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have you gone over the throttle body and changed the idle position solenoid. They go bad and that acts like a cork , when it is stuck. It creates the air bypass , when you are idling. Take the intake snorkel off and look at it, clean it. take off the throttle body and you will be amazed at how much crud is under the butterfly plate. That crud gets into the throttle position sensor and the idle solenoid valve. I changed those sensors ( everything is $40.00 each for those ) and all idle issues went away. Have not been back for 2 years. Remember that fuel injected engines have an air pedal. it's not a gas pedal. If air is cut off at idle the computer can't compensate for it beyond a certain point. If it is running rough, then you also have to look at loose wires, spark plugs, head gasket integrity.
Since you changed the sensor we will assume it is working. When you disconnect it you put the computer in limp mode which stops it from using sensor values. If code 300 is the only code we will assume the coolant temp sensor and others are ok. I would check fuel pressure with a gauge next.
Dirty throttle body . Does it run good when you step on the gas pedal ? driving down the road . Don't guess , do fuel pressure testing . Videos on youtube showing how , fuel pressure tester under $30 most part stores .
The engine in your car is designed to run smoothly with maximum power output and produce as minimal emissions as possible. When your car's engine is not performing properly it can cause, low gas mileage, low power output, increased emissions and possible internal engine damage if left untreated. This troubleshooting guide is designed to isolate the malfunctioning cylinder and troubleshoot to repair the problem. Before we start we need to know one of two things; is the engine running poorly at idle only and seems to be ok under power? Or does the engine run fine and it's just the engine idle condition that is the problem. If your engine is idling rough please visit, engine misfires at idle If your engine cranks over but won't start visit engine wont start. If your engine won't crank over visit engine wont crank over. If your engine is running rough all of the time or intermittently you are in the right place. Below we have created a guide to aid diagnoses and repair procedure for most common rough engine running problems. Car Repair Guide - READ COMPLETELY BEFORE BEGINNING
Step 1: Anytime you have a problem with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, air bag) inspect all fuses in the under hood power distribution center and under dash fuse panels using a test light. If all fuses test ok continue to the next step.
Step 2: To check for problems with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system) and the fuses test ok a trouble code scan is needed to identify any system trouble. Use a simple scanner tool to retrieve trouble codes and check if they relate to the specific problem, like an ignition coil failure code. If a trouble code is present but does not pertain to the immediate problem like an EVAP code ignore it until a later time, after the engine is running properly. The reason we repair non-related codes after the engine is running properly is because sometime false codes can be triggered by a rough running engine. Once the engine is running properly the code present might cycle and turn itself off. You might say "if the engine isn't running right shouldn't it have a check engine light and a trouble code?" Sometimes conditions occur that will not be detected by the computer, example: if the intake or exhaust valve operation fails the computer cannot detect the failure because the problem is not sensor related, so the engine doesn't run smooth and the computer thinks everything is ok with no codes. If the trouble code retrieved relates to a cylinder misfire like an injector driver or ignition coil failure first repair these problems then re-test system. If no trouble codes are present proceed to the next step.
Step 3: The spark plugs in your engine are used to ignite the compressed fuel air mixture. If the condition of the spark plugs are fouled by excessive fuel or carbon the engine will not start, backfire or run rough. Remove all spark plugs to inspect the condition of the plug. Please use this spark plug condition reference guide to see how the spark plugs are operating.
Step 4: Check for broken or dilapidated vacuum hoses on and around the engine, your car's engine is designed to run on a system that can hold vacuum. Vacuum hoses are typically connected to the engine intake manifold and will supply engine vacuum to various accessories like power brakes. Some cars are designed with a larger vacuum transfer hose like Ford that connects the intake manifold to the IAC (idle air control) motor. A broken or dilapidated vacuum line or air intake boot can cause the engine to lose vacuum which will allow the engine to run rough and die. Inspect all engine and accessory vacuum lines to look for missing, torn or dilapidated lines and replace as needed. Also have a helper rest their foot on the gas pedal just enough to keep the engine running. Check the engine when it is running to listen for any whistling noise coming from the engine that is not usually present. Follow the noise and inspect vacuum lines in that area. Also, when the engine is running it will pull inward a broken or weak piece of the hose to create a larger vacuum leak. Check the integrity of all vacuum hoses at each end of the hose. Typically this is where a vacuum hose fails. If all vacuum hoses check "ok" proceed to the next step.
Also check here: http://www.2carpros.com/articles/engine-misfires-or-runs-rough