Cooling & overheating

Coolant expansion tank cracked

A cracked plastic expansion tank (reservoir) often leaks only when the system is pressurized and hot, making it easy to miss during a quick cold inspection, and it's a common failure point since the plastic becomes brittle with age.

Difficulty
Easy
Time
20–40 minutes to replace
DIY cost
$20–$50
Pro cost
$100–$250

Symptoms

  • Coolant level in the tank drops but no leak is visible when cold
  • Fine hairline cracks visible on close inspection of the plastic tank
  • Leak more apparent after the vehicle has been driven and is warm
  • Possible coolant residue around the tank's mounting area

Likely causes

  • CommonPlastic tank becoming brittle and cracking with age and heat cyclingUV exposure and years of heat cycling make the plastic more prone to hairline cracks, especially around seams or mounting points.
  • PossibleCap seal not properly sealing to the tankA leak can look like it's from the tank body when it's actually the cap not sealing properly against it.
  • Less commonOvertightened or cross-threaded cap causing a crackExcess force on the cap can crack the neck of the tank.

What to do

  1. Inspect the tank closely, including in bright light, for fine cracks, especially around the seams and near the cap.
  2. Check if the leak is worse or only present when the system is warm and pressurized versus cold.
  3. Check the cap's seal separately, since it can be a simpler and cheaper fix if that's the actual source.
  4. Avoid overtightening the replacement cap, which can create new problems.
  5. Have a mechanic confirm whether it's the tank, the cap, or a nearby hose causing the apparent leak.
  6. Replace the cracked expansion tank, since plastic tanks generally can't be reliably repaired.
  7. Refill and bleed the system as needed after replacement, and monitor for a few days to confirm the leak is resolved.

Tools and parts

  • Flashlight
Coolant expansion tank/reservoirSearch: “coolant reservoir tank for [make model year]”

When to call a pro

See a mechanic if you can't clearly tell whether the tank, cap, or a hose is the source, since a pressure test can pinpoint it quickly.

Safety

Generally low risk

Generally low risk for a careful person. Stop if anything looks different from what this guide describes.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.