Do I need to do a Valve Job?
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Do I need to do a Valve Job?
Most motors will go at least 80,000 miles before "needing a valve job"! But what are the signs? How do I determine if I really need it? Do I do it myself or pay a local machine shop.
Most likely you own a MINI because you enjoy driving it and in fact probably by 80,000 or so miles you can actually feel the car. The performance of the cylinder head valves can have a big impact on the feel with the car. The valves take a lot of abuse in modern motors even if you are not into seeing how many times you can hit the red-line on the tachometer. Modern motors are delicate balance between performance, economy and most important emissions. On the positive side of emissions is that fact that a motor that runs 'lean' will emit fewer contaminants that a motor that runs rich (fat)! All modern motors are designed to run as lean "as possible". The 'as possible' part of the equation exists because as you get leaner and leaner the explosion in the cylinder gets hotter and hotter! A lean running motor will perform best just before it melts itself into oblivion! The factory does it best to keep everything just on this fine edge. As time goes on, just like any knife edge that gets dull after daily use, the valve performance is degraded by heat and shear use. The results can mean lower compression and oil consumption. A "brunt valve" can often result when an owner or mechanic simply fails to feel the gradual loss of performance.
Once you have recognized that need for a valve job and removed the cylinder head a quick visual inspection can greatly increase your understanding of the results of all the daily abuse the valves take. The valves are operating to get the gas into the combustion chamber and the exhaust out of it. On the intake side, the valves see’s a relatively cool mixture of air and fuel as it proportions it into the combustion chamber and then try to seal the opening so that the resulting explosion is confined to propelling the piston (and thus the crank and thus the transmission and thus the car) to it's desired performance. On the exhaust side the valve is not only required to maintain a very closed and sealed exit during the combustion but also then must pass all of this super heated waste out the exhaust pipe. A visual inspection of the valves will typically reveal the intakes will have a deposits on the back side of them from all the impurities in the fuel while the exhaust will always have a large very encrusted back and front side of the valve face. The super heated air still contains impurities but now is so heated they melt and almost become part of the metal of the valves. The Valve face edge is the critical part of the valve that must maintain a perfect seal around the port to attain the compression needed for performance. This seat area is not only subjected to physical abuse as the seat opens and closes but also must close while the impure cold air is allowed in or the super hot impure air is exhaust. These impurities are often caught in the "seat area" and compressed as the valve attempts to maintain the needed seal. A visual inspection of this seat area will typically reveal light and dark spots were the various impurities were caught. The resulting lack of perfect seal will in itself also result in further degradation of the seal area as hot gases are allowed to squeeze by the fractional opening with more impurities and more degradation.
So what do you do? Cleaning the valves is often and option. A number of methods can be used but my performance is the start on the wire wheel and finish by putting them into a lath for final cleaning and polishing. If you have the head off the motor at this point you have done so mostly likely for performance issues and thus the nest step of renewing the seat area of the valves is most critical. A visual inspection of this area will reveal anything from small discolored spots, small dents in the surface or completed edges that have been burnt away. Small discolored spots can be resolved with a easy to use lapping compound. Valves with small dents or imprints can be cured by have the valve edge cut and the mating surface in the cylinder head also surfaced. If the pitting is anywhere close to severe the only real solution is new valves.
But remember that the other half of the issue can be oil consumption. While the seat area controls combustion, the valve stem, stem seals and valve guides control oil consumptions thru the top end of the motors. (Piston rings control oil consumption from the bottom side of the motor.) Replacing valve stem seals during a 'valve job' is an automatic must do and often times new valves guides need to be pressed into the cylinder head and then reamed to size. The final factor in controlling the oil is the wear that occurs on the valve stem itself. A worn stem that is severely worn can typically be seen by the naked eye but to be sure a digital caliper is the best method. Clearly a worn stem can only be resolved by valve replacement.