Valve problems are a sign of serious issues with your car or truck's engine. Properly sealed valves are crucial to your engine's compression. A problem with the valve seals or guides will mean that you will have to completely rebuild your vehicle's motor in order to restore it to good working condition. If you believe your vehicle is having valve problems, you should take it to a certified professional mechanic and have it thoroughly evaluated.
Open the bonnet and check your oil. If there is too much clearance between the valve guide and the valve stem, it will cause the engine to consume oil. If the oil is continuously running low regardless of how often you are adding it, and you have not noticed any areas where oil appears to be leaking out of your engine, there is a chance that your valve seals and guides are failing.
Start your car. Watch the tail pipe for exhaust. If an excessive amount of exhaust is coming out the tailpipe and it is thick and bluish or grey in colour, it is being caused by burning oil. This is where the oil your vehicle is consuming is going.
Listen to your car running. If you can hear a tapping or rattling noise coming from within the engine, there is a good chance the noise is coming from the valves. Valve noise stems from problems within the engine and with the valves themselves.
Determine if your check engine light is on. If it is, hook your car up to an OBD II error code diagnostic reader to the vehicle's power train control module and scan the PCM for error codes. Problems with your vehicle's valve seals or guides will come up under specific code numbers. Codes P0075 through P0086 are generic exhaust valve codes (see resources) and brand specific valve codes can be determined by looking up the codes using the make of the vehicle (see resources). These will give you a good idea of where to start looking for your valve problem.
Disassemble your engine and manually inspect the valves, valve seals and guides. This is a complicated process that should not be attempted by anyone who is not a certified mechanic because taking an engine apart incorrectly can severely damage the components. Also, you have to understand what your valve seals and guides should look like in good condition in order to determine if there is a problem with them. You should look for areas that appear dented, damaged, burnt or to be leaking an excessive amount of oil.