What is a "valve job" and when is it necessary?

How much should a valve job cost, and why?

What is the PCV valve and what does it do?

What is the EGR valve and what does it do?

My engine won't start anytime and in any temperature, but the starter works normally. What seems to be wrong?

The engine starts but quits right after the key is released from the start position. What gives?

My engine starts but quits after a few seconds of running. Why?

A hissing sound is heard from the engine. What is this?

Clunking from under the car when gears change. What's up.

Gray smoke from the exhaust. Why?

White smoke or water vapor from the exhaust. Why?

Car stalls when stopped quickly. Why?

There is a rotten egg smell coming from the exhaust. What is it?

There is a strong gas smell coming from the exhaust. What is it?

Red fluid under the car by the transmission. What is it?

Slow engagement of gears. What's happening?

Steering wheel jerks. Why?

Clunking from under the car when the car while accelerating. What's going on?

My clutch chatters and feels "jerky" when I start out from a dead stop. Do I need a new clutch?

What is a four-stroke engine?

Temperature gauge rises into the Red zone ?

The diagnosis of a burned valve is usually the result of a compression test. If a cylinder shows little or no compression, it frequently means the exhaust valve is not sealing. The valve may or may not be actually burnt (melted), but have other physical damage such as cracks or areas where pieces of metal are missing or eroded away from the valve face.

The cure for this condition is to remove the cylinder head, replace the bad valve and reface (or replace) the valve seat. As a rule, the head is usually given a complete valve job at the same time because the rest of the valves and guides probably need attention, too. If one exhaust valve has failed, the rest are probably on the verge of failure if they haven't already started to leak.

Why valves burn: There are several reasons why valves burn. One is normal wear. As an engine accumulates miles, the constant pounding and thermal erosion wears away the metal on the face of the valve and seat. The exhaust valve sheds most of its heat through the seat, so when the face and seat become worn and the area of contact is reduced, the valve starts to run hot. Eventually the buildup of heat weakens the metal and pieces of it start to break or flake away. Once this happens, it forms a hot spot that accelerates the process all the more. The valve begins to leak and compression drops. The result is a weak or dead cylinder and a noticeable drop in engine power, smoothness and performance. A bad exhaust valve will also increase exhaust emissions significantly because it allows unburned fuel to leak into the exhaust. High hydrocarbon (HC) emissions, therefore, may also be an indicator of a burned valve. An exhaust valve can also burn if the valve lash closes up for some reason (improper lash adjustment, cam or lifter wear, a bent push rod, worn rocker arm or cam follower, etc.). The lack of lash (clearance) in the valvetrain prevents the valve from closing fully, which causes it to leak compression and overheat. Valve burning can also be caused by any condition that makes the engine run hot or elevates combustion temperatures. This includes cooling problems, abnormal combustion like detonation or preignition, loss of exhaust gas recirculation (EGR), retarded ignition timing or lean fuel mixtures.

Before we describe all the steps that a typical valve job involves, we should warn you that some shops don't necessary do all the steps. In other words, you get what you pay for. A "cheapie" valve job might skip a lot of things that saves you a few dollars in the short run, but may end up costing you a lot more in the long run. So look for a shop or service facility that does quality work. A valve job typically begins by disassembling, cleaning and inspecting the cylinder head. Cast iron heads are "Magnafluxed" to check for hairline cracks. This involves applying a strong magnetic field to the head and sprinkling iron powder on it. Cracks disrupt the magnetic field and attract the iron powder, making invisible cracks easy to see.

Cracks are bad news because they can leak coolant into the combustion chamber damaging the cylinders and/or causing the engine to lose coolant and overheat. If cracks are found in any critical areas of the head, the head must either be repaired or replaced. Cracks in cast iron heads are most often repaired by "pinning" (installing a series of overlapping threaded pins). Cracks in aluminum heads are very common and can often be repaired by welding. If a head has been repaired (pinned or welded), most shops will usually pressure test the head afterward to make sure there are no leaks. Some may also apply a sealer compound to the inside of the water jackets as added insurance against future leaks.

Once the head passes this point, it is also checked for flatness. The surface of the head must be flat to seal the head gasket against the block. Excessive warpage, roughness or any damage can cause the head gasket to fail. If the head exceeds the maximum allowable out-of-flatness specs, it must be resurfaced or replaced. Usually there's enough metal in the head to allow for a certain amount of resurfacing. But on many import aluminum cylinder heads, the amount of resurfacing that's possible is minimal.

Overhead cam aluminum cylinder heads are often found to be warped (usually the result of overheating). If the condition cannot be corrected by resurfacing, the head can often be straightened by heating it in a special oven and then bending it until it is straight.

Next come the valves, guides and seats. The guides are checked for wear. They're almost always worn, so they either need to be replaced, relined or knurled (a process whereby grooves are cut into the inside diameter of the guides to decrease the bore size). Few shops knurl guides anymore. Most install new guides, guide liners or bore out the old guides to accept new valves with oversized stems. Aluminum heads have cast iron or bronze guides that can be replaced but most cast iron heads do not.

If the valves are to be reused, they will be inspected, checked for straightness then refaced. Many shops automatically replace all the exhaust valves to reduce the risk of failure (exhaust valves run much hotter than intakes and are much more likely to fail).

The seats in the head are either cut or ground to restore the sealing surface. If a seat is cracked or too badly worn to be refaced, the seat must be replaced. If that isn't possible (as is the case on many late model cast iron heads because the casting is too thin), then the entire head must be replaced. All aluminum heads have hardened steel seats that can be replaced.

The valve springs are all inspected and tested to make sure they are still capable of maintaining proper pressure. The spring retainers, keepers and other hardware is likewise inspected. Any worn or damaged components are replaced. New valve guide seals are always used.

The least expensive valve job would be one on a domestic V8, V6, straight six or four cylinder engine that required no crack repairs or head straightening, no new valves, springs or other hardware, and no special tools. On the other hand, if you have an import car with dual overhead cams, four-valves-per-cylinder and a lot of fancy valvetrain hardware, you might be looking at a total bill of several thousand rupees. Parts typically cost much more for import engines as well as performance engines and diesels. The labor to do the valve work is also much more involved on these overhead cam multi-valve heads. (Of course, they didn't tell you that when you bought the car. All they stressed was the fantastic performance of overhead cams and multiple valves.)

The main component in the PCV system is the PCV valve, which is usually located in the valve cover. A hose connects the PCV valve to the intake manifold. A second hose between the air cleaner and crankcase or other valve cover (V6 or V8 applications) provides fresh air to help flush the vapors out of the crankcase. Some engines have a separate air filter for the PCV breather hose located inside the air cleaner.

The PCV valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that's siphoned from the crankcase into the intake manifold. This is necessary because air drawn through the valve from the crankcase has a leaning effect on the fuel mixture much the same as a vacuum leak. So air flow through the valve must be controlled within certain limits. At idle, air flow is reduced because little blowby is produced. When the engine is cruising and vacuum is high, airflow through the PCV valve is at a maximum to purge the blowby vapors from the crankcase.

It's important to note that PCV valves are sized for specific engine applications. The wrong PCV valve for an application can flow too much or too little air causing driveability problems. Varnish deposits can clog the valve, so replacement for preventative maintenance is recommended (every 50,000 miles usually).

The EGR valve consists of a poppet valve and a vacuum diaphragm. When vacuum is applied to the EGR valve diaphragm, it pulls the valve open allowing exhaust to pass from the exhaust manifold into the intake manifold. Some engines have "positive backpressure" EGR valves, while others have "negative backpressure" EGR valves. Both types contain a second diaphragm that modulates the action of the valve. This prevents the valve from opening unless there is a certain level of exhaust backpressure in the system. EGR valves are calibrated for specific engine applications. The wrong valve may flow too much or not enough exhaust and cause emission, driveability and detonation problems.

EGR valves do not normally require maintenance or replacement for preventative maintenance. But the valve can become clogged with carbon deposits that cause it to stick or prevent it from closing properly. Dirty EGR valves can sometimes be cleaned, but replacement is necessary if the valve is defective.
Some newer engines are so clean from a NOX emissions standpoint that no EGR valve is required.

Ported vacuum switch: Another component that may be a part of your vehicle's EGR system is a "ported vacuum switch" (PVS), which may also be called a "thermal vacuum valve" or "temperature vacuum valve" (TVS). The switch controls the passage of vacuum that operates the EGR valve. This device is a heat-sensitive switch that remains closed until the engine's coolant reaches a certain temperature. The PVS screws into the intake manifold, thermostat housing or engine so the heat-sensing element is in contact with the engine's coolant. Inside the switch is a wax plug that pushes a sliding plunger to uncover or block vacuum ports in the switch. As the engine heats up, the wax expands and pushes the plunger up until it uncovers or blocks the vacuum port. At this point, vacuum to the device that the switch controls is either applied or blocked. Severe engine overheating can damage the switch, making replacement necessary.