How to Fix a Car That Runs or Idles Rough

How to Fix a Car That Runs or Idles Rough

I have been a mechanic for the past 20+ years at Young Ford and prior to that City Chevrolet. If your car or truck is running or idling rough then it is most likely going to be one of two things – a spark plug problem or a dirty throttle body that is robbing the engine of air and oxygen which it needs for proper combustion.

Two Main Problems: Bad Spark Plugs & Dirty Throttle Body

There are other problems that can cause a car to run rough but these two are what I primarily have found on customers cars over the past 25 years. The easiest thing to check and fix would be your spark plugs. If your car is over five years old and has over 50,000 to 80,000 miles or more on it then it would benefit from a tuneup anyway as a major component of any tuneup is to change out the spark plugs.

Check Spark Plugs

So, to stay cheap on your fix, start with replacing the spark plugs. Some mechanics will tell you that you need to use OEM (Original Equipment Manufacturer) spark plugs and that non-OEM or aftermarket spark plugs are bad or will not work as good for your vehicle. I find that to be hogwash. Any new spark plug will work better than an old one. Also don’t waste your money and pay extra for two to 4 prong or premium spark plugs.

You should be able to buy spark plugs for $2-$3 each. So, for a 6 cylinder engine that would be $12 to $18 plus tax for the set. Use a spark plug specific attachment for your ratchet wrench. Every ratchet wrench set that I’ve ever seen comes with one in the set. Then just be sure not to strip the threads upon inserting and tightening the new spark plugs.


Also as a tip do one at a time so it is easy to keep track of which wire goes to which spark plug. If you do them all at once is very easy to mess up and have misfiring on your engine. The easiest way to avoid this is to just do one at a time. Besides it doesn’t really take long to change out the spark plugs on any engine – so there’s no need to rush or shortchange yourself and cause a real problem with stripped threads.

After you have replaced all the spark plugs and wires than crank up your engine and take your car or truck for a ride. Does it run and idle smoother or is it still running or idling rough? If it’s still idling rough then you can pretty much bet that your problem lies with a dirty throttle body. As I said earlier a dirty throttle body starves your engine of necessary oxygen and will definitely make it run and sound rough.

Not Spark Plugs? Check Throttle Body

To find the throttle body first followed your air intake duct to where it connects to it. Disconnect the air intake duct (the flexible part of the intake hose unless you have a cold air intake installed then it is just its connection point to the throttle body. Be careful when moving this duct out of the way as it will probably have several electrical connections and other hoses attached to it. Remove these as necessary and use colored tape to mark for an easier re-installation.

Clean a Throttle Body

Once you have access to the inside of the throttle body use a spray cleaner specifically designed for throttle bodies or carburetors and spray it in there and use a clean shop towel to remove any dirt, carbon deposits or gunk. It may not seem that dirty when you’re cleaning it as compared to other parts on your car like brake rotors, but dirt and carbon deposits left in the throttle body basically guarantee that your car is going to run rough and will not idle properly.

Now, once you’ve thoroughly cleaned out the throttle body, removed any dirt or carbon soot buildup, and wiped it dry start reassembling the air intake duct and it’s connection to the throttle body. Be sure and attach all of the electrical connections and hoses that you had to detach to get access to the throttle body. If any of these are connected wrongly you could end up with a engine check light coming on.

Ready to Ride

Next take your car out and give it a test spin. In over 99% of the cases of a car or truck idling or running rough these two items are the cause – bad spark plugs or a dirty throttle body. And guess what? These repairs are easy to do for any weekend or backyard mechanic and can be done in less than 1 to 2 hours. A car dealership like mine will probably charge you $600 or more to do the same.

So, by doing this yourself you’ll save yourself probably around $560 or more. You will also increase your fuel economy as a dirty throttle body or bad spark plugs can easily reduce your fuel economy by 10 – 20% or more. Your only cost is the price of the spark plugs plus tax. And it is very easy to do anyone can easily do these repairs. So, now that you know how to fix a car that runs or idles rough, there is no excuse to be driving around wasting gas mileage in a rough running car – get out there and fix it!

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