The PCV valve is designed to suck crankcase vapors from the crankcase into the intake manifold and burn them with the fuel mixture so that they don't pollute the environment. In high mileage engines the crankcase vapors are increased due to excessive blow-by. Therefore there are a lot more vapors and general oily junk that are sucked into the intake manifold. On carbureted engines this junk is continuously flushed away by the gas vapors which have lots of good detergent properties if you are using a decent fuel. Therefore there is no build-up of oily carbonized residue in the intake manifold.. On the other hand, fuel injected engines introduce the gasoline way down by the intake valves and there is no gasoline inside the intake manifold "above" the injection point and thus nothing to flush away the crud that builds up inside the intake manifold around the place where the PCV Valve dumps all of its garbage!

The result is a gradual build-up of thick oily carbonized material that eventually chokes an engine until it will no longer perform the way it used to. Introducing "in-the-tank" cleaning agents like Gumout etc. can do nothing since the fuel never reaches the region where the crankcase garbage build-up occurs.

You can remove the throttle body and peer into the intake manifold to see if you can see the area where the PCV tube does the damage to see if there is a problem however I advocate just removing the tube from the intake manifold and introducing carburetor cleaner into the port. The engine will run very poorly with the tube removed completely however if you restrict the opening so that it is only the size of a pencil lead then you can stick the nozzle of a can of carb cleaner into the hole with the engine running at a fast idle and squirt the cleaner into the opening.

You need to do this slowly since the release of all that buildup can choke an engine to death so give a squirt, rev it up a bit and then give another squirt. It is possible to remove too much at a time and foul the spark plugs so do it slowly. You should see a noticeable amount of smoke coming out of the tailpipe unless the catalytic converter is very thorough in doing its thing in burning up the smoke.

When you are finished you should see improved performance and better gas mileage. If the junk is too carbonized from being on there too long then there is nothing short of removal of the glop by brute force means like the sharp end of a long chisel and a vacuum hose to suck out all the mess that you loosen up. Sometimes it is necessary to remove the intake manifold to remove all of the residue.

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