Hydraulic Heat vs Engine Heat (When Your Excavator Overheats for the Wrong Reason)

Most excavators don’t die because their engines are weak.

They die because their engines are forced to absorb heat that never belonged to them in the first place.

Overheating is one of the most misunderstood failure modes in excavators. When temperature rises, the reflex response is always the same:Radiator. Coolant. Thermostat. Fan.

And when those don’t fix it, the engine gets blamed.

In reality, a large percentage of “engine overheating” complaints are not cooling failures at all. They are hydraulic heat overloads—where the hydraulic system quietly generates excessive heat and dumps it into the shared cooling system until the engine becomes collateral damage.

This blog explains how that happens, how to diagnose it correctly, and why confusing hydraulic heat with engine heat is one of the fastest ways to destroy a perfectly good excavator.

The Foundational Problem: One Cooling System, Multiple Heat Sources

Modern excavators don’t have isolated cooling systems.

They have a stacked cooling package:

• Hydraulic oil cooler

• Engine radiator

• Charge air cooler

All relying on:

• The same airflow

• The same fan

• The same shrouding

• The same ambient conditions

That means heat is cumulative.

If the hydraulic system generates excess heat, the engine cooling system must absorb it—whether it wants to or not.

This is where most diagnostics fail: technicians treat overheating as a single-source problem, when in reality it’s often a heat management problem.

Engines Don’t Usually Overheat Randomly

Internal combustion engines are predictable heat generators.

Engine heat output correlates closely with:

• RPM

• Load

• Fuel rate

• Ambient temperature

If an engine overheats:

• At high RPM

• Under sustained engine load

• Regardless of hydraulic activity

Then you likely have a genuine cooling system issue.

But many machines overheat under hydraulic load, not engine load.

That distinction matters.

Hydraulic Systems: Masters of Invisible Heat

Hydraulics are brutally efficient at turning energy into motion—and brutally efficient at turning inefficiency into heat.

Heat is generated every time hydraulic oil:

• Passes through a restriction

• Leaks internally

• Is forced over worn surfaces

• Is dumped across relief valves

The problem is that hydraulic systems can:

• Lose efficiency gradually

• Generate more heat as they wear

• Continue working while overheating internally

Unlike engines, they don’t immediately stall or alarm.

They just get hotter.

How Hydraulic Heat Enters the Engine Cooling System

Hydraulic oil coolers are usually mounted upstream of the radiator.

That means:

• Air is heated by the oil cooler first

• The radiator receives pre-heated air

• Cooling capacity is reduced before the engine even gets a chance

If hydraulic oil temperature climbs:

• The oil cooler rejects more heat

• Fan demand increases

• Radiator efficiency drops

Eventually, the engine runs hot—even if nothing is wrong with it.

The engine becomes the victim, not the cause.

Restricted Oil Coolers: The Perfect Disguise

One of the most common hydraulic heat causes is a restricted oil cooler.

Externally:

• Dust

• Oil mist

• Debris

• Fines

Internally:

• Oxidised oil

• Sludge

• Metal particles from worn components

Externally clogged coolers reduce airflow.Internally restricted coolers reduce heat transfer.

Both cause hydraulic oil temperature to rise.

And when hydraulic oil runs hot, engine coolant follows shortly after.

From the operator’s seat, it looks like an engine problem.

It isn’t.

Why Radiator Work Often Fails to Fix Overheating

Radiators get cleaned or replaced all the time.

And sometimes, nothing changes.

That’s because radiators remove heat—they don’t stop it being created.

If the hydraulic system is dumping excessive heat into the cooling stack:

• A new radiator may delay overheating

• A bigger fan may help temporarily

• Higher coolant flow may mask the issue

But none of these fix the source.

You cannot out-cool internal hydraulic leakage.

Fan Capacity: When Hydraulics Exceed the Design Limit

Cooling fans are sized for expected heat loads.

When machines are new:

• Hydraulic components are tight

• Leakage is minimal

• Oil stays cooler

As machines age:

• Pumps leak internally

• Valves bypass

• Motors bleed pressure

Heat generation increases.

Eventually, the hydraulic system produces more heat than the fan and cooling stack were designed to remove.

At that point:

• Fan runs constantly

• Engine derates

• Temperatures spike under load

The cooling system hasn’t failed.It’s being overwhelmed.

Swing Motors: The Forgotten Heat Source

Swing motors are one of the most overlooked contributors to hydraulic heat.

They operate:

• Continuously

• Under high torque

• With frequent direction changes

Worn swing motors:

• Leak internally

• Dump pressure into case drain

• Convert energy directly into heat

Because swing still works “normally,” the motor escapes suspicion.

But the heat doesn’t disappear—it goes straight into the oil.

Travel Motors: Heat Under Load, Not Speed

Travel motors generate enormous heat when:

• Operating on slopes

• Counter-rotating

• Fighting track resistance

• Running with worn internals

High case drain flow is common in worn travel motors.

That excess leakage:

• Raises oil temperature

• Loads the oil cooler

• Pushes heat into the radiator

Engines get blamed for what the undercarriage started.

Why Hydraulic Heat Rarely Triggers Alarms Early

Most machines monitor:

• Engine coolant temperature

• Sometimes hydraulic oil temperature

But alarms are conservative.

Hydraulic oil can:

• Run too hot for long periods

• Thin excessively

• Increase wear

• Generate more heat

All without triggering shutdowns.

By the time alarms appear, damage is already happening.

Diagnosing Heat Correctly: The Only Way Forward

Correct overheating diagnosis starts with a simple question:

Where is the heat being created?

Not:

• What part is hot?

• What alarm is active?

But:

• What system is generating excess energy loss?

Step 1: Compare Temperature Rise Rates

Watch:

• Hydraulic oil temperature

• Engine coolant temperature

If hydraulic oil temperature climbs first, the hydraulics are the source.

Step 2: Reduce Hydraulic Load

Idle the machine.Minimise hydraulic functions.

If temperatures stabilise:➡️ Hydraulic heat confirmed.

Engines don’t suddenly cool because hydraulics stop working.

Step 3: Case Drain Testing

Test:

• Main pumps

• Swing motor

• Travel motors

High case drain = internal leakage = heat generation.

This is one of the most reliable diagnostics available—and one of the most ignored.

Step 4: Pressure Drop and Relief Activity

Excessive relief valve operation creates heat instantly.

Listen for:

• Constant relief noise

• Stall behaviour

• Jerky functions under load

Relief valves don’t just waste pressure—they cook oil.

Why Engines Die After “Overheating” Complaints

Hydraulic heat doesn’t kill engines quickly.

It kills them slowly.

Prolonged elevated coolant temperatures cause:

• Oil breakdown

• Loss of lubrication

• Head gasket fatigue

• Liner distortion

• Bearing wear

When the engine finally fails, it looks guilty.

The hydraulic system walks away clean.

The Most Expensive Diagnostic Mistake

Replacing an engine without diagnosing hydraulic heat.

The new engine:

• Has tighter tolerances

• Produces less internal heat

• Has less margin for abuse

It fails faster than the old one.

Then the myth begins:

“These engines are junk.”

They aren’t.

They were murdered.

Brand Reality (Across the Board)

Different brands manage heat differently:

• Volvo derates aggressively

• Komatsu masks heat with load control

• CAT absorbs heat until it can’t

• Doosan and Hyundai tolerate abuse longer

But physics doesn’t care about branding.

Hydraulic heat will overwhelm any engine if left unchecked.

The Rule That Saves Machines

Never diagnose overheating by replacing cooling components alone.

Always ask:

• Is the heat being created—or just not removed?

Cooling systems remove heat.Hydraulics often create it.

Confusing the two is fatal.

Final Thought

Excavators don’t usually overheat because their engines are weak.

They overheat because worn hydraulic systems quietly turn efficiency into heat—and dump it into the one system that can’t refuse it.

Diagnose the heat source.Fix the cause.Then fix the cooling.

Anything else is just guessing with expensive parts.

Hydraulic Heat Decision Tree

Is Your Excavator Overheating Because of Hydraulics or the Engine?

START: Machine Is Overheating

⬇️

STEP 1: WHEN Does the Temperature Rise?

A️⃣ Overheats at high RPM even with minimal hydraulic use

➡️ Go to ENGINE COOLING PATH

B️⃣ Overheats mainly during digging, swinging, traveling

➡️ Go to HYDRAULIC HEAT PATH

🔥 HYDRAULIC HEAT PATH (Most Common)

⬇️

STEP 2: Which Temperature Rises First?

A️⃣ Hydraulic oil temperature rises before coolant

➡️ Hydraulic heat confirmed → Continue

B️⃣ Coolant rises first

➡️ Mixed issue → Check both paths

STEP 3: Reduce Hydraulic Load Test

• Idle machine• Minimal joystick input• No travel / no swing

Result:

• ✅ Temperature stabilizes➡️ Hydraulic heat source confirmed

• ❌ Temperature continues climbing➡️ Check ENGINE COOLING PATH

STEP 4: External Cooler Inspection

Check:

• Hydraulic oil cooler blocked externally?

• Oil mist + dust contamination?

• Bent fins / poor airflow?

Findings:

• ❌ Blocked → Clean/repair → Retest

• ✅ Clean → Continue

STEP 5: Internal Cooler Restriction Check

Signs:

• Hot hydraulic oil

• Cooler warm but ineffective

• Oil dark, oxidised, burnt smell

If YES:

➡️ Internal restriction likely → Flush or replace cooler

If NO:

➡️ Continue

STEP 6: Case Drain Testing (CRITICAL STEP)

Test:

• Main hydraulic pumps

• Swing motor

• Travel motors (both sides)

Results:

• ❌ High case drain on any component➡️ Internal leakage = heat generator FOUND

• ✅ Case drain normal➡️ Continue

STEP 7: Component-Specific Heat Sources

Swing Motor

• Constant use?

• Weak braking?

• High case drain?

➡️ Common hidden heat source

Travel Motors

• Overheats during travel or slopes?

• Machine pulls unevenly?

• One motor hotter?

➡️ Often fail in pairs

STEP 8: Relief Valve Abuse Check

Listen for:

• Constant relief noise

• Operators holding functions at end stroke

• Jerky movement under load

➡️ Relief valves dumping pressure = instant heat

HYDRAULIC CONCLUSION

If any of the following are true:

• High case drain

• Hot hydraulic oil under load

• Overheating disappears when hydraulics stop

➡️ DO NOT TOUCH THE ENGINE➡️ Hydraulic system is killing it

🧊 ENGINE COOLING PATH (Less Common, But Real)

⬇️

STEP 2E: Overheating With Minimal Hydraulic Use

Check:

• Coolant level & pressure

• Thermostat operation

• Fan speed / clutch engagement

• Shrouding & airflow integrity

STEP 3E: Radiator Performance Test

• Clean radiator properly• Measure inlet vs outlet temperature

Results:

• ❌ Poor delta → Radiator issue

• ✅ Good delta → Continue

STEP 4E: Engine Load Check

• Over-fueling?• Restricted exhaust?• Turbo issues?• Aftercooler blockage?

➡️ Engine heat production issue

ENGINE CONCLUSION

Only blame the engine if:

• Overheating occurs without hydraulic load

• Cooling stack tests fail

• Fan & airflow are insufficient

⚠️ THE GOLDEN RULE (PRINT THIS)

Never replace an engine for overheatinguntil hydraulic heat has been ruled out.

Most engines die innocent.

Why This Decision Tree Saves Money

• Stops repeat overheating after engine replacement

• Prevents blaming radiators for hydraulic wear

• Identifies hidden swing & travel motor killers

• Separates “cooling failure” from “heat creation”

This is how professionals diagnose.Flippers guess.

The Hydraulic Heat Survival Guide

How to Keep Hydraulic Heat From Quietly Killing Your Excavator

Most excavators don’t die dramatically.

They don’t explode.They don’t seize on the spot.They don’t fail right after a warning light.

They die slowly.

And in a frightening number of cases, the killer is hydraulic heat—not engine failure, not bad luck, not “these machines are junk.”

Hydraulic heat is silent, cumulative, and brutally expensive if you don’t understand it. This guide exists to do one thing:

Keep your excavator alive by stopping hydraulic heat before it destroys everything around it.

What Hydraulic Heat Really Is (And Why It’s So Dangerous)

Hydraulic systems are energy converters.

They turn:

• Engine power → hydraulic pressure

• Hydraulic pressure → movement

Any inefficiency in that process becomes heat.

That heat comes from:

• Internal leakage

• Pressure drop across worn components

• Relief valve activity

• Oil being forced through restrictions

Unlike engines, hydraulic systems can:

• Generate increasing heat as they wear

• Keep functioning while cooking themselves

• Kill other systems without obvious failure

Hydraulic heat doesn’t announce itself.It accumulates.

Why Hydraulic Heat Is an Engine Killer

Most excavators use a shared cooling stack:

• Hydraulic oil cooler

• Radiator

• Charge air cooler

That means all heat—engine and hydraulic—must be rejected by the same airflow and fan.

When hydraulic oil runs hot:

• The oil cooler dumps massive heat into the air

• The radiator receives pre-heated air

• Engine cooling efficiency collapses

The engine didn’t create the heat.But it’s forced to absorb it.

Over time, this leads to:

• Chronic high coolant temperatures

• Oil breakdown

• Bearing wear

• Head gasket fatigue

• Premature engine failure

The engine gets blamed.The hydraulics walk free.

The Hydraulic Heat Death Spiral

This is how machines quietly die:

1. Hydraulic components wear

2. Internal leakage increases

3. Hydraulic oil temperature rises

4. Oil thins and oxidises

5. Leakage increases further

6. Even more heat is generated

7. Cooling system becomes overloaded

8. Engine runs hot for long periods

9. Engine fails

10. Machine is declared “tired”

This isn’t bad luck.It’s physics.

The Biggest Myth: “It’s Just Running a Bit Warm”

There is no such thing as “a bit warm” hydraulically.

Hydraulic oil temperature has compounding effects:

• Every 10°C increase halves oil life

• Thin oil leaks more

• Leaks create more heat

• Heat accelerates wear

By the time you feel the problem, the damage is already done.

Where Hydraulic Heat Comes From (The Real Sources)

1. Worn Main Pumps

Worn pumps don’t always lose pressure immediately.

Instead, they:

• Bypass oil internally

• Convert power directly into heat

• Show high case drain

A pump can “still work” while acting like a heater.

2. Valve Bank Wear

Valve spools wear gradually.

This causes:

• Pressure drop

• Constant bypass

• Flow inefficiency

Valve banks can generate huge heat without obvious performance loss—especially early.

3. Swing Motors (The Silent Offender)

Swing motors:

• Work constantly

• Change direction frequently

• Handle high torque

When worn:

• Internal leakage skyrockets

• Case drain increases

• Heat output climbs continuously

Because swing still feels “normal,” it often escapes suspicion.

4. Travel Motors (Heat Under Load)

Travel motors generate heat when:

• Climbing

• Counter-rotating

• Fighting track resistance

Worn motors:

• Leak internally

• Dump heat into the oil

• Overload oil coolers

They often fail—and overheat—in pairs.

5. Relief Valve Abuse

Relief valves are heat machines.

Every time oil dumps across a relief:

• Pressure is converted straight into heat

Common causes:

• Holding functions at end stroke

• Poor operating habits

• Incorrect valve settings

Relief noise = oil cooking.

Why Cooling System Fixes Often Fail

Radiators get cleaned.Fans get replaced.Coolant gets flushed.

Sometimes nothing changes.

Why?

Because cooling systems remove heat—they don’t stop it being created.

If hydraulics are producing more heat than the system was designed for:

• Bigger radiators only delay the problem

• Stronger fans just mask it

• New engines die faster

You cannot out-cool internal leakage.

How to Tell If Hydraulic Heat Is the Real Problem

1. Overheating Happens Under Hydraulic Load

If temperatures rise mainly during:

• Digging

• Swinging

• Traveling

And stabilise at idle → hydraulic heat.

Engines don’t cool down just because hydraulics stop.

2. Hydraulic Oil Temperature Rises First

If hydraulic oil temperature climbs before coolant:➡️ You’ve found your culprit.

3. New Radiator Didn’t Fix It

Radiators don’t half-work.

If airflow and coolant flow are restored and overheating remains, the heat source is upstream.

4. Case Drain Is High

Case drain testing is the truth serum of hydraulics.

High case drain = internal leakage = heat.

Survival Rule #1: Monitor Hydraulic Oil Temperature Like a Hawk

Most operators watch coolant temperature.

Few watch hydraulic oil temperature closely.

That’s a mistake.

Hydraulic oil temperature is an early warning system.Coolant temperature is a late alarm.

If oil temperature is climbing:

• Damage is already happening

• Engine trouble is coming next

Survival Rule #2: Treat Oil Coolers as Critical Components

Oil coolers fail in two ways:

• Externally clogged

• Internally restricted

External clogging reduces airflow.Internal restriction reduces heat transfer.

Both lead to rising oil temperature.

Cleaning only the outside is often not enough.

Survival Rule #3: Case Drain Testing Is Not Optional

Any high-hour machine should undergo:

• Pump case drain testing

• Swing motor case drain testing

• Travel motor case drain testing

This isn’t overkill.It’s survival.

Ignoring case drain is how machines die quietly.

Survival Rule #4: Don’t Ignore “It Still Works”

Hydraulic systems can:

• Work

• Dig

• Travel

• Swing

And still be destroying themselves with heat.

Performance loss comes after thermal damage—not before.

Survival Rule #5: Operator Behaviour Matters More Than You Think

Bad habits create heat:

• Riding relief valves

• Slamming controls

• Holding stalled functions

• High idle with no load management

The machine pays the price—even if the operator doesn’t.

The Most Expensive Mistake of All

Replacing an engine without addressing hydraulic heat.

The new engine:

• Has tighter tolerances

• Produces less internal heat

• Has less thermal margin

It fails faster than the old one.

Then people say:

“These engines are rubbish.”

They aren’t.

They were murdered by hydraulics.

Brand Doesn’t Save You

Some brands:

• Mask heat better

• Derate more aggressively

• Absorb abuse longer

None are immune.

Physics doesn’t care if it’s Volvo, CAT, Komatsu, Doosan, or Hyundai.

Hydraulic heat kills them all eventually.

The Golden Survival Principle

If you don’t control hydraulic heat, it will control how long your excavator lives.

Not hours.Not brand.Not luck.

Heat.

Final Words: How Machines Actually Survive

Excavators that live long lives share three traits:

1. Hydraulic oil temperature is monitored

2. Case drain is tested before failure

3. Heat sources are fixed early—not after engines die

That’s it.

Hydraulic heat doesn’t need to kill your machine.

But if you ignore it, it absolutely will.