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The car starts and runs smoothly no hesitation on acceleration but when idling there is a fluctuation in the RPM and the engine starts misfiring. changed the plugs and coils and the problem is still there. Any thoughts? what should i do next?
 

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Since you replaced 2 of the main causes already(plugs, coils), I would check for vacuum leaks first. After that look at the injectors.
 

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Discussion Starter #3
can it be because
Since you replaced 2 of the main causes already(plugs, coils), I would check for vacuum leaks first. After that look at the injectors.
Thanks for the response, can it be because of the fuel delivery failure somewhere along the road from tank all the way to the tip of the injector and not the injector?
 

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Just to add to the above, random/multiple misfire could be from a weak failing fuel pump, bad fuel, bad MAP or crank position sensor.

Regardless, I would drive it as little as possible with a P0300 FLASHING check engine light. You can ruin the catalytic converter on the car if it's getting hit with too rich or too lean of exhaust.
 

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My light is not flashing it is just there and sometimes its all gone and no light is on. Another thing i forgot to add is, my car does not have any hesitation on acceleration. the only problem that I have is rough idling, harder to start the car and the check engine light is on. also, after a long drive day(like lets say after I drive with it for more than 8-10 hours non-stop the hesitation becomes worse and it stops and turns off and I cannot turn it back on but if i wait for 15-20 mins and i start it, it will turn back on. there is also no steam coming from the exhaust.

I dont think it is the air delivery system failure as most likely the air delivery failure just makes a rough idle and not stalling and being hard to start the car. and it is not the ignition as I replaced the plugs and coils and the previous plugs were dry and were in good shape anyway. I think the problem is most likely from the fuel part of the story (either the injectors or somewhere along the road from the tank up until the injector itself). any thoughts or similar experiences?
 

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I've never once seen a P0300 code without a flashing check engine light.

I'm sure you know this, but if it's a coil or plug problem, the misfire will be pinpointed to only the effected cylinder.
 

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Exactly and because it is random misfire it sure isnt the ignition part. It is either fuel( most probably) or rarely the air side. I am gonna have a mechanic run an advanced full diagnostics on it.
 

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'...it sure isnt the ignition part.'

Not so, any variance in power to the coil power supply could do it. Usually there is one feed to all the coils for power.

If it were fuel related you should be getting running lean codes, Not necessarily though. Somebody needs to read the sensor1 O2 output at warm idle, it hanging low might tell you.
 

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Agree with the above in that you need some more sophisticated diagnostics here, but if the problem only happens at idle, it could be vacuum leak too. Loose hose, intake manifold gasket(s), etc..
 

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Yep, vacuum leak is the most common form of idle missing although most 300 series codes are tied to ignition system like crank or cam sensor. Anything tied to fuel will be able to be seen in fuel trims or O2 sensor action. A vacuum leak shows readily on a O2 sensor check.
 
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