Diagnosing Excessive Crankcase Pressure in the CAT 277B: Why the Oil Filler Cap Blows Off

[MikePhua]

What Causes the Oil Cap to Blow Off
When the oil filler cap is forcefully ejected—sometimes even before the engine runs—it’s a strong indicator of elevated crankcase pressure. In the CAT 277B skid steer, this typically points to a malfunctioning crankcase ventilation system or internal engine blow‑by. Rather than contamination or overfilling, top causes include:

• Blocked or frozen crankcase breather or PCV (Positive Crankcase Ventilation) valve
  
• A stuck or malfunctioning SOV (Shut‑off Valve) inside the breather system
  
• Engine blow‑by due to worn piston rings or cylinder wear, generating excess crankcase pressure
  

Cold Weather and Breather Freeze‑Up
Chronic issues with the cap typically surface in sub‑freezing temperatures. Condensation inside the breather hose or foam element freezes, blocking airflow and trapping pressure. The engine then builds internal pressure, pushing the cap off and sometimes causing oil to leak around the valve cover or eject from the exhaust.
Operators report melting the blockage with warm air temporarily restores venting—but long term, a system modification is needed.
Service Bulletin and Fix: Replace Valve with Straight Tube
A service bulletin and update procedure replaces the original SOV or valve system with a plain, straight breather tube. That modification greatly reduces risk of freeze‑ups and unwanted pressure build‑up:

• Remove standard breather and valves
  
• Plug any unused ports or hose fittings
  
• Install a straight‑through hose from valve cover to lower engine area or belly pan to allow condensation to drain safely
  

This retrofit is advised especially for early 277B models that still use the sealed breather system. Using a larger hose and avoiding routing into air intake ensures uninterrupted airflow and eliminates pressure traps.
Symptoms and Diagnostic Steps
Before removing the entire housing, perform these checks:

• Inspect breather hose or element for icing, oil buildup, or blockage
  
• Feel the valve cover area immediately after stopping—excess pressure often pushes oil around the cap
  
• Replace the oil cap with a known good OEM seal and torque to spec; if it still blows off quickly, the root cause is ventilation
  
• If a straight tube retrofit has been done, examine hose size and ensure it's unobstructed
  

Worn Seals vs. Ventilation Blockage
While engine internal wear (blow‑by) can pressurize the crankcase, most cases in the 277B point to vent blockage, especially in cold climates. If venting is clear and the issue persists, consider engine compression testing to evaluate piston ring integrity or head gasket failure. However, this remains less common in low-hour skid steers with sudden oil cap blow-offs.
Preventive Measures and Seasonal Tips
To keep the system functioning all year:

• Inspect and clean breather hoses and elements regularly
  
• In cold climates, retrofit with straight breather tubing as recommended by service bulletins
  
• Use proper oil cap torque and ensure gasket seating
  
• After winter jobs, check for condensation in hoses—dry or replace if damp remains
  
• Monitor for oil leaks or exhaust smoke—which may indicate overflow from venting issues
  

Real‑World Stories
A skid steer operator in Canada noticed the rubber filler cap repeatedly popped off on -20 °F mornings. Letting the machine warm up indoors didn’t help, and after retrofitting to a straight tube breather, the problem disappeared completely.
Maintenance crews in northern states have found that engines dripping oil from the muffler during winter turns out to be frozen breather foam that blocked the vent. Once melted and cleaned, and the system modified, the skid steer ran cleanly without symptom recurrence.
Summary: Key Takeaways

• Excessive crankcase pressure, not oil overfill, is candidate number one when the filler cap blows off
  
• Frozen or blocked breather systems (especially in cold weather) are the most common culprit
  
• A service bulletin retrofit replaces the breather valve with a straight tube, mitigating freeze risk
  
• If venting is clear and cap still ejects, investigate internal engine blow‑by via compression or leak‑down testing
  
• Preventive inspections, especially of breather routing and hose condition, reduce recurrence and downtime
  

By understanding how blow‑by vs. vent blockage manifests—and applying the proper system modification—a properly ventilated CAT 277B stays safer, cleaner, and avoids sudden oil loss during operation.