Menu

Home / Symptoms / Losing coolant

Cooling

My diesel is losing coolant with no visible leak. Why?

Applies to: Duramax, Power Stroke, all diesel engines · Pickups, Semi trucks, RV / diesel pushers, School buses, Vans & cars, Tractors / ag

DuramaxPower StrokeAll dieselsPickupsSemi trucksRV / diesel pushers+ moreCooling

Short answer

When coolant disappears without a puddle, it is usually going somewhere inside the engine. On EGR-equipped diesels a leaking EGR cooler is a well-known suspect and can mimic a head gasket. Testing comes before teardown.

Safety first. Diesel injection systems run at extremely high pressure; a pinhole leak can inject fuel through skin. Exhaust and turbo parts stay very hot long after shutdown. Never open a hot cooling system. If you are unsure, stop and see a qualified diesel technician.
Duramax LML diesel V8 display engine on a stand
A Duramax LML display engine. Duramax coolant-loss bulletins point to testing the EGR cooler first. Photo: Dana60Cummins, CC0, via Wikimedia Commons.

Start with the easy checks

  1. Confirm it is really dropping: check cold, same spot, over time.
  2. Look closely for external weeps, dried coolant crust and hose damage.
  3. Watch for white smoke on startup and a sweet smell.
  4. Check for milky oil.

Likely causes, most common first

EGR cooler leak
Coolant leaks into the exhaust path and can reach cylinders. The AERA Duramax bulletin says to test the EGR cooler before engine repair if coolant is found in cylinders.
Head gasket or cracked head
Coolant into a cylinder or combustion gas into the coolant.
Oil cooler failure
Can mix fluids; on the 6.0 Power Stroke a clogged oil cooler is linked to EGR cooler failures.
Hidden external leaks
Behind covers or evaporating on hot parts.

These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.

Duramax note

The AERA Engine Builders Association bulletin on 2002-2016 6.6L Duramax coolant loss says not all coolant loss is a head gasket: EGR cooler leaks can let coolant reach the right-bank cylinders, so the cooler must be pressure tested first.

When to stop and see a diesel tech

  • Any coolant in the oil or cylinders.
  • Pressurized cooling system, bubbles in the reservoir.
  • You need a pressure test of the EGR cooler.

Questions

Can an EGR cooler make it look like a blown head gasket?

Yes. An AERA bulletin for Duramax engines warns that EGR cooler leaks can put coolant into cylinders and lead to misdiagnosis.

Cooling

How do I know if my 6.0 Power Stroke EGR cooler is failing?

Power StrokePickupsVans & carsTroubleshoot →Smoke

What does white smoke from my diesel mean?

All dieselsPickupsSemi trucksRV / diesel pushers+ moreTroubleshoot →Cooling

Why does my diesel run hot when towing?

All dieselsPickupsRV / diesel pushersTroubleshoot →

Where this comes from

Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.