I have a 2001 Nissan Altima and when I turn off the engine, you can literally hear the engine stuttering out. Before, you turned off the motor and it stopped silently. Now it goes plup, pluuup, pluuuuuup and only then is it out. Any idea?
I'm also thinking there is crud built up in your engine. Prior to taking it to a mechanic, there's something you can try first. Pick up two cans of Seafoam. The first can, dump into your fuel tank. The second can you are going to have to do some work with. First, this is a two person job. Second, pull the vacuum hose from the brake booster (not from the engine side). If you have a small funnel, put it into the end of the vacuum hose. Have someone start the car. They will probably need to rev the engine during the next part, but only enough to keep it running. With the engine running, slowly dump about 1/2 of the can into the funnel and let it get sucked up by the engine. This will cause volumes of white smoke, but not to worry. This is normal. When you get down to about 1/2 to 1/3 of the can left, dump the rest into the funnel and as soon as it is sucked in, have your helper shut the car off. Let the engine sit for about 10-15 minutes. After the wait is over, start the car back up. This may be a little difficult and again will cause a bunch of smoke. Keep the engine running until the smoke disappears. You have now successfully completed a Seafoam treatment. This will clean all of the carbon out of the intake tract, from the valves, and from the combustion chamber. This should eliminate your dieseling issue. If it doesn't, then it's time to take it to the shop.
This sounds like what is called "dieseling." Basically, you are getting continued combustion even without the spark plugs firing. This can happen when the engine is dirty and/or hot, and fuel continues to flow to the cylinders. You car is fuel injected which means after you turn it off, the fuel supply should also be cutoff. However, if the injector nozzle is leaking, fuel can continue to enter the cylinder for a time and that could be the cause.
At any rate, there could be any number of causes and you really ought to take the car to a qualified mechanic.