Wednesday, August 16, 2017 Detailed Auto Topics
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Modern automotive engines use an overhead valve design. Valves require lubrication which allows an opportunity for leakage. Valve cover gaskets help to keep the engine oil where it belongs and prevent leaks.

Valve cover gaskets

They make valve covers from metal or plastic and in many designs. To seal them to the cylinder head, engineers use a valve cover gasket. The oil pump pressurizes engine oil and uses it to lubricate the engine valves and the mechanisms that operate them. Because engine oil clings to surfaces, any opening allows oil to leak out.

Small bolts are normally used to hold the valve covers in place. Because the valve cover and cylinder head are not precise surfaces, we need a gasket as the seal. The flexible valve cover gasket seals any imperfections between the valve cover and the cylinder head.

When we install a valve cover gasket the cover and the head surface should be clean and dry. This allows the valve cover gasket to bond to both surfaces, producing the needed seal. Oil on either surface prevents the bond and will allow the valve cover gasket to leak.

Other seals on valve covers

Many engine types have holes in the valve covers though which the spark plugs pass. With this design, they use spark plug-tube seals to prevent leakage around the spark-plug tubes. These seals may also leak and we normally replace them with the valve cover gaskets.

With overhead cam engine types, an opening for the camshaft may also pass through the valve cover and have a seal. As with the valve cover gasket and spark plug-tube seals, we replace these whenever we must remove the valve cover.

A plugged PCV valve can cause valve covers to leak

Any opening in the valve cover presents an opportunity for leakage. Many designs use a grommet to seal the bolts that retain the valve cover. Replacing these grommets with the valve cover gaskets helps prevent leaks. Rubber grommets may also seal the positive crankcase ventilation or PCV inlet and outlet. Replacing these, the PCV valve and any hoses that are hard or cracked prevent problems.

Heat affects valve cover gaskets

The cylinder heads get very hot when the engine runs. They design valve cover gaskets of material to resist this heat. Continued exposure to high heat will cause valve-cover gaskets to become hard. They add chemicals to the engine oil to help prevent seal hardening. These chemical additives break down with use and replacing the oil is necessary. Extending oil changes may allow seals and valve cover gaskets to fail.

Symptoms of valve-cover gasket failure

Severe leakage onto exhaust causes burning oil odor

The most common symptom with a valve-cover gasket failure is oil dripping under the vehicle. They often position the exhaust manifold under the valve cover, so another common symptom is an oil burning odor. Oil leaking passed the valve cover gasket may get onto the exhaust manifold. The heat of the exhaust causes the oil to burn and we smell an odor.

Other symptoms can include an engine misfire, when oil leaks into the spark plug tubes. Engine oil will ruin the spark plug wires or coils and cause the engine to misfire. Leaking engine oil may also destroy the starter, the alternator and often cause damage to rubber suspension components.

How difficult is it to replace valve-cover gaskets?

On most inline engines, the valve cover gasket is very easy to replace. With V-type engines valve cover gaskets may be much more difficult. Often they place the intake manifold over the valve covers. With this type design, we may have to remove the intake manifold to access the valve cover gasket. Replacing valve cover gaskets on many transverse mounted V6 and V8 engines may take several hours.

Preventing valve-cover gasket leakage

The most common cause of failure with valve cover gaskets is a lack of maintenance. With extended oil changes, we may deplete engine oil additives that help keep valve cover gaskets pliable. Without this protection, gaskets and seals get hard and may start to leak.

Poor cooling system maintenance may cause engine temperature to rise. Elevated temperature can cause valve cover gaskets to fail. The positive crankcase ventilation or PCV system is also important to engine sealing. When we do not properly vent engine crankcase pressure, oil is forced past gaskets and seals.

Do not tighten valve-cover gasket fasteners

If oil leaks from a valve cover, do not be tempted to tighten the retaining bolts. Oil leaks because the valve cover gasket is broken or too hard to seal. Tightening the valve cover bolts will only damage the gasket and will not make it seal. Stop leak products are not effective and often cause additional damage. When the valve cover leaks, replacement of the valve cover gasket and other seals if present, is the only repair.

Look for the cause

Hardened valve cover gasket has cracked

When we replace valve cover gaskets, we should always look for the cause of the failure. For instance, valve cover gaskets that are very hard suggest we have not replaced the engine oil often enough.

Sometimes a close inspection will reveal a cracked valve cover. Cracks result from over tightening, mechanical impact, excessive heat and sometimes poor design.

They sometimes paint metal valve covers before they assemble the engine. If the paint comes loose, the valve cover gasket has nothing to stick to. We should remove any peeling paint before replacing the valve cover gasket.

Valve cover-tube seals

Cross section showing spark plug tube seal

When the spark plugs pass through the valve cover, they use a seal to prevent oil leaks. The spark plug tube-seal may be part of the valve cover gasket on some designs. Tube seals are a separate part on other valve covers and press into the cover. We replace these by knocking the old one out and pressing the new seal into place. When buying valve cover gaskets, asking for the tube seals may be necessary. They sometimes package these with the valve cover gasket but often do not. They may also sell the bolt grommets separately.

A few engines, such as some Nissan and Infinity designs, do not offer these seals without buying a new valve cover. This is expensive, but cost less than still having a leak after the job is complete.

Torque the valve cover bolts

We should tighten the small bolts that retain the valve cover to the specified torque and in a specific pattern. This information varies by the vehicle and is available in service data for the specific model. They specify most valve cover bolt-torque in inch-pounds. Confusing this with the more common foot-pound specification will cause major problems.

With valve-cover gasket replacement, a proper job depends on diagnosing the cause and replacing all needed parts. Time spent gaining understanding of the problem will often prevent an expensive rework.





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