Engine Rattle After Oil Ingestion in a Skid Steer

[MikePhua]

Engine noise is one of the most alarming symptoms an equipment owner can face, especially when it appears suddenly after a hydraulic failure. When a skid steer ingests hydraulic oil through the intake system, the consequences can range from temporary smoke to catastrophic internal damage. Understanding why this happens, what components are at risk, and how to diagnose the resulting symptoms is essential for preventing further failure and making informed repair decisions.

Background of the Bobcat S175 and Its Engine
The Bobcat S175 skid steer was introduced in the mid‑2000s as part of Bobcat’s popular S‑series lineup. Powered by a Kubota V2203 diesel engine, the machine became widely used in construction, forestry, and agriculture. The S‑series sold in large numbers—industry estimates place total production of the S175 and its close variants well above 50,000 units globally.
Kubota’s V2203 engine is known for reliability, but like all diesel engines, it is vulnerable to hydrolock, overfueling, and air intake contamination. These risks increase when hydraulic hoses fail near the cooling or intake system.
Terminology notes

• Hydrolock occurs when a liquid enters the combustion chamber, preventing the piston from completing its stroke.
  
• Connecting rod links the piston to the crankshaft; bending it causes severe engine imbalance.
  
• Runaway describes a diesel engine accelerating uncontrollably when burning an unintended fuel source such as oil.
  

How Hydraulic Oil Enters the Engine
On certain skid steer designs, the air intake snorkel is routed through the engine compartment near the hydraulic cooler. When a hydraulic hose bursts, oil can spray into the intake path. If the air filter becomes saturated, the engine may draw in oil mist or even liquid oil.
This can lead to several dangerous conditions:

• Overfueling due to oil acting as an uncontrolled fuel source
  
• Hydrolock if enough liquid enters a cylinder
  
• Severe smoke as the engine burns off the oil
  
• Loss of power due to restricted airflow
  
• Internal damage from excessive cylinder pressure
  

A real‑world example involved a machine producing thick clouds of smoke and nearly stalling before being shut down. Even after repairs and fluid changes, the engine continued smoking and developed a noticeable rattle.

Why the Engine Continues Smoking
If the engine oil level remains stable, persistent smoke usually indicates residual hydraulic oil still being burned off. Oil trapped in the intake manifold, intercooler (if equipped), or air filter housing can take hours of operation to clear.
However, smoke combined with low power and mechanical noise suggests deeper issues.

Understanding the Rattle
A rattle after oil ingestion often points to internal mechanical damage. The most common causes include:

• Bent connecting rod
  
• Damaged piston
  
• Worn wrist pin
  
• Cracked piston skirt
  
• Bearing damage from shock loading
  

Even without a full hydrolock, extreme overfueling from oil ingestion can create enough pressure to deform internal components.
One mechanic noted that a diesel does not need to fully lock to bend a rod; a sudden spike in cylinder pressure is enough. Another pointed out that it is surprising the engine did not run away, given the amount of smoke produced.

Air Filter Damage and Intake Contamination
Many owners underestimate how much oil can pass through an air filter. While filters block dust, they do not stop liquids under pressure. Water ingestion has been known to destroy large diesel engines, including multi‑cylinder industrial units.
In one documented case, a 12‑cylinder engine suffered multiple bent rods after water entered the intake during a storm. Oil behaves similarly when forced through the filter media.
Replacing both primary and secondary air filters is essential after any oil ingestion event.

Field Diagnosis and Remote Operation Challenges
In remote work environments—such as machines operating 100 miles into the bush—owners often face difficult decisions. Continuing to run a damaged engine risks catastrophic failure, but transporting the machine for inspection can be costly and time‑consuming.
Common field checks include:

• Inspecting air filters for oil saturation
  
• Checking intake piping for pooled oil
  
• Monitoring engine oil level for consumption
  
• Listening for changes in rattle frequency or intensity
  
• Performing a cylinder cut‑out test if possible
  

If one cylinder is dead, as in the case described, internal damage is almost certain.

When a Rebuild Becomes Necessary
A dead cylinder combined with a loud rattle typically indicates a bent connecting rod. Once a rod bends, the piston no longer reaches the correct height, causing:

• Loss of compression
  
• Misfire
  
• Increased blow‑by
  
• Imbalanced engine operation
  
• Accelerated wear on bearings and crankshaft journals
  

Repair options include:

• Rebuilding the existing engine
  
• Installing a remanufactured long block
  
• Swapping in a replacement Kubota V2203 engine
  

Rebuild costs vary widely, but a full replacement often totals around $8,000 including labor, fluids, and ancillary parts.

Manufacturer Involvement
In some cases, equipment manufacturers may contact owners after severe failures, especially when the failure mechanism relates to design vulnerabilities such as intake routing. While not common, customer service departments sometimes offer guidance or goodwill support.

Preventing Future Oil Ingestion
Several practical modifications can reduce the risk of recurrence:

• Rerouting the intake snorkel away from the hydraulic cooler compartment
  
• Enlarging drain holes in the intake area to prevent fluid pooling
  
• Inspecting hydraulic hoses regularly for abrasion and heat damage
  
• Installing protective sleeves on high‑pressure lines
  
• Replacing aging hoses proactively rather than reactively
  

A shop foreman once reported that rerouting the intake on a fleet of skid steers reduced oil ingestion incidents by nearly 90%.

A Related Case of Engine Seizure
Another operator experienced a seized engine on a Case 1835B skid steer. The machine had been parked for a month, and when attempting to start it, the engine would not turn over. The owner suspected the hydraulic pump might have locked the engine.
In such cases, technicians recommend:

• Checking the starter for engagement failure
  
• Inspecting the ring gear through the starter opening
  
• Testing voltage at the starter during cranking attempts
  
• Examining hydraulic filters for signs of pump failure
  

A locked hydraulic pump can theoretically prevent engine rotation, but it usually leaves clear evidence in the hydraulic system.

Conclusion
An engine rattle following hydraulic oil ingestion is a serious warning sign. While smoke alone may clear with time, mechanical noise combined with power loss almost always indicates internal damage such as a bent connecting rod. Early shutdown, thorough inspection, and proper intake system maintenance can prevent catastrophic failure. For machines operating in remote areas, proactive hose replacement and intake rerouting are especially important. With proper diagnosis and timely repair, even a severely stressed engine can be restored to reliable service.