Symptoms Of Bad Valve Seals And Piston Rings And Replacement Cost

Do you believe that your car has bad valve seals or piston rings? What are the symptoms and consequences? Here's a guide, including the replacement cost.

Valve seals and piston rings are vital parts of your engine. They keep the pressure in and the motor oil out.

If the valve seals or the piston rings become faulty, they will require replacement.

You’ll immediately notice some obvious symptoms: low power and increased oil consumption.

In this simple guide, I will explain what to watch out for.

So without any further ado, let’s get into it.

Table of ContentsShow

What Are Valve Seals?

Car Valve Seals

Within an engine, valves open and close. Intake valves allow the air (or air/fuel mixture, depending on the engine type) into the chamber. Exhaust valves allow the exhaust gases out through your exhaust pipe.

Cylinders typically have one or two valves intake and exhaust valves each.

Valve stem seals are made from hardened rubber.

Their main jobs are:

  1. They sit around the valve stem and prevent pressure from escaping.
  2. They stop oil from entering the combustion chamber through the cylinder head.

Valve seals are like any other ordinary rubber seal, but they’re pretty diminutive – about the size of your fingertip.

What Are Piston Rings?

Piston Ring

The pistons rise up and down within the engine cylinders. Piston rings surround each one of them.

Most pistons have three piston rings. The top two rings are responsible for sealing compression in the combustion chamber. The bottom one’s primary function is to be an oil seal.

Piston rings have a few significant functions:

  1. They prevent pressure from escaping the chamber.
  2. They stop oil from splashing into the chamber and being burned.
  3. They stabilize the piston as it rises and falls within the cylinder.
  4. They help regulate temperature, distributing heat from the piston to the cylinder walls. From there, it moves into the coolant and dissipates into the atmosphere.

Symptoms Of Bad Valve Seals And Piston Rings

Bad valve seals and piston rings seal the top and bottom of the combustion chamber. As mentioned above, they keep pressure in and oil out.

Unsurprisingly, you’ll notice compression- and oil-related symptoms when they wear or break.

Here are some of the main symptoms you’ll notice when valve seals or piston rings wear or break.

  • Increased oil consumption
  • Reduced engine power

Increased Oil Consumption

Is Engine Or Motor Oil Flammable

If the valve seals or piston rings become ineffective, oil can go into the combustion chamber.

As a result, your car’s oil consumption will dramatically increase. You’ll notice the following:

  • Gray/blue smoke from your exhaust pipe. When oil burns, it creates a film. That’s what you see in the smoke.
  • Low oil pressure warning lights or messages.
  • Coolant overheating on the engine temperature gauge.
  • Noticeably low oil levels at the manual dipstick or electronic sensor.

When your car suffers from increased oil consumption, the leak could be anywhere in the system. Other possible causes include a blown head gasket or a leaking oil filter.

However, it could be happening due to a worn valve seal.

Reduced Engine Power

The powerful engine of a car

When the piston rings or valve seals go bad, the pressure within the chamber leaks out.

When the spark plug ignites the air and fuel, this explosion is the force that drives the piston down. If there’s somewhere for these gases – this pressure – to escape, there isn’t as much power forcing the piston down.

With worn or broken valve seals or piston rings, that’s just what’s happening.

As a result, your engine’s performance will take a severe hit. You’ll notice the following:

  • Sluggish under acceleration
  • Difficulty reaching higher speeds
  • Rough idle
  • Stalling
  • Shaking and shuddering from the imbalanced engine

The best way to check this is a pressure test, also known as a vacuum test. This uses simple air pressure and connects via the spark plug seat.

Severe pressure loss is more likely caused by broken piston rings than valve seals. They have a more significant, more impactful job.

Other spots within your cylinder could be causing this pressure leak. The cylinder wall could be cracked, for example. However, piston rings and valve seals are an excellent place to start.

How To Know Whether The Problem Is Piston Rings Or Valve Seals

auto spark plugs
Do you see oil residue on the spark plug? If so, it’s most likely bad valve seals.

Piston rings are more likely to go wrong than valve seals.

If your pressure test fails, you will likely find problems with the piston rings.

If the pressure test leaks its vacuum slowly, it’s more likely to be the valve seals.

One of the most obvious symptoms of bad valve seals is significant signs of oil residue on the spark plug. It could also mean bad piston rings, but valve seals are more likely if there’s a lot of oil on the top of the plug itself.

What is the best way for a mechanic to see whether the problem is valve seals? They need to remove the cylinder head to look for themselves.

Oil in the cylinder head isn’t a sign of bad valve stem seals, contrary to what you might have heard. Oil is supposed to be there. When you turn the engine off, it naturally pools in certain spots.

The valve seals’ jobs aren’t to prevent oil from getting into the cylinder head. They’re to stop oil leaking from the head into the combustion chamber.

Replacement Cost For Bad Valve Seals And Piston Rings

Cost and Price

If the valve stem seals are intact and in decent condition, but the cylinder still isn’t holding pressure, you’ll need an engine rebuild. The mechanic should first rule out a blown head gasket, of course.

You should only use a certified, trusted mechanic for this job. Unless you know your way around cars, this isn’t something you should attempt yourself.

The only conclusive way to know whether the piston rings or something else is the primary issue is to disassemble your engine.

An engine rebuild is the only way to replace the piston rings. Rebuilding an engine isn’t a cheap job. In most cases, you’re looking at at least $3,000. The larger and rarer your motor, the greater the impact on your wallet.

Replacing the valve seals means removing the cylinder head from the block. The camshaft(s) (if overhead), rocker arms and pushrods (if you have an older engine), and spark plugs need to come out. Mechanics will then switch out the seals. Expect to pay $1,000 to $1,500 for this job.


high oil pressure

You have a serious problem if you notice low power combined with falling oil levels. It could stem from piston rings or valve seals, although other causes are possible.

Get your engine inspected and repaired as soon as possible. The longer it’s left, the more damage will be done to other components.

It’s expensive, but that’s unfortunately unavoidable.

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!