How to Tell if Your Hydraulic Pump Is Failing

And why the pump is often not the problem

This is one of the most common calls we get. The machine has gone slow, the operator is complaining, and somebody has already decided it needs a pump.

Sometimes it does. Often it doesn’t.

A main pump is one of the most expensive components on an excavator, and replacing one that was never faulty is an expensive way to find out the problem was somewhere else. Worse, if the real cause is contamination or a starved suction line, the new pump goes the same way as the old one.

Here is the sequence we work through before anyone spends money.

The symptoms people describe

Four descriptions come up again and again.

“The machine has gone slow.” The most common, and the least specific.

“It works fine cold, then slows down once it warms up.” This one is worth paying attention to. Oil thins as it heats, and internal leakage past worn components gets worse with it.

“It’s quick enough, but there’s no breakout force in the bucket.” Speed and power are different problems. A machine that moves fast but won’t dig is telling you something different from one that moves slowly.

“The engine dies the moment we load the hydraulics.” This one gets misdiagnosed as a pump more than any other, and it very often isn’t. The usual cause is an underpowered engine, and most of the time that traces back to the fuel side — either not enough fuel getting through, or the engine not making boost. The pump is doing exactly what it should. There simply isn’t enough power behind it.

Questions to answer before you touch anything

Before any testing, work through these. The answers usually narrow it down considerably.

Did it come on suddenly, or get worse over time?

 A gradual decline points at wear. A sudden failure points at something breaking, blocking or leaking.

Is the whole machine affected, or just one circuit? 

This is the big one. If only one function is bad and everything else is normal, the pump is unlikely to be your problem — a single circuit fault sits further downstream.

Which functions are worst, and which are fine? 

Be specific. Boom, arm, bucket, swing, travel — rank them.

Is it the travel side, the swing side, the implement side, or all three?

Does it happen hot, cold, or both?

Does using more than one function at once make it worse? 

Combined movements demand more flow, and a struggling pump shows up faster under that load.

Can you hear the engine lugging while the hydraulics are working, or is it not lugging at all? 

An engine that isn’t lugging under hydraulic load is telling you the pump isn’t demanding power, which points at the control side rather than worn internals.

Any unusual noises from the pump?

Is the oil temperature normal, or is it running hot?

Has anything been worked on recently?

Cylinders replaced, valve banks redone, hoses changed, anything opened up.

Has the machine been serviced recently? Was the oil changed or topped up?

Are there any warning lights or fault codes? 

These often point in the right direction before you’ve picked up a gauge.

Did a hose burst recently, and did somebody top up the oil afterwards? 

Worth asking specifically. It tells you what may have gone into the system.

The checks, in order

1. The tank

Open it and look inside.

•           Is the level correct, or is it low?

•           Is the oil milky, foaming or aerated?

•           Any sign of water contamination?

•           Is the viscosity right for the machine and the conditions?

•           Is the oil running abnormally hot?

Every one of these affects the whole circuit, and none of them are the pump’s fault.

2. The suction side

This is where a lot of “pump failures” actually live.

Check the suction line for crushed sections, loose clamps, damaged O-rings, and any slightly wet patches where oil is weeping.

A small leak on the suction side matters more than it looks. 

Air is far less dense than oil, so it will pull into the system faster than oil will leak out. A weep that looks minor can be feeding air straight into the pump.

Then check the suction strainer for blockage, and look for contamination sitting in the tank.

3. The breather

Remove the breather and check it isn’t restricted. A blocked breather creates a vacuum in the tank, and the oil can’t move freely no matter how good the pump is.

4. The engine

If those are all clear, look at the engine before going any further. A pump cannot perform properly behind an engine that isn’t performing properly.

•           Does it lug correctly under load?

•           Is the hydraulic system pulling the engine down dead?

•           Have the fuel filters been changed?

•           Is the fuel supply side right?

•           Is there a puff of smoke under load that then clears?

•           Is the boost coming in as it should?

If the engine dies when the hydraulics load up, work the fuel side properly before you look any further at the pump. Filters, supply, and whether boost is genuinely coming in. An engine that can’t make power will make a perfectly good pump look faulty every time.

5. Mechanical drive

Listen for noise at the transfer box and confirm the pump is being driven correctly — that the splines aren’t jumping.

6. Fault codes

Check the machine’s fault codes and any errors showing on the platform. Electrical inconsistencies often point straight at where the problem sits.

Now the pressure tests

Only once the checks above are clear does testing make sense.

Pilot pressure first

Check the pilot pressures against spec. If they’re out, look at the filters, check the solenoids, and inspect the pilot relief system before going further. Low pilot pressure will make a perfectly good pump behave like a failing one.

Pump pressure

Take P1 and P2 and start testing the internals.

A low P1 does not automatically mean the pump. It can be the regulator or the control side failing to move the pump to the correct angle. The pump may be mechanically fine and simply not being asked to do its job.

Check the standby pressures and the servo control pressure. Then put all the readings together and look at the picture as a whole rather than reacting to one number.

The flow test

This is the final test, and the one that confirms it.

Run a flow test on the pump, and check the case drain to see how much internal leakage is taking place inside it.

Only when you have results from all of these can you say with confidence that it’s the pump, and not something else in the system causing the problem.

Why this matters

Every step above is cheaper than a main pump. Most of them are free.

We would rather tell a customer their suction line is drawing air, or that the real problem is on the fuel side of the engine, than sell them a pump they didn’t need. Partly because it’s the right answer, and partly because a new pump fitted to a system that’s still contaminated or still starved will fail again, and then nobody is happy.

If you’ve worked through this and the tests point at the pump, send us the machine make, model and serial number along with a photograph of the pump data plate.  We’ll identify the exact unit and quote — new, reconditioned or used, depending on the machine and how long you’re keeping it.

And if you’re partway through the diagnosis and want a second opinion, send us what you’ve found. We’ve seen most of it before.