A wheel loader was brought in with a maddening symptom. On cold mornings, the hydraulic pump would let out a horrific, screaming whine when the operator pushed the pedal to the floor. If the operator let off the pedal and eased into it slowly, the pump was quiet. Once the hydraulic oil warmed up, the machine operated perfectly fine for the rest of the day.
The shop assumed the main piston pump was worn and replaced it. The symptom did not change. We then looked at the suction hose-the large, low-pressure hose running from the hydraulic tank to the pump. From the outside, the hose looked perfect. No bulges, no leaks.
We disconnected the hose from the pump and peered inside with a bore scope. The inside of the hose was collapsed. Suction hoses have a rigid internal wire helix to keep the tube from collapsing under pump vacuum. Over years of use, microscopic moisture in the hydraulic oil had migrated through the inner rubber and rusted that steel helix completely away.
When the oil was cold and thick, the pump required massive suction to pull it from the tank. Without the internal wire helix to support it, the atmospheric pressure outside the hose crushed the inner rubber tube flat, choking off the flow and causing the pump to cavitate and scream. When the oil warmed up and thinned out, the pump didn't need to pull as hard, and the hose stayed open just enough to feed the pump. We replaced the $80 suction hose, and the cold-morning whine vanished forever.
