# Rough Idle Issue



## leftoverture (Nov 16, 2008)

Hello! I am working on a 99 Altima with 182000 miles on it. It is not my car so I know virtually nothing about the car's history. The engine light is on and it idles rough. I am getting codes P0400 (EGR flow malfunction), P0325 (knock sensor circuit malfunction), and P0171 (system too lean). This car belongs to my son's fiance and she doesn't have much money so can't really afford to throw parts at it, so hoping that someone might have some input as to where to start with the diagnosis and any tips for diagnosing this. Thanks!


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## rogoman (Dec 16, 2004)

The P0400 code (EGR flow malfunction) can be caused by any of the following:
- EGR valve stuck closed
- Missing or cracked vacuum hose(s)
- EGR solenoid valve not working
- EGR temperature sensor not working 

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Here are the possible causes for a P0171 condition, the first two being the most likely:

● Intake system vacuum leaks
● Incorrect fuel pressure
● Exhaust gas leaks
● Lack of fuel
● Mass air flow sensor

The P0171 code (lean mixture) many times is caused by a major intake system vacuum leak which can also cause a rough idle. Because of a lean condition, this can also set the P0325 code (knock sensor). Fixing the P0171 condition many times will clear the Po325.

To test for a vacuum leak, use a vacuum gauge for good accuracy; most auto parts stores sell vacuum gauges. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source, usually somewhere on the intake manifold. 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 body and at the MAF for cracks or loose clamps.

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The fuel pressure may be incorrect. Tee-in a temporary fuel pressure gauge to the fuel rail input. The readings at idle should be approximately as follows:
- with vacuum hose connected to the fuel pressure regulator: 34 psi
- with vacuum hose disconnected from the fuel pressure regulator: 43 psi


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## leftoverture (Nov 16, 2008)

Thank you!!! That will really help point me in the right direction.


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## smj999smj (Jan 1, 2006)

Ignore the knock sensor code for now, as it will not cause a drivability issue and, as mentioned already, is likely caused by the rough running condition. Two areas I would look at first is a stuck open, EGR valve or a leaking intake manifold gasket. If the EGR valve checks out okay, then try spraying carb cleaner around the intake manifold gasket area with the engine idling and see if there are any flares or surges in the RPM, which would be a good indication of a bad intake manifold gasket. Failing intake gaskets were not as common on L30 Altimas (which had oval shapped intake ports) as they were on U13 Altimas (which had triangular intake ports), but they should still not be overlooked. Rogoman's advice is spot on!


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## leftoverture (Nov 16, 2008)

Found the vacuum line going into the EGR modulator valve was leaking (cracked in two places, actually). Replaced that and then throwing a P0300 and a P0304. Both misfire codes. So I replaced plugs and wires. No more codes. Idle is better but still not perfect, but a good improvement was achieved. Going to leave well enough alone for the day. I did not detect any intake gasket leaks using the carb cleaner method, so that is good also.


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## RestoRider/AltimaGXE (Apr 14, 2017)

I just finished up a diagnosis on a returning CEL after reset. When the radiator was changed the battery was disconnected for 24 hours.
CEL was reset then, After repairs were made drove another 15 minutes or so ,CEL came on again. Puzzled I was looking around checking for leaks
in the cooling system just as a follow up. Noticed a really hard rubber line going from the distributor end of the valve cover into the intake plenum
just in back of the MAF sensor. date code 112700. Being hard as rock I pulled back the clamps and when I pulled the hose off there was a huge
crack on the backside out of view. I took it to Advance auto and got another hose to replace it and all is good now. No CEL and no codes
My code scanner turned up nothing.

So intake leaks can be rather easy to find if you use the right theory . And I thought intake manifold gasket too. Not yet though only 97 K

jake


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