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Car idle is rough Inspection Service & Cost
A rough idling engine can be caused by a number of issues, some of them are serious while others tend to be minor, but the symptoms remain pretty much the same. The car will feel rough and bouncy when the engine is running. The car will also idle below its regular speed, display inconsistent RPMs and may produce a shaking, skipping or slipping sound when the vehicle is running.
While a rough idling engine may seem to be a simple inconvenience it often indicates a deeper problem within the engine. The vehicle should be inspected and repaired as soon as possible because small problems have a way of turning into expensive repairs.
How this system works:
The idle speed of an engine is basically the rotational speed the engine runs on when it is un-coupled from the drivetrain and the throttle pedal is not being depressed. The idle speed is measured in the revolutions per minute of the crankshaft.
When an engine is running at idle speed it generates enough power to smoothly operate equipment such as the water pump, alternator, and power steering but not enough power to move the vehicle itself. A passenger car will usually idle between 600 RPMs and 1000 RPMs. A properly functioning idle should run smoothly without skipping or slipping.
Common reasons for this to happen:
Dirty Fuel Injectors: The fuel injection system injects fuel into the cylinders, which creates a mix of air, and fuel to ignite and burn. Fuel injectors have tiny nozzles to spray the fuel into the cylinder and they can become clogged over time. clogged or failing fuel injector creates a lack of fuel in the vehicle's engine. This can cause a rough idle, it can also cause symptoms such as slow acceleration or the car not feeling as if it has enough power. If the problem is addressed early, it is possible to clean the injectors, which will restore them to full function. If this condition is not addressed in a timely manner the injectors will need to be replaced. Incorrect Idle Speed: While the average idle speed falls between 600 to 1,000 RPMs, if your vehicle is experiencing a rough idle it could be due to an incorrect idle speed setting. A trained mechanic can easily adjust the idle speed, and it should stay at the proper speed. If an adjusted idle speed becomes inconsistent or changes at random intervals there may be a bigger problem that needs to be explored. Vacuum Leak: If the vacuum system has a leak, it can seriously affect the ability of the vehicle's computer to regulate the air to fuel ratio. This can lead to a rough idle and if the problem is not addressed the car may experience slow acceleration and a lack of power. Vacuum leaks should be inspected and repaired immediately. Incorrectly Installed or Damaged Plugs: Spark plugs are responsible for creating the spark that allows the vehicle to burn fuel. If spark plugs are improperly installed or malfunctioning, the idle speed can be affected. The vehicle's engine may vibrate or there may be slipping or straining sounds coming from the engine. Defective or Clogged Fuel Pump: A rough idle can be related to fuel delivery issues. The fuel pump, which is responsible for pulling fuel from the gas tank to the fuel injectors, can become clogged or defective. If this happens the engine will not get enough fuel, which can cause a rough idle, sputtering, stalling and even slow acceleration. Clogged Fuel Filter: A clogged fuel filter can cause similar problems. The job of the fuel filter is to screen out contaminants in the fuel, over time it will become clogged and need to be replaced. A rough idle is one symptom of a clogged fuel filter. Failing Electrical Components: A problem or failure in the ignition system or various electronic components can cause a rough idle. If this is the case, the problem will usually get worse as RPMs increase. Common culprits include the ignition control module, plug wires, coils, and spark plugs. Defective Airflow Sensor: A defective airflow sensor can be responsible for a rough idle. A mass airflow sensor detects the amount of air coming into the fuel injection system and sends that information to the vehicle's computer. The computer uses that data to deliver the proper amount of fuel to the air in the vehicle. Over time these sensors can malfunction or become dirty. One of the first symptoms of a malfunctioning airflow sensor is a rough idle. The car may also accelerate slowly and even stutter or stall as the problem progresses. Dirty Oxygen Sensor: Oxygen sensors measure how rich or lean the gases are as they exit the combustion chamber. Depending on the results, the amount of fuel entering the engine is adjusted by the vehicle computer. The ultimate goal is to maintain an ideal mixture that produces the lowest emissions. A dirty or failing oxygen sensor will usually trigger the check engine light and can lead to a rough idle, lower fuel efficiency and failed emission test.
Hi. The most common cause that I have found for an " only idle misfire" is low compression. The engine needs at least 100 psi of "dry " compression to idle smoothly in most cases and if any cylinders are higher or lower by more than 20 percent of any other cylinder, you will also have an idle misfire condition felt. Should you perform a compression test, make sure to disable the igntion system completely, not just move the ignition wires out of the way as you will still have spark and the fuel injectors will still inject fuel in nearby proximity to possible ignition source. Always perform two tests, "dry" and "wet" compression. "Dry" is warm engine with spark plugs removed. "Wet" is after performing "dry" test add a few drops of light oil into cylinders and perform compression test again to see if the number rise. If they rise, you have worn piston rings or cylinder walls. If they do not rise, you likely have worn valve seats, misadjusted valves, or head gasket issues, possibly cracks in the cylinder head. If you need to know how to compression test with a tester in detail, message me back and I can help. I'm assuming you may already know due to the fact you state you replaced all the parts likely by yourself so you probably have strong technical merit.
I have a malibu 3.1 and it idles very good at about 650 rpms, when warm. it sounds like your problem may be a bad coil. a multible misfire can be-- wires, coils, vacuum leak in a hose. but change fuel filter and check the coils one may be bad. i think at auto zone they are a reasonable cost. GOOD-LUCK!
You could have a clogged fuel filter, if you haven't changed it in awhile. You could have a leak in the intake system somwhere also. When you slow down, you get less power to the engine. It may need spark plugs and wires as well, if in need for a tune up.
You may need to check the engine timing, that would be the first thing,, 4.3 is also bad about leakes in the fuel lines that are under the intake manifold. this is part of the fuel injection system, it will cause it to rough idle, but will accelerate fine, but when come to stop, will have a rough idle.
The air fuel mixture sensor is probably starting to fail. You need to have a simple diagnosis done at the shop and then have it replaced - they are usually not much a reputable mechanical shops. When the motor warms up your air/fuel mixture changes hence the rough idle etc. Hope this helps.