High idle, rough idle
Any time you see a high idle, rough idle high, or hunting idle, it’s time to suspect a vacuum leak. Extra air leans out the exhaust mixture, which is caught by the oxygen sensor and reported to the powertrain control module
(PCM). To correct the lean condition, the PCM adds fuel. The extra fuel and extra air are the equivalent of pushing down on the accelerator. As the engine speeds up, the PCM notices that you really haven’t pushed down on the gas (watches the throttle position sensor to determine this) and it starts to scale back on the gas. That’s why you get a “hunting” idle that goes up and down—the computer is shadow boxing the problem. The problem can also be caused by a sticky idle air control valve.
Always start your high idle diagnostic procedures by checking for vacuum leaks. Check all rubber and plastic hoses first. Then, try spraying carb cleaner around any vacuum ports. Listen for a change in engine speed.
Unfortunately, many manufacturers have experienced problems with bad intake manifold gaskets. The Nissan Altima is no different. If you do an internet search on this, you’ll see that failed intake manifolds gaskets are a common failure point on the 2.4L engine. So, if you can’t find a vacuum leak at any of the locations listed above, try spraying carb cleaner around the intake manifold gasket. If you see it getting sucked in or hear a change in engine speed, it’s time to examine that area more closely.
If you don’t plan on doing this repair yourself and you’re fairly certain you might have a bad intake manifold gasket, it might pay for you to take it to a pro and have them perform a smoke test on it. They pressurize the engine with smoke, and watch to see where it comes out.
© 2012 Rick MuscoplatPosted on by Rick Muscoplat