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If your car has a mas air flow sensor (usualy located right aroung air filter or in air passage after the air filter) check and clean the sensor. This looks like a small spring on the end of 2 wires. BE CAREFUL not to damage the sensor. I have seen where the sensor gets dirty over time and this can cause a rough idle.
Sounds like you have cylinder misfire. Any of the large auto parts stores can run an OBDII check and tell you which cylinder is misfiring (not firing). The engine is running with one cylinder out causing the rough idle. The faster the car goes the less perceptible the misfire is. That is why at 30mph the car seems to run smooth.
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.
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 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.
transmission probably needs fluid and filter change. as for the the engine not running smootly, it may be in need of a tune up, you know, changing air filter, spark plugs, spark plug wires... or it could be a weak fuel pump.