How to diagnose valve recession: a checklist for mechanics

Valve recession is a common issue that can affect engines, leading to reduced performance and increased emissions. If you suspect that your engine may be experiencing valve recession, it’s crucial to check and address the problem promptly. This guide will provide you with a comprehensive step-by-step process on how to check valve recession, ensuring accurate diagnosis and timely repairs.

What is Valve Recession?

Valve recession occurs when the valve seats wear down, causing the valves to sink deeper into the cylinder head. This can lead to a loss of compression, reduced engine power, and increased emissions. It’s typically caused by prolonged exposure to high temperatures and pressure, often due to improper valve adjustment or worn valve guides.

Symptoms of Valve Recession

• Reduced engine power
• Increased emissions
• Rough idling
• Difficulty starting the engine
• Backfiring
• Excessive oil consumption

How to Check Valve Recession

1. Gather Necessary Tools

• Valve spring compressor
• Dial indicator
• Micrometer
• Valve stem height gauge
• Cleaning supplies

2. Remove Cylinder Head

• Disconnect the battery and drain the coolant.
• Remove the intake and exhaust manifolds.
• Unbolt the cylinder head and carefully lift it off.

3. Clean Valve Seats

• Use a wire brush or solvent to remove any carbon buildup or debris from the valve seats.

4. Check Valve Stem Height

• Install the valve stem height gauge on the valve stem.
• Measure the distance from the top of the valve stem to the bottom of the valve retainer.
• Compare this measurement to the manufacturer’s specifications.

5. Measure Valve Seat Depth

• Install the dial indicator on the valve seat.
• Slowly rotate the valve while monitoring the dial indicator.
• Record the maximum depth reading.

6. Calculate Valve Recession

• Subtract the valve stem height from the valve seat depth.
• This difference represents the amount of valve recession.

7. Inspect Valve Guides

• Use a micrometer to measure the diameter of the valve guides.
• Compare the measurements to the manufacturer’s specifications.
• Worn valve guides can contribute to valve recession.

Tolerance Levels

The acceptable tolerance for valve recession varies depending on the engine type and manufacturer. However, as a general rule of thumb:

• Less than 0.002 inches: Minimal recession, likely not a problem.
• 0.002 to 0.004 inches: Moderate recession, may require attention.
• Over 0.004 inches: Significant recession, repairs are necessary.

Repairing Valve Recession

If you find excessive valve recession, it’s important to repair the problem promptly. This typically involves:

• Replacing worn valve seats
• Replacing worn valve guides
• Adjusting valve clearances
• Resurfacing the cylinder head

Basics You Wanted To Know

1. Can I check valve recession without removing the cylinder head?

In some cases, it may be possible to use a borescope to inspect the valve seats and measure the valve stem height without removing the cylinder head. However, this method is less accurate and may not provide a comprehensive diagnosis.

2. What causes valve recession?

Valve recession is primarily caused by high temperatures and pressure, often due to improper valve adjustment or worn valve guides. Other factors such as poor lubrication, detonation, and excessive carbon buildup can also contribute to the problem.

3. How often should I check for valve recession?

The frequency of valve recession checks depends on the engine type, driving conditions, and maintenance schedule. As a general rule, it’s recommended to inspect the valves every 50,000 to 100,000 miles or as per the manufacturer’s recommendations.