Weak Valve springs in an engine can cause a variety of driveability and performance problems. Every valve in the cylinder head has at least one spring. The valve spring exerts pressure against the valve retainer to keep the valve closed. When the rocker arm in a pushrod engine, or the cam follower in an OHC engine moves and pushes the valve open, the pressure exerted by the spring increases. The pressure maintains valve lash in the valvetrain, and pushes the valve shut again as the rocker arm or follower rotates away from the valve.
To maintain correct valve operation at normal engine speeds, all of the valve springs must exert a certain amount of spring pressure. In most stock engines, the pressure exerted by the valve springs when the valves are fully closed or seated is around 70 to 90 pounds. When the valves are pushed all the way open by the valvetrain, the pressure exerted by the springs may go as high as 200 pounds or more. In high performance racing engines, very stiff valve springs are typically used so the engine can rev higher without the valves floating. Closed spring pressures may range from 200 to 3oo pounds, and open pressures as high as 1000 pounds or higher!
Valve float occurs when the springs can't keep up with the engine's speed. At high RPM (anything above 5000 RPM), the valves are opening and closing very rapidly. The camshaft rotates at half the speed of the crankshaft, so at 5000 RPM, the cam is spinning around at 2500 RPM, and pushing the valves open over 40 times a second! If the valve springs are weak, the springs may not be able to push the valves all the way shut before the next opening cycle begins. If the valves fail to close, the cylinders will lose compression causing the engine to misfire and lose power.
In addition, valve float can also allow hydraulic lifters to "pump up." Hydraulic valve lifters have an internal piston that is supported by oil pressure. Oil pressure pushes the piston up, forcing the pushrod to remain tight against the rocker arm. This maintains zero lash in the valvetrain for quiet operation. If the engine experiences valve float at high RPM, it can allow the pistons inside the lifters to move up slightly as they try to take up slack in the valvetrain. When the engine slows down, the lifters are over-filled with oil and may prevent the valves from fully closing, causing the engine again to lose compression and misfire. Eventually the lifters will settle down and normal operation will return.
Valve float due to weak valve springs is bad not only for performance, but also for the valves themselves. If a valve remains open too long in an engine with close piston-to-valve clearances (as is the case with most "interference" engines), one or more valves may actually hit a piston, causing serious engine damage. A valve may be bent, or the piston may crack or shatter as a result of the contact.
Valve float can also cause exhaust valves to run hot, burn and fail. Exhaust valves are cooled when the valve closes and rests on its seat. Heat is conducted away from the valve into the seat and cylinder head. If the engine is running at high speed and the valve is not fully seating because of weak valve springs, the exhaust valves can overheat. This can lead to burning or cracking in the head of the valve. A burned valve will leak compression, causing a steady misfire in the affected cylinder.
Symptoms of Weak Valve Springs
Weak valve springs will typically cause misfiring and a loss of power as engine speed increases. The engine may start and idle fine, or run normally up to 3500 to 4000 RPM. But at higher speeds, it may misfire and lose power because the valves are floating because the springs are weak and unable to keep up with the speed of the valvetrain.
If a valve spring has failed (broken), it may not be able to hold the valve shut at all, causing a steady miss and loss of power at all engine speeds. Worse yet, a broken valve may allow the keepers that hold the spring retainer in place to drop out, allowing the valve to drop down into the cylinder head and combustion chamber. This will usually destroy the valve and/or piston and cylinder head, causing a catastrophic engine failure.
Diagnosing Weak Valve Springs
If your engine is misfiring and losing power at higher engine speeds, the problem may be weak valve springs or fuel starvation due to a weak fuel pump, dirty fuel injectors, clogged fuel filter or fuel line obstruction. Bad gas can also cause high speed misfires as can a weak ignition system.
To narrow down the list of possible causes of your engine performance problem, connect a vacuum gauge to a vacuum port on the intake manifold. Then start the engine and observe the vacuum gauge readings at idle and at progressively higher engine speeds.
If weak valve springs are causing your misfire problem, the vacuum readings on the gauge will oscillate as engine speed increases.
However, a rapid vibration or variation (up to 4 in Hg) in the vacuum gauge reading at increased engine speed can also be caused by a leaking intake manifold gasket or head gasket, burned valves or ignition misfire. You need to rule out these other possibilities before even thinking about replacing the valve springs.
One very important point to keep in mind about valve springs is that they all lose tension and weaken as the miles add up. By the time original equipment valve springs have 100,000 or more miles on them, they have probably lost up to 10% or more of the tension they had when they were new. Most engine experts recommend replacing valve springs that have lost 10 percent or more of their original tension.
The rate at which springs age are affected not only my miles, but also by engine speed and temperature. Valve springs that seldom see the high side of 3500 RPM will last a LOT Lot longer than valve springs which are constantly being revved to much higher engine speeds. Racers often have to replace their valve springs at the end of every season, or in some cases after every major race. That's because the valves in a performance engine are much stiffer and experience much greater loads from high speed operation than those in a typical passenger car engine.
How To Check for Weak Valve Springs
You can't tell if a valve spring is weak just by looking at it. You can line up all the valve springs once they have been removed from the engine to see if they are all the same height. Any spring that is shorter than the others has lost tension and should be replaced. Of course, if they are all weak, they all may have lost height, making it difficult to pick out the weak springs.
The only way to know for sure if the valve springs are okay is to remove them from the engine and test each one individually on a valve spring test stand. The test stand has a gauge that show much force it takes to compress the spring a certain distance. If a spring fails to meet the pressure specification (which will vary depending on the engine application), the spring is weak and needs to be replaced.
Valve springs should also be replaced if they are out-of-square. If the spring leans to one side more than about 1/32 inch when placed against a 90 degree square, the coils have deformed and the valve spring needs to be replaced.
Valve springs should also be inspected closely for signs of damage (such as nicks, scratches or corrosion), for coil binding (shiny spots between adjacent coils that have been making contact), or shiny or worn areas on the ends of the springs. Replace the valve springs if they have any of these defects.
Replacing Weak Valve Springs
As a rule, high mileage valve springs should always be replaced as a complete set.
On many engines, it is possible to replace the valve springs without having to remove the cylinder heads. The trick is to remove the spark plugs and pressurize the combustion chamber with air, or to snake a piece of rope or rubber hose into the combustion chamber with the piston at Top Dead Center (TDC). This will hold the valves shut while the springs are compressed and removed with a spring compressor.
Stock replacement springs should be fine for a typical passenger car application. If stiffer performance springs are installed so the engine can rev higher, additional modifications may be required such as stronger push rods and/or rocker arms. Also, if the application is an older pushrod engine with a flat tappet camshaft (no roller rockers), stiffer springs will increase friction on the cam lobes. This will require using an oil additive that contains ZDDP to prevent cam failure or premature lobe wear.
If you are replacing a stock camshaft with an aftermarket performance camshaft, new valve springs are a must -- especially in a high mileage engine.
Check Installed Spring Height
The installed height of new valve springs should be checked to make sure they are all within specifications. The installed height of the springs can be affected by seat wear, or by valve or seat machining. If the installed spring height is too long, the spring may not exert enough pressure against the valves, acting like a weak spring. If the installed spring height is too short, there is a risk of coil bind with a high lift camshaft and possible valvetrain damage.
If the installed spring height is too tall, it can be corrected by installing a shim under the valve spring.
Lexus Valve Spring Recall
In July 2010, Toyota issued a recall to replace potentially defective valve springs on a number of Lexus sedans and the Toyota Crown with 3.5L V6, and 4.6L and 5.0L V8 engines. The U.S. Lexus recall will include the 2007 & 2008 GS350, 2007 & 2008 GS 450h, 2008 GS460h, 2006 to 2008 IS 350, 2007 & 2008 LS 460 and LS 460 L, as well as 2008 LS 600h L. The problem is that the wire used to make the valve springs was contaminated during the manufacturing process, resulting in defects that may cause some valve springs to fail. The fix involves replacing the valve springs in these engines with all new springs. There is no cost to the vehicle owner for this repair.