What Are the Problems Caused by Bad Valve Adjustment?

by Phillip JamesUpdated August 08, 2023
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Vehicle engines are precision machinery; the movement of many parts must be synchronized carefully to make the engine perform properly. Intake and exhaust valves are critical components for optimum engine performance. These valves contain mechanisms that must be adjusted correctly to allow for changes in temperature and compensate for material wear. Running a vehicle with incorrectly adjusted intake and exhaust valves can cause damage to the rest of the car. Here are some problems that can come from a bad valve adjustment.

Poor Engine Performance

All of the intake and exhaust valves in an engine must open and close at correct intervals to allow the engine to run smoothly. There is a small amount of valve clearance, called lash, between the valve itself and the mechanism that activates it. If the valve lash is set incorrectly (too little or too much clearance), the engine may respond by rough idling or misfiring. Get a certified mechanic to do a clearance adjustment and see if that fixes the issue.

Loss of Power

There are two different types of valves that can degrade an engine's ability to make maximum power, a common cause of a bad valve adjustment. Intake valves control when and for how long an air-fuel mixture is allowed into the combustion chamber, and must be synchronized with the speed of the pistons to allow the maximum amount of mixture into the engine. Exhaust valves perform a similar function, except their purpose is to allow exhaust gases to leave the engine through the exhaust system. If the valve stems aren't adjusted correctly, the engine will not burn fuel at peak efficiency. Fuel consumption, engine power and mileage then dramatically decrease.

Damaged Valves

The most serious result of incorrect valve lash adjustment is damage to the valves and related components. Setting the clearances loosely causes the rocker arm and other parts of the valvetrain to hammer together, damaging valves and creating a rattling or tapping noise. Tight valve clearance prevents valve covers from completely closing (or not closing for enough time), which may cause extreme overheating and complete valve failure. Always keep your engine valves adjusted according to the manufacturer's specifications.

Video: Common Problems - Tight Exhaust Valve Clearances | Honda S2000

Helpful comments on this video:

  • The correlation between clearance and lift is often misunderstood by many people, even in youtube videos. If you have a valve lift of, say, 9mm, what importance do you think a valve that has been misadjusted by 0.2mm will lift? Either 8.8mm or 9.2mm, for example, and these are the extremes. None of these will effect performance in any noticeable manner. The real problem is when the clearances are so tight that the valve might not fully seal, and let gases on either side escape slightly. An exhaust valve is especially in danger if this occurs, because the running engine's piston might try to scavenge the hot burnt exhaust gas through an exhaust valve that is a tiny bit open before the actual valve lift, and the very small cross section will result in high gas flow which may burn the valve to a degree that it loses the metal tempering (gets blue/purple, and weakens).
  • Hey man, I really appreciate your videos! I've used several of them to guide me along. I feel about 90% sure I could do a valve adjustment, (I was able to do everything else so far without issue, a TCT for example) but I'm just a little worried as it involves more serious maintenance.

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