Why Do Some Engines Run Better With the Dipstick Pulled

[MikePhua]

The Role of Crankcase Ventilation in Engine Performance
Across all categories of internal combustion engines—whether in trucks, cars, or heavy equipment—the crankcase ventilation system plays a critical role in maintaining pressure balance and removing blow-by gases. Blow-by refers to combustion gases that escape past the piston rings and enter the crankcase. If not properly vented, these gases can build pressure, contaminate oil, and disrupt engine performance.
Most modern engines use a Positive Crankcase Ventilation (PCV) system, which routes these gases back into the intake manifold to be re-burned. Older engines or heavy-duty machines may use open breather systems or oil separators. When the dipstick is pulled, it can act as an unintended pressure relief point—especially if the PCV system is clogged or malfunctioning.
Terminology Note: “PCV valve” regulates the flow of crankcase gases into the intake. “Blow-by” is the leakage of combustion gases past the piston rings into the crankcase.
Why Removing the Dipstick Affects Idle Quality
In some engines, pulling the dipstick improves idle smoothness or reduces stalling. This usually indicates excessive crankcase pressure or vacuum imbalance. When the dipstick is removed:

• Pressure is vented, reducing resistance on rotating components
  
• Vacuum leaks caused by faulty PCV valves are temporarily bypassed
  
• Oil mist and vapor are allowed to escape, reducing intake contamination
  

A mechanic in Alberta once diagnosed a stalling issue on a diesel generator. When the dipstick was pulled, the engine stabilized. The root cause was a collapsed PCV hose that had created a vacuum lock in the crankcase. Replacing the hose restored normal operation.
Common Causes of Crankcase Pressure Problems
Engines that run better with the dipstick pulled often suffer from:

• Clogged PCV valves or breather filters
  
• Worn piston rings increasing blow-by
  
• Oil overfill causing foaming and pressure spikes
  
• Blocked oil separators or catch cans
  

In heavy equipment like excavators or loaders, crankcase pressure can also be affected by:

• Long idle periods without load
  
• High ambient temperatures
  
• Poor maintenance intervals
  

A fleet manager in Texas reported that several of his skid steers ran rough until the dipstick was pulled. After inspecting the PCV systems, he found that dust ingress had clogged the breather filters. Switching to high-flow filters and cleaning the hoses resolved the issue.
Diagnostic Strategies and Field Testing
To determine if crankcase pressure is affecting performance:

• Use a manometer to measure pressure at the dipstick tube (should be slightly negative or near zero)
  
• Inspect PCV valve for movement and flow
  
• Check for oil leaks around seals and gaskets
  
• Perform a compression test to assess ring wear
  

If pressure exceeds 1 PSI, it may indicate excessive blow-by or restricted ventilation. In one case, a bulldozer in Chile showed 2.5 PSI at the dipstick tube. After replacing the PCV valve and cleaning the separator, pressure dropped to 0.3 PSI and idle stabilized.
Risks of Operating With the Dipstick Removed
While pulling the dipstick may temporarily improve performance, it introduces risks:

• Oil vapor can escape and contaminate the environment
  
• Dirt and debris may enter the crankcase
  
• Oil level readings become unreliable
  
• Safety hazards from oil spray near hot components
  

Operators should treat dipstick removal as a diagnostic step—not a permanent solution.
Preventive Maintenance and System Upgrades
To avoid crankcase pressure issues:

• Replace PCV valves every 1,000 hours or 30,000 km
  
• Clean breather filters quarterly
  
• Use synthetic oil to reduce vapor formation
  
• Install catch cans or oil separators in high-duty applications
  

Some heavy equipment operators retrofit dual-breather systems to improve ventilation. In 2021, a mining crew in Australia added auxiliary breathers to their haul trucks, reducing seal failures and improving fuel economy.
Conclusion
Engines that run better with the dipstick pulled are signaling a deeper issue—usually related to crankcase pressure or ventilation. Whether in a pickup truck or a 40-ton excavator, maintaining proper airflow through the crankcase is essential for performance, longevity, and emissions compliance. By diagnosing and correcting ventilation faults, operators can restore smooth operation without resorting to temporary fixes. The dipstick may offer a clue—but the real solution lies in understanding the system behind it.