How to Check a Stuck Engine Valve

by Chris Stevenson
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Every engine has intake and exhaust valves. Valves open to intake fuel or expel exhaust. They close when their cycle completes. Valve stems sit in long tubular guides that provide a track for them to move up and down in. The camshaft lifts and drops the valves singularly, or they have rocker arms, springs, lifters and push rods to assist them. Sometimes valve stick open or closed, causing an engine miss. Stuck valves can have serious consequences on engine performance and engine life. They must be attended to quickly to avoid catastrophic engine failure. Diagnosis should be performed immediately.

Step 1

Look for any "Check Engine" light that illuminates on the instrument panel during normal engine operation and driving. If you see warning light with an "Emissions" indicator it could be mean the problem directly relates to a frozen valve. O2 (oxygen) sensors can pick up a faulty air-fuel mixture in the exhaust system.

Step 2

Listen for a noticeable engine miss just after start-up. A cold engine will magnify any sticking valve problem because the valve stem and guide have minimum clearance when cold. You might detect an intermittent miss or hesitation that goes away after the engine warms up.

Step 3

Stay alert for any engine overheating that appears on your dashboard indicator lights. A stuck exhaust valve in the closed position produces extreme cylinder temperatures. Listen for a pinging or rattling engine noise, in association with pre-ignition (after-burning of fuel). Hot spots on the valve face and piston top cause this type of noise.

Step 4

Notice any unusual smell originating from the undercarriage of the vehicle in the vicinity of the catalytic converter. A rotten egg or strong sulphur smell indicates a saturated catalytic converter that cannot burn away a rich fuel mixture. An intake valve stuck in the open position allows too much fuel to pass through the exhaust system without proper burning.

Step 5

Remove the valve covers of the engine, using an appropriate socket and wrench. Remove the single valve cover in the case of a four-cylinder or straight-six engine. Disconnect the main coil wire, or the plug wires at the coil pack location. Have an assistant crank the engine over. Watch all of the valve springs on the head for movement. Every valve spring should move up and down with regular rhythm. A valve spring that does not move, chatters or moves intermittently indicates a sticking valve.

Step 6

Remove the spark plug wires from each cylinder with the spark plug wire removal tool. Keep the plug wires in the proper order and location. Remove all the spark plugs from their sockets with a wrench and plug socket. Check the spark plug electrodes. They should all have a light tan appearance. Any plug that looks wet, black, crusted and dark brown indicates an improper fuel mixture or overheating problem, associated with a stuck valve (provided the plug receives proper spark).

Step 7

Screw in the compression gauge at one of the cylinder locations. Make sure the coil or coil pack wire remains disconnected. Have your assistant crank the engine over six or seven times and stop. Read the psi (pounds per square inch) on the gauge and record it.

Step 8

Test all the cylinders with the compression gauge in the same manner and write down the numbers. All the cylinders should read high, with none reading lower than 30 pounds or less than the others. An exhaust or intake valve that is stuck open or partially open will cause a noticeable drop in cylinder compression. A zero reading in a cylinder points to a valve stuck in a wide-open position.

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