Excessive Blow-By: Understanding the Indicators

Measuring Excessive Blow-By In Your Engine

Excessive blow-by doesn’t occur without cause. It’s a red flag indicating potential issues such as worn piston rings, cylinder wall glazing, or improper ring installation. When blow-by surpasses acceptable levels, it allows combustion gases and unburned fuel to contaminate the crankcase oil, leading to sludge formation and accelerated engine wear.

How Much Blow-By Is Too Much?

Engine builders agree that a new engine should have around 1% blow-by or less. Once blow-by exceeds 10%, it’s a clear indication of trouble. While the engine might continue to run, the situation will only deteriorate, resulting in further power loss and potential mechanical failures.

For more information on blow-by, see this article.

Measuring Blow-by

As you search the topic, you’ll find lots of advice on determining if your engine has excessive blow-by. The most common “diagnostic technique” is the “dancing oil fill cap and smoke test.” This test involves unscrewing the oil fill cap and loosely placing it on the engine while it’s idling at full operating temperature. Supposedly, the escaping gasses should merely raise the oil cap. However, it’s generally accepted that the engine has excessive blow-by if the cap dances up and down above the valve cover or blows off. In addition, any sign of smoke is considered a sign of excessive blow-by.

This rudimentary test should NEVER be used as conclusive evidence of a worn out engine!

The misleading aspects of the visual test

The cap dance test is not scientific and is often improperly used. The part DIYers skip is the preliminary inspection of the PCV system and a test of the intake manifold vacuum prior to conducting the cap dance test.

• It assumes the PCV system is working properly— If you haven’t fully tested the PCV system prior to conducting the cap dance test, the results are invalid. If the PCV valve/hose is restricted somehow, you’ll get a false positive  If you haven’t first confirmed that the PCV system is working properly, then all bets are off of the cap dance results.

• It assumes smoke is smoke— The crankcase is always filled with oil mist, exhaust gasses, and moisture vapor. When you remove the cap, crankcase pressure will always force the oil mist and exhaust gasses out of the oil fill port. So, a light, hazy mist is typical for all engines and does not indicate excessive blow-by. In other words, smoke is not always smoke. To qualify as excessive, the smoke has to be heavy and thick, not hazy.

• It assumes there are no vacuum leaks— Just as a blown-off cap may lead you to falsely conclude your engine has severe wear problems, a vacuum leak can cause you to conclude your engine is in good shape. Always check for vacuum leaks and use an intake vacuum gauge first to confirm proper intake vacuum readings. This is especially important on diesel engines equipped with a vacuum pump where a vacuum leak can cause a false positive on the cap dance test.

• It assumes the leak is due to worn piston rings— While tests like compression and leak-down assessments are invaluable, they often fail to detect issues such as leaky valve guides. A compromised valve guide can allow boost and drive pressure to enter the crankcase. Identifying the cylinder head as the source of significant blow-by can prevent unnecessary and costly complete engine overhauls.

Affordable Tools for Precise Excessive Blow-By Measurement

A Dwyer flow meter (manometer), for example, is a plastic meter with a floating

Dwyer flow meter

ball that can precisely measure the amount of blow-by in your engine. They cost about $50 and can be purchased directly from the manufacturer at Dwyer-inst.com.  The Dwyer flow meter gives you a reading in cubic feet per minute (CFM). Connect it to the valve cover, observe the ball’s position, and record the rate of flow.

A Compression Gauge

By performing occasional compression tests, you can track the condition of your engine and its sealing capabilities.

A Cylinder Leak Down Test

To pinpoint the cause of excessive blow-by, it’s essential to identify its root cause using a cylinder leak down tester.

Common culprits include:

Worn or Damaged Piston Rings: Over time, rings can wear out or break, losing their sealing capability.
Cylinder Wall Issues: Glazing or scoring on cylinder walls can prevent rings from sealing properly.
Incorrect Ring Installation: Improperly gapped or oriented rings can lead to inadequate sealing and increased blow-by.

©, 2025 Rick Muscoplat