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Diagnosing An Idle Issue Without An Engine Light or Error Code, Pt 2

Fuel Injectors

In the first part of this series on idle issues, we focused on air intake. The other half of the fuel:air equation is the fuel. We started with air because it's both more common and easier to diagnose and fix. Fuel, especially in most modern engines with fuel injection, can be trickier.

Fuel Injection Problems

Most idle issues where fuel (too much or too little) is the issue are likely to be due to a fuel pressure regulator. It's also one of the easiest things to check. The fuel pressure regulator is usually situated directly on top of the engine near the beginning of the fuel rail. On most vehicles, getting to the regulator is relatively simple and requires no more than removal of a few plastic intake parts.

Checking the regulator is even easier - at least, for a quick check. Your first check will start with something simple (and cheap). Find the vacuum line going to the regulator and remove it. If fuel is leaking out, you very likely have a faulty regulator. The vehicle will usually run better without that vacuum line if this is the case.

To fully check the regulator, of course, requires that you do a full fuel pressure check. Explaining how to do one is beyond the scope of this article, but luckily, we've covered it before.

The other likely suspects are the fuel injectors. They can become dirty or faulty over time. Faulty injectors usually set off an error code, but not always. With dirty fuel injectors, you have two choices. You can replace them or clean them. Which you choose will depend on your repair budget and how much time you have. On some vehicles, the injectors are relatively cheap, so new ones are easier and better than cleaning. On most, cleaning will be the lower cost option.

Either way, there is no good, easy way to check fuel injectors. There are spray tests and the like, but they are not generally do-able for a DIY mechanic. Cleaning is quick and easy, so if the fuel pressure regulator was not your problem, cleaning the injectors shouldn't hurt. We've explained how to do it before.

Before you start pulling and cleaning injectors, though, do a quick power balance test. This looks for slight misfires that may not be registering with the ECM. Sometimes you can do this with a scan tool. Most of the time, you can't count on that. We will talk about how to do a power balance test without special tools in an upcoming article.

In part three of this series, we'll talk about the PCV and EGR systems.