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Just acquired a 2000 altama
It had no active codes when I bought it. However there was a stored p0303 code when I put my cheap scanner on it. Inckeaeed all codes and took for a drive. I changed plugs as a precaution. In the last year it has had plugs, wires cap and rotor installed. And a few other small odds and ends including motor mounts.
There are now no codes showing however when the car is in park idling you can hear what appears to be an audible misfire. I read maybe a fuel injector however I’ve had injectors fail on cars before and they usually shake way worse than what I’m seeing here. any help would be appreciated!
 

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The P0303 tells you that there was a misfire on multiple cylinders. This could be an indication of dirty injectors, vacuum leaks in the intake system or incorrect fuel pressure:

* Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. The readings at idle should be as follows:
  • with vacuum hose connected to the fuel pressure regulator: 34 psi
  • with vacuum hose disconnected from the fuel pressure regulator: 43 psi
  • There may be a major intake system vacuum leak. To check the intake system for a vacuum leak, 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 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.
  • Dirty fuel injector(s). Run some good injection cleaner, like BG products 44K, through the system; give the cleaner about a week or two to do it's job.
 
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