Hydrovac Hydraulic System Overheating: Causes and Solutions

When your hydraulic system starts running hot, you've got a problem that's actively getting worse. Every degree above normal is doing damage — breaking down the oil, wearing seals, and killing pump life. I've rebuilt hydraulic systems that looked like they'd been tortured, all because someone ignored that temperature gauge.

I've been working on hydrovac hydraulic systems for over 25 years. Let me tell you what causes overheating and what you can actually do about it before you cook a $15,000 system.

Why Overheating Happens

Your hydraulic system generates heat during normal operation — that's physics. Fluid under pressure moving through restrictions creates friction, friction creates heat. The system is designed to handle this with a cooler and adequate fluid volume.

When the heat generated exceeds the system's ability to dissipate it, temperature climbs. Eventually it climbs high enough to cause damage.

What Gets Damaged:

• Hydraulic fluid breaks down (loses lubricating properties)
• Seals dry out and fail (causing leaks)
• Pump internals wear rapidly (metal expanding at different rates)
• Valves stick or malfunction
• Hoses degrade faster

Normal operating temperature for most hydrovac hydraulic systems is 120-160°F. Above 180°F you're in the danger zone. Above 200°F you're causing active damage.

The Most Common Causes

Let me go through these in order of how often I see them:

1. Low Fluid Level

This is number one with a bullet. Low fluid means less oil to absorb and transfer heat, plus the pump works harder to move what's left. It's a double whammy.

How to Check: Look at your sight glass when the system is warm and running. Fluid should be visible in the middle of the sight glass. Can't see it? You're low.

Why It Happens:

• Leaks (the obvious one)
• Consumption during operation (some bypass is normal)
• Someone didn't refill after a repair

The Fix: Top it off with the correct fluid. Find and fix any leaks. This sounds simple but I've seen trucks run low for weeks because "the sight glass is kinda foggy anyway."

2. Failed or Clogged Hydraulic Cooler

Your hydraulic cooler works like a radiator — hot oil flows through it, air or water cools it, cooled oil returns to the system. If the cooler can't do its job, heat has nowhere to go.

Symptoms:

• System runs hot even at light loads
• Cooler isn't warm to the touch (means no flow)
• Visible damage or clogging on cooler fins

What Fails:

• Fins clogged with mud, debris, or bugs
• Internal passages plugged
• Bypass valve stuck open (oil skips the cooler)
• Air cooler fan not running
• Water-cooled systems: water supply issues

The Fix: Clean those fins — really clean them. Pressure wash the cooler from back to front (opposite of airflow) to push debris out, not further in. Check that the fan is actually spinning when the system's running. For water-cooled systems, verify water flow through the heat exchanger.

I had a Maryland contractor whose truck overheated every summer. Finally pulled the cooler and found a plastic bag sucked onto the back side, blocking half the airflow. Problem there for who knows how long.

3. Wrong Hydraulic Fluid

Not all hydraulic oil is the same. Using the wrong viscosity or type causes overheating because the system has to work harder to move fluid that's too thick, or doesn't get proper lubrication from fluid that's too thin.

Common Mistakes:

• Using motor oil (NO!)
• Mixing different fluid types
• Using summer weight in winter or vice versa
• Using cheap fluid that doesn't meet specs

The Fix: Drain and refill with manufacturer-specified fluid. For most hydrovacs, that's AW46 hydraulic oil for moderate climates. Colder areas might need AW32, hotter areas AW68. Check your operator's manual.

4. Internal Pump Bypass (Worn Pump)

When hydraulic pumps wear internally, fluid bypasses instead of being pushed into the system. This bypass flow generates tremendous heat because pressure energy converts to heat energy.

Symptoms:

• System is hot AND weak (slow operation, low power)
• Pump is very hot to the touch
• Gradual loss of system performance over time

The Test: Measure system pressure with a gauge. If pressure is lower than spec even at full RPM, the pump is worn. Compare to specs (most systems run 2000-3500 PSI at full load).

The Fix: Pump rebuild or replacement. This isn't a field repair. But if you're running an overheating system with a worn pump, you're just making everything worse. Shut it down and get it repaired properly.

5. Relief Valve Problems

The pressure relief valve protects your system from over-pressure by dumping excess flow back to tank. If it's set too low or is leaking, it dumps constantly — and all that bypassed flow turns to heat.

Symptoms:

• System runs hot at idle or light load
• Pressure never reaches spec
• You can hear fluid rushing back to tank

The Fix: Relief valves can be adjusted (carefully!) or replaced. This requires a pressure gauge and knowledge of your system specs. Done wrong, you can cause dangerous overpressure situations. Get professional help if you're not sure.

6. Restricted Return Lines

Your hydraulic fluid has to get back to the tank. If return lines are kinked, clogged, or undersized, the backpressure creates extra heat.

Check For:

• Kinked or crushed return hoses
• Clogged return filter (these often get forgotten)
• Collapsed hose internally (looks fine outside, blocked inside)

The Fix: Replace damaged hoses. Change your return filter — when's the last time you did that? Some systems have return filters that rarely get attention.

7. Running Too Hard for Too Long

Sometimes overheating isn't a failure — it's just pushing the system beyond its design limits.

Causes:

• Continuous high-pressure operation without breaks
• Operating in extreme ambient heat
• System undersized for the work being demanded

The Reality: Hydraulic systems need some rest. If you're running boom, tank lift, and water pump all at max constantly, the system generates more heat than it can shed. Build in some downtime during heavy operations.

Emergency Response: It's Overheating Right Now

If your temperature gauge is climbing and you're mid-job:

1. Reduce the load. Stop using the highest-demand functions.
2. Check fluid level immediately. Top off if low.
3. Let it idle. Fluid keeps circulating, cooler keeps working, load goes away.
4. Check the cooler fan. Is it spinning? Is airflow blocked?
5. Feel the cooler. If it's not warm, oil isn't flowing through it.

If temperature keeps climbing: Shut down. Seriously. Running an overheated hydraulic system causes exponential damage. Whatever job deadline you're trying to meet won't matter if you scatter pump shrapnel through the whole system.

Long-Term Prevention

Daily:

• Check fluid level
• Look for leaks
• Glance at temp gauge during operation

Weekly:

• Clean cooler fins (especially in dusty conditions)
• Check fan operation
• Inspect hoses for damage or kinks

Monthly:

• Change or check filters (including return filter)
• Verify fluid condition (dark, burnt smell = bad)
• Check system pressure

Annually:

• Consider fluid analysis
• Have a pro inspect the whole system
• Scheduled maintenance inspection

Fluid Analysis: Worth Doing

For trucks running heavy hours, hydraulic fluid analysis is cheap insurance. A lab tests your fluid and tells you:

• Contamination levels
• Wear metals (indicates pump/cylinder wear)
• Water content
• Viscosity breakdown

It's like a blood test for your hydraulic system. Catches problems before they cause failures. I recommend it at every major service interval for fleet trucks.

When to Call for Help

Get professional diagnosis when:

• You've checked the basics and it's still overheating
• Pump is making unusual noises
• Pressure readings are abnormal
• You've had repeated overheating episodes
• System performance is degrading

I provide mobile hydraulic repair throughout Pennsylvania, New Jersey, New York, and the DC area. Hydraulic diagnosis is something I do regularly — I've got the gauges and experience to pinpoint problems quickly.

Related Problems

Hydraulic overheating often connects to other issues. Check out my guides on:

• Boom rotation problems (hydraulic-powered)
• Debris tank won't dump (hydraulic lift)
• Complete troubleshooting guide

The hydraulic system powers most of your hydrovac's critical functions. Take care of it and it takes care of you. Ignore that temperature gauge and you'll be shopping for a new pump — or worse.

Call me at 272-296-9637 if you need help diagnosing an overheating system. Better to fix it now than rebuild it later.