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Hey folks, I’m trying to determine the cause of a random misfire in my Altima. I’ve checked my plugs and coils and that’s not it. Cleaned my vvt solenoids. Two things I do know, my valve covers are leaking and the intake manifold runner control valve is hissing like there is an air leak. Would that in itself be enough to cause such rough idling, random misfires and serious decreases in fuel economy? I’m not throwing any other codes other than p0300 so I don’t know where to focus. Any ideas are much appreciated!
 

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Any unmetered air coming in behind the MAF will be a problem and can lead to a random misfire. You may need a intake gasket set. Also check your fuel injectors for that hissing sound. If the o-rings fail around the injectors and you can get vacuum leaks from there. Also check the tube just before the throttle body that can leak
 

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Hey folks, I’m trying to determine the cause of a random misfire in my Altima. I’ve checked my plugs and coils and that’s not it. Cleaned my vvt solenoids. Two things I do know, my valve covers are leaking and the intake manifold runner control valve is hissing like there is an air leak. Would that in itself be enough to cause such rough idling, random misfires and serious decreases in fuel economy? I’m not throwing any other codes other than p0300 so I don’t know where to focus. Any ideas are much appreciated!
Here are some possible causes for a P0300 fault code:
Improper or dirty spark plugs
Insufficient compression
Incorrect fuel pressure
Intake system vacuum leak
Incorrect PCV hose connection
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Major intake system vacuum leaks will cause engine misfires, rough idle and affect the MPG. Here's the best way to test for vacuum leaks. Attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
 

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Two things I do know, my valve covers are leaking and the intake manifold runner control valve is hissing like there is an air leak.
The runner actuator has a diaphragm, if there's a hole in it then it will behave like any other vacuum leak when the VIAS solenoids are active. Try popping the hose off it and plugging the nipple and the hose, if the hiss goes away and it suddenly runs better then you've found your culprit. Since your VC's are leaky, one other thing to check is whether they're filling the spark plug wells with oil. That causes the coil boots to get soft and eventually they'll start arcing to the sides of the tubes.
 
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