. If you see the temperature gauge climbing into the red, pull over and shut off the engine immediately to prevent catastrophic damage like a warped cylinder head or blown head gasket.
A stuck-open thermostat is less dangerous but still a problem. It allows coolant to flow to the radiator constantly. This prevents the engine from ever reaching its proper operating temperature, especially in cooler weather. Signs of a stuck-open thermostat include the temperature gauge staying on the low end, poor heater performance (blowing lukewarm air), and a potential decrease in fuel economy.
Choosing the Right Aftermarket Thermostat for Your CJ5
For a vintage vehicle like the 1975-1983 CJ5, new OEM (Original Equipment Manufacturer) thermostats are generally not available. The good news is that the aftermarket offers many options, but quality can vary. The standard factory-specified temperature for most CJ5 engines of this era is 195°F.
Aftermarket Brand Quality Tiers
| Tier | Brands | Assessment |
|---|---|---|
| Good (Stock Replacement) | Crown Automotive, Omix-Ada, Gates | Crown and Omix-Ada are well-known brands specializing in Jeep replacement parts that meet original specifications. Gates is a long-standing, reputable general aftermarket brand. These are all solid, reliable choices for a daily driver or stock restoration. |
| Performance Upgrade | Mr Gasket, Edelbrock, Mishimoto | These brands offer high-flow thermostats, sometimes with lower temperature ratings (like 180°F). A high-flow design can improve coolant circulation, which is beneficial for modified engines or heavy-duty use like towing or slow-speed rock crawling. Mishimoto is a popular choice for performance cooling system upgrades. |
| Use With Caution | Motorad, Modern Stant | Historically, Stant SuperStats made in the USA were considered top-tier. However, Stant was acquired by Motorad, and many enthusiasts report that new parts sold under the Stant, and other store brands, are now re-boxed Motorad units with a different design and more frequent failures. While Motorad's "Fail-Safe" thermostats are designed to fail in the safer open position, reports of them failing prematurely or not regulating temperature correctly are common in forums. |
Known Issues, Recalls, or TSBs
Searches for official recalls or Technical Service Bulletins (TSBs) from the NHTSA regarding the engine coolant thermostat for the 1975-1983 Jeep CJ5 did not yield any specific results. Failures are typically due to age and wear rather than a specific manufacturing defect.
Frequently Asked Questions (FAQ)
Should I use a 195°F or 180°F thermostat?
For a stock engine, stick with the factory 195°F thermostat. This temperature ensures the engine runs efficiently and burns off contaminants in the oil. A lower temperature 180°F thermostat may be used in heavily modified engines or dedicated off-road rigs that see extreme, low-speed use, but for most applications, it will cause the engine to run too cool.
Do I need a new gasket?
Yes. Never reuse the old thermostat housing gasket. A new gasket is cheap and essential for preventing coolant leaks. Make sure the metal surfaces on both the engine and the thermostat housing are perfectly clean before installing the new gasket.
Does it matter which way the thermostat is installed?
Absolutely. The thermostat must be installed with the spring-loaded, cylindrical part facing the engine and the flat, flanged part facing the radiator hose. Installing it backward will block coolant flow and cause immediate overheating.
What is a "high-flow" thermostat?
A high-flow thermostat, like those from Mr. Gasket or Edelbrock, has a larger opening that allows more coolant to pass through when it opens. This can help improve cooling system performance under heavy load, but it is not essential for a stock engine in good working order.
Technical Specifications
OEM Part Numbers: 83501426 J3189874
Standard Opening Temperature: 195° F (90.56° C)
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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.