From Mechanical Database
In a combustion engine the valve seat is the surface against which intake and exhaust valves rest when they are closed. It is a very critical component in the operation of a combustion engine for if it is not properly positioned, oriented, or formed during manufacturing valve leaking will occur, resulting in a loss of compression.
Valve seats are usually formed by first press-fitting a cylindrical piece of a hardened metal alloy, such as Stellite, into a cast depression in a cylinder head above each valve stem position, and then machining several conical-section valve seat surfaces to form the valve seat into a shape that will match the mating surface of the valve.
An improperly installed or machined valve seat can happen several ways:
- Incomplete seating during press fitting.
- Distortion of the circular valve seat surfaces.
- Tilt of the machined surfaces relative to the valve guide axis.
- Deviation of the valve seat surfaces from concentricity with the valve guide holes.
- Deviation of the machined conical section of the valve seat from the cone angle of the valve.
Automated quality control of inserted and machined valve seats has traditionally been very difficult to achieve until the advent of digital holography which has enabled high-definition metrology for measuring all of these listed deviations.
Dropped valve seat
Due to manufacturer quality issues certain engines are more prone to dropping a valve seat during operaiton. This usually occurs when the engine is hot, from heat distortion, due to Aluminum's ability to expand 5 times the rate of steel. The aluminum material that holds the valve seat will expand and hold the valve seat with less pressure. As the valve seat becomes less restrained, stress from regular operation can work it lose completely, disbanding it from the aluminum head. Once this occurs the movement of the valve will usually break the valve seat insert into pieces, effectively eliminating any compression in the corresponding cylinder, and creating a lot of damage from the loose metal debris inside the combustion chamber.
Most of the damage happens to the piston and cylinder head, as they are made of aluminum. If the engine continues to run, the metal debris can make it's way into all the other cylinders. This happens because due to lack of compression in the effected cylinder, rather than being expelled into the exhaust, the metal debris gets pulled back in to the intake manifold with vacuum created by the other cylinders. Usually the engine will continue to run, although very harshly, for some time before complete failure.
The best way to prevent dropping a valve seat is to keep the cooling system in perfect working condition to prevent extra heat expansion in the event of overheating the engine past the normal operating temperature. In most cases valve seats drop randomly without warning or due to any specific conditions.
One way to lower the chances of this happening is to have a deeper and more tempered valve seat design fitted into the cylinder head during the rebuild process. The latest valve seats have also incorporated a reduced leading edge chamfer which gives a greater contact area and heat transfer. The contact area is the greatest factor in valve seat reliability. When replacing the valve seats it is best to chose a machine shop that is an AERA member and to follow the specs of AERA tech bulletins for the specific job.
Occasionally pieces of the valve seat can become lodged in the manifolds and re-installing them can create more problems. Therefore once the valve seat has been repaired, both the intake and exhaust manifolds should be thoroughly cleaned prior to reinstalling.
Caution should be used when injecting Seafoam or other liquids that can cool parts of the engine too quickly. The best time to do this is when the engine is cold. When the engine is hot the aluminum head has already expanded and loosened it's grip on the valve seat, cooling the valve seat with a liquid will further increase the difference in size between the parts, in some cases resulting in immediate failure.
Diagnosis & repair
Upon failure, the engine will have a very loud ticking noise from the broken valve seat bouncing around inside the cylinder. This can sometimes be mistaken for valve tick, however the sound will be twice as loud and easily heard 50 ft away. Without having to disassemble the engine, valve seat failure can be determined by performing a compression test. The cylinder in which the valve dropped will reveal to have no compression. The same cylinder will also have a damaged spark plug from the metal debris, as can be seen furthest on the right. Metallic particles can also be seen in the oil after running the engine for a little bit, the oil will look gray rather than it's natural golden brown.
If the issue is diagnosed in time, and the engine has not operated for more than a minute or so with valve seat failure, it may be possible to repair the condition by replacing the piston and cylinder head. The cast iron walls of the engine block will usually contain minimal damage and continue to serve the car well. Depending on the severity of the damage, honing of the cylinder walls may be required.
Ford Escort & Focus problems
More frequently than others, the 1.9 & 2.0 SOHC CVH engines used in Ford Escorts and Focuses have been known to drop valves. Most often this happens in the fourth cylinder, and sometimes the third. The issues is on vin number P engines, due to the PM valve seat inserts.
The following pictures are of valve seat failure damage that occured in a 1997 Mercury Tracer with the 2.0 engine after injecting Seafoam. The valve seat would have probably failed soon anyways. Amazingly the car drove for 40 miles, prior to scrapping the engine the compression readings read (1-4 cylinder, left to right) 140, 130, 110, 0. The pictures demonstrate how failure in only one cylinder can transfer the debris to all the others, damaging them as well.
Another instance in a Ford Focus.
- Ford Escort & ZX2 section for the entire index of all Ford Escort and ZX2 related articles.