Rough Idle Troubleshooting and Diagnostic

An idling engine is often where trouble can be found, since there is no load placed on the engine and only minimal air and fuel are injected. If there's anything wrong with anything, it will often show as a rough idle first, since under load there's so many things that can cover up a small trouble. If your engine's idle isn't smooth or stable, here's what you can look for. If your engine is actually down on power, you may have a cylinder that isn't firing at all: to find which one, drop a dab of water on your exhaust ports with the engine running at temperature, the one(s) that doesn't sizzle isn't firing.

Fuel Delivery

Engines need fuel. Duh. Here's how to diagnose an issue with the fuel system.

  1. Fuel Pump: with the engine off, turn the key to "On", without starting, and listen for a hum coming from the rear of the car that only lasts for a few seconds. That's the fuel pump getting primed. If you don't hear it, investiate the fuel pump: it's either dead or not receiving power.
  2. Fuel Pressure: there's a fuel pressure tester you can buy, they're rather inexpensive. Measure your fuel pressure with the engine running, make sure it's within spec. If it isn't, look at the fuel pressure regulator, look for fuel leaks and look for obstructions. Well, you can't really look for obstructions, but there's fuel line cleaners you can use, if you suspect a blockage.
  3. Fuel Filter: you can't really tell what condition it is, just how old it is. They are inexpensive and easy to replace, and should be replaced regularly anyways. Just remember to relieve your fuel system pressure first, we wouldn't want to spray gasoline everywhere, would we. Especially if the engine and exhaust are still hot.
  4. Fuel Injectors: injectors can get dirty, and their internals can also fail. You can always disconnect their electrical connector and take a resistance reading, make sure they are within spec. The best way to clean them would be to remove them and soak them in the appropriate cleanser, just remember to relieve the fuel pressure first.

Air

Engines need to breathe, because gasoline won't burn itself. Here's how to diagnose the intake system.

  1. Air Filter: check it's condition. Paper filters are inexpensive to replace, high performance filters need to be cleaned (there's a cleaning kit you can buy). While you're in there, inspect the intake duct for blockage between the air box and the outside: leaves, rocks and dead mice can sometimes be found inside.
  2. Mass Airflow Meter (MAF): not all engines have these, but the ones that do can have an issue with it. Despite the air filter's best efforts, the MAF can get dirty. There's a cleaning spray you can use, just be gentle, it's a delicate and expensive sensor. Make sure the electrical connection is solid, and make sure it is receiving power (using a multi-meter). While you're in there, inspect the rest of the intake duct. Small cracks, especially between the MAF and the engine can cause extra air to enter the engine without the sensor reading it. Electrical tape can be used to plug these.
  3. Throttle Body: it's a big plate that rotates when you press the accelerator. Make sure it can move freely and smoothly (you can turn it by hand). While you're in there, inspect the throttle cable, make sure it is attached securely at that end and that the cable isn't pinched anywhere.
  4. Idle Air Control Valve (IACV): this component is often overlooked. Basically, when the engine is idling, it still needs air even though the throttle body is mostly closed. That's where the IAC valve comes into play: it opens and closes to allow just the right amount of extra air to keep the engine alive. They often get dirty and their little plunger can get stuck. There's a spray cleaner you can use.

Spark

Gasoline and air need a spark in order to burn. Here's how to diagnose the ignition system.

  1. Spark Plugs: this is the business end of the ignition system. Removing the spark plugs for a visual inspection can reveal a lot about the internal condition of the engine, whether it runs too hot, too cold, too rich or too lean. Spark plugs are inexpensive and should be replaced regularly.
  2. Spark Plug Cables: also called ignition wires, these little guys are often overlooked. Attaching a timing light to each one can tell you if spark is being delivered smoothly and consistently, and you can also measure their internal resistance with a multi-meter, make sure it's within spec. Lastly, inspect them for cracks and chafing, and make sure they are securely connected at both ends.
  3. Distributor: this only applies to older cars, but the distributor is a regular maintenance item that can cause a rough idle. Inspect the cap and rotor for wear and replace them when necessary.

Compression

There's not much to do here. A compression tester is inexpensive and easy to use. Test each cylinder's compression, make sure it's within spec and that all cylinders are pretty even (within 10% of eachother). If one of your cylinders is reading a lot lower than the other ones, drop a dab of motor oil in the cylinder and redo the test. Motor oil will make a good seal between the piston and the cylinder wall, if that restores compression, you have worn out piston rings; if motor oil doesn't restore compression, the issue may lie with the valves not closing completely.