Valve Springs, have a very basic job, but how they do it, is very important.
By the time they have 100,000 miles on them, they have probably lost up to 10%, of their tension.
The main function of valve springs, is to keep the valves closed, to build up engine compression. The second function, is to maintain specific pressure, on all moving parts to follow the camshaft lobe.
Valve springs make sure that the proper amount of pressure, is applied constantly, to prevent valve “float” or bounce.
So, the camshaft, pushrods, lifters and rocker arms, do all the work, in opening the valves. But, when their job is done, the valve springs take over, to make sure the valves keep closed.
Furthermore, the valve springs also, push back against all the other parts. Including, the rocker arm, pushrod and lifters, to maintain pressure on the cam lobe. In most stock engines, the pressure exerted by the springs, when the valves are closed, is around “85 pounds”.
But, when the valves are pushed, all the way open by the valve train. Then, the pressure exerted by the springs, may go as high as “200 pounds”.
So, at some point, the valve spring will wear out and start causing issues.
Possible Failure Signs Include:
What Can Cause Valve Springs To Fail
So, to maintain correct valve operation, all of the valve springs, must exert a certain amount of spring pressure. Consequently, too low or week valve spring pressure, may cause valves to not completely seal or “float” under higher (RPM). While, too high a pressure, may cause premature wear, of other valve train parts.
Cracked Or Broken Valve Springs
A broken valve spring in an engine, can cause many different drivability and performance problems. Also, broken springs can cause, excessive valve noise, compression loss and severe internal engine damage. The actual breaking of the valve springs, is not always the most serious consequence.
Actions following the breaking, cause the most serious damage to the engine. When valve springs break, they may collapse just enough to allow the valve, to drop into the cylinder. As a result, the piston may strike it, causing severe damage, to the piston cylinder head, and other nearby parts.
What Can Cause “Floating” Valves
Valves can “float”, 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 rotates, at 2500 (RPM), pushing the valves open, over 40 times a second!
So, if the valve springs are weak, they may not be able to push the valves all the way shut. Before, the next opening cycle begins. If the valves don’t completely close, the cylinders will lose compression, causing the engine to misfire and lose power.
What Can Cause Hydraulic Valve Lifters To “Pump Up”
In addition, valve “float” can also allow hydraulic lifters to “pump up”. First, oil pressure operates an internal piston, inside the hydraulic valve lifter. Then, oil pressure pushes the piston up, forcing the pushrod to remain tight, against the rocker arm. This maintains, zero lash in the valve train, for quiet operation. But, if the engine experiences valve “float” at high (RPM). Then, the piston inside the lifter, moves up slightly, as they try to take up slack in the valve train.
When the engine slows down, the lifters are overfilled with oil and may prevent the valves from fully closing. Consequently, causing the engine again to lose compression and misfire. Eventually, the lifters will settle down and normal operation will return.
Valve “Float” Damage, On Interference Engines
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). Then, one or more valves, may actually hit a piston, causing serious engine damage.
The resulting damage, may be:
- Bents valves
- Cracked pistons.
- Damaged cylinder walls
- Cracked heads or block
Valve “Float”, Causing Burnt Valves
Valve “float”, can also cause exhaust valves, to run hot burn and fail. Exhaust valves, are cooled when the valve closes and rests on the valve seat. Then, the 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, 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.
Diagnosing Weak Or Broken Valve Springs, With A Vacuum Gauge
So, you can narrow down the list of possible causes, of your engine performance problem. Start by, connecting 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 problem, the vacuum readings on the gauge, will oscillate as engine speed increases.
However, a rapid vibration or variation in the vacuum gauge reading, at increased engine speed, can also be caused by:
- Leaking intake manifold gasket
- Leaking head gasket
- Burnt valves
- Ignition misfire
A broken spring, will cause the gauge needle to drop sharply, each time the affected valve tries to close. You need to rule out these other possibilities, before even thinking about, replacing the springs.
Thank You !