How to Replace a Diesel Coolant Reservoir

The coolant reservoir, or degas bottle on many diesels, does more than hold spare coolant; on pressurized-degas systems it separates air from the coolant and is part of the sealed cooling circuit. When it cracks, the cap fails, or the seams weep, you get coolant loss, air in the system, and overheating. Replacing it is an easy job, but bleeding the system afterward is where people go wrong. Here is the full procedure.

Signs the reservoir needs replacing

• Cracks or crusty coolant trails on the bottle, seams, or fittings
• A cap that no longer holds pressure, causing overheating or boil-over
• Persistent air in the cooling system and unexplained coolant loss
• Brittle, discolored plastic from years of heat cycles

Step-by-step replacement

1. Let the engine cool completely before opening the cap; a hot system is pressurized and will scald.
2. Drain or siphon coolant to below the reservoir, saving clean coolant for reuse if it is fresh.
3. Label and disconnect the hoses at the reservoir, catching residual coolant. Note any sensor connectors, such as a low-coolant sensor.
4. Remove the mounting hardware and lift the old reservoir out.
5. Transfer any sensor to the new reservoir if required, using a new seal.
6. Install the new reservoir, reconnect the hoses with good clamps, and reconnect any sensor.
7. Refill with the correct coolant, install a new cap if your kit includes one, and bleed the system.

Bleeding is the critical step

On a degas-style system, air must be purged or the engine will run hot and the level will read wrong. Fill slowly, open any bleed screws until air-free coolant flows, run the engine with the heater on hot, and top off as the level drops when the thermostat opens. Recheck the level and the cap seal once the system has cooled. A correct cap that holds rated pressure is essential, so replace a questionable cap.

Do not overlook the cap

The pressure cap is easy to ignore, but it sets the boiling point of the whole system by holding pressure. A weak or wrong cap causes overheating and boil-over even with a brand-new reservoir. When you replace the bottle:

• Use a cap rated for your system's pressure, not just one that fits.
• Inspect the cap's rubber seals for cracks and swelling.
• Confirm the reservoir is the correct pressurized-degas type for your engine, not a plain overflow bottle.
• Check the hoses and clamps you disturbed, replacing any that are hard or cracked.

Get the cap and the bleed right and a reservoir swap is a lasting, inexpensive fix.

Frequently asked questions

What is a degas bottle?

It is a pressurized reservoir that separates air from the coolant and is part of the sealed cooling circuit on many diesels, unlike a simple overflow bottle.

Why do I keep getting air in my cooling system?

A cracked reservoir, a failed cap, or incomplete bleeding all introduce or trap air. Fix the leak, use a correct cap, and bleed thoroughly.

Do I need to replace the cap too?

If the cap is old or does not hold rated pressure, yes. A weak cap causes overheating even with a new reservoir. Need a reservoir or cap? Email sales@coredieselparts.com.