Valve Guide Wear – What requires repair

Valve guide wear is referencing too much clearance in between valve stem and guide.

As guides begin to wear, the movement side to side may make valves seat poorly.

Valve leakage, bringing on a loss of compression, is an additional explanation for engine misfires.

It’s highly recommended that guides be inspected regularly, and fixed or replaced if found to be outside manufacturer’s specs.

Finding worn valve guides should almost always be expected when rebuilding a cylinder head.

Due to the fact that there is ongoing friction between the stem and guide, the guides normally experience a great deal of wear.

Exacerbating the problem, positive valve seals found on later model engines keep adequate lubrication from reaching the guides.

Side influences on the valve stem brought on by alterations in valve train geometry or by direct acting overhead cams affect guide wear even further.

WORN VALVE GUIDES:

When the guides are worn or there is excessive clearance in between the valve stem and guide, the engine will probably consume more oil. This is applicable to both intake and exhaust guides.

Consumption of oil may be a more significant issue for the intake side because:

  • Consistent direct exposure to engine vacuum.
  • Oil may also be taken down past the exhaust guides by suction in the exhaust port.
  • The circulation of exhaust past the guide produces a venturi effect that may pull oil down the guide.

Oil infiltrating the exhaust system on later model autos equipped with catalytic converters might trigger overheating in the converter, causing damage.

CHALLENGES ON THE INTAKE SIDE:

  • Oil drawn past worn intake guides, into the engine, may foul spark plugs.
  • It will make the engine produce higher than usual emissions of unburned hydrocarbon (HC).
  • Contribute to a quick accumulation of carbon deposits in the combustion chamber and on the back side of the intake valves.
valve carbon build up photo

Photo by asgw

Carbon deposits in the combustion chamber can increase compression to where detonation happens under load. Deposits on the back side of the intake valves in engines with multi-port fuel injection may instigate hesitation and idle issues due to the interference of deposits with appropriate fuel distribution.

If the carbon build-up makes the valve stick open, this may also trigger misfires.

Insufficient valve cooling and early valve failure is an additional issue that may be triggered by worn guides or those with too much clearance. About 75% of the heat from a common valve is carried to the seat, with 25% carried up the stem and out via the guide.

On later model engines, with three-angle narrow seats, the degree of heat transfer that happens through the stem is even higher since less heat may be conducted through the seat. So if the guide is worn, the valve might run hot and burn.

Worn guides can likewise pass air. “Unmetered” air pulled through the intake ports past the guides produces an effect comparable to worn throttle shafts on a carburetor. The additional air decreases intake vacuum and distorts the air/fuel calibration of the engine at idle. The consequence can be a lean misfire challenge and rough idle.

VALVE FAILURE:

Worn guides may also be a contributing factor to valve breakage. The guides support and center the valves as they operate. A worn guide will enable the valve to wobble a little as it opens, causing it to shift off center of the seat. The metal becomes distressed and the valve head breaks off from the stem over time.

Intake valve stem-to guide clearance for the majority of passenger vehicles varies from.001” to.003”. Due to the excess heat, exhaust will be.002” to.004”. Diesel motors typically have looser specifications on both intake and exhaust guides than gas engines.

ENGINE SMOKING:

cold car engine photo

Photo by cosmic_spanner

When blue smoke is visible on startup of a cold engine, you can usually attribute the issue to excessive valve guide clearance or worn valve seals. If the engine smokes consistently or when accelerating, the problem is probably with the piston rings.

Either way, it’s recommended that professional diagnosis is performed before investing in engine repairs as they can be quite costly.

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ABOUT NASH’S AUTOMOTIVE

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