Engine Coolant Thermostat Guide for the 1989-1993 Dodge D350
Keep your first-generation Cummins running at the right temperature by choosing the correct replacement thermostat.
- A failing thermostat can cause your D350's Cummins engine to overheat or run too cold, both of which are harmful.
- For maximum reliability, many owners prefer the genuine Cummins OEM thermostat (P/N 5292738).
- Among aftermarket brands, Gates and Mishimoto are popular choices, while Motorad has a poor reputation in owner forums for this application.
- Replacement is a straightforward DIY job that takes about an hour and requires basic tools.
Symptoms of a Failing Thermostat
A bad thermostat on your D350 will typically fail in one of two ways: stuck open or stuck closed. Both conditions can cause problems for your 12-valve Cummins engine.
- Engine Overheating: If the thermostat is stuck closed, coolant can't get to the radiator. Your temperature gauge will climb into the red zone, which can lead to serious engine damage if not addressed immediately.
- Engine Too Slow to Warm Up: A thermostat stuck open will constantly circulate coolant, even when the engine is cold. This makes it take a very long time for the engine to reach its normal operating temperature, especially in winter. This can cause poor heater performance and reduced fuel efficiency.
- Temperature Gauge Fluctuates: A temperature gauge that swings up and down erratically can point to a thermostat that is sticking or opening and closing at the wrong times.
- Coolant Leaks: While less common, a stuck-closed thermostat can cause excessive pressure in the cooling system, potentially forcing coolant out around the thermostat housing gasket or other weak points.
Choosing the Right Aftermarket Thermostat
When shopping for a new thermostat for your 1989-1993 D350, you have several options. The choice between OEM and aftermarket, as well as the temperature rating, depends on your needs and budget.
OEM vs. Aftermarket Quality
For critical engine parts on the 5.9L Cummins, many experienced owners recommend using the genuine Cummins OEM thermostat. The OEM part (Cummins P/N 5292738) is known for its reliability and consistent performance, operating at the factory-specified 180°F. Aftermarket parts can be a good value, but quality varies by brand. Some may not perform as consistently as the OEM unit.
Aftermarket Brand Tiers
- Premium Aftermarket (Mishimoto): Mishimoto is a well-known performance brand that offers direct-fit thermostats for the first-gen Cummins. They provide options for lower (174°F) and higher (190°F) opening temperatures to suit different climates or uses. A lower temperature may help in hot climates or during heavy towing, while a higher temperature aids faster warm-ups in the cold. They come with a lifetime warranty, but owner reviews are mixed; some see benefits while others notice little difference.
- Standard Aftermarket (Gates): Gates is a widely available and generally respected aftermarket brand. They are a common choice and often considered a reliable alternative to OEM parts.
- Budget/Economy (Motorad): While widely available, Motorad thermostats have received negative feedback in owner forums. Some users have reported issues with these parts failing to let the engine warm up properly or have concerns about them failing in the stuck-closed position. Caution is advised with this brand for this application.
- Performance (Mr. Gasket): Mr. Gasket offers performance thermostats, but most available information relates to gasoline engines like the Hemi, not the 12-valve Cummins. Given the lack of specific positive reviews for the D350, it may be better to stick with a brand proven for this diesel engine.
Pro Tip: Choose the Right Temperature
The stock 180°F thermostat is ideal for most situations. Running a diesel engine too cold can negatively affect performance and efficiency. Only consider a lower temperature thermostat if you live in a very hot climate or tow heavy loads constantly. A higher temperature thermostat can be beneficial for trucks that operate in consistently freezing weather.
Cost of a New Engine Coolant Thermostat (1989-1993 Dodge D350)
| Part Type | Price Range |
|---|---|
| New Aftermarket Thermostat | $8.47 - $56.13 |
| New OEM (Cummins) Thermostat | $80 - $100 |
| Shop Labor Cost | $90 - $150 |
Note: Prices are estimates and can vary based on location, brand, and supplier.
Frequently Asked Questions
Is it hard to replace the thermostat on a 1st Gen Cummins?
No, it is a relatively simple job for someone with basic mechanical skills. It involves partially draining the coolant, removing the upper radiator hose and thermostat housing, swapping the part, and refilling the coolant. The estimated time is about 45 to 90 minutes.
Do I need a new gasket?
Yes, you should always use a new gasket or seal when replacing the thermostat to prevent leaks. Many aftermarket thermostats, like those from Mishimoto, include a new gasket in the box. Always clean the old gasket material off the housing and engine surfaces completely before installing the new one.
Are there any recalls for the thermostat on my D350?
No, searches of the NHTSA database and other sources did not find any recalls or technical service bulletins (TSBs) specifically for the engine coolant thermostat on the 1989-1993 Dodge D350.
Which way does the thermostat go in?
The thermostat must be installed in the correct orientation. The spring-loaded side should face the engine. If it has a small jiggle pin or bleeder valve, it should be positioned at the top (12 o'clock position) to help bleed air out of the system.
Technical Specifications
OEM Part Numbers: 5292738 3917324 3972071 3283589
Standard Opening Temperature: 180°F (82°C). Housing Bore Diameter: 54.25 mm.
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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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