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As long as we're on the topic of too much air, we should also discuss too much fuel. The most common fuel system problems that can cause idle issues are a malfunctioning fuel pressure regulator or dirty or leaking fuel injectors. Either one of these can cause too much fuel to enter the intake, thus affecting the idle.

An easy check for a fuel pressure regulator is to just remove the vacuum line going to it. If you see fuel leaking out, replace it and recheck for your idle problem. A failed fuel pressure regulator will not always exhibit this symptom, but it's pretty easy to check for when doing your diagnosis. To properly evaluate a fuel pressure regulator would require hooking up a fuel pressure gauge and observing the pressure with the vacuum line connected and disconnected. I'll cover this more in the article on engine performance.

I also mentioned fuel injector problems. Fuel injectors can get dirty and cause an idle issue. A dirty fuel injector does not atomize fuel like a clean one. As a result, it can cause inconsistent fuel delivery, which can upset the idle speed. This is difficult to check for. If I suspect dirty fuel injectors, I hook them up to my fuel injector cleaning kit and clean them out. If it gets better, I chalk it up to dirty injectors.

Fuel Injector Internals

You might also try pinching off the fuel return line briefly with the engine running. This will elevate fuel pressure; if you have dirty injectors this might straighten them out. If the idle smoothens out when you do this, it could indicate that you have a fuel delivery issue.

Dirty Fuel Injector Spray

A leaking injector might be easier to find. A leaking injector often causes a misfire on the cylinder it's leaking into. You can find this cylinder by doing a power balance test. This is a simple test to tell you if all cylinders are putting out the same amount of power. This video demonstrates this process.

Another thing to look for is a wet spark plug. If you remove a spark plug and it's wet with fuel, this could indicate a leaking fuel injector. If you see fuel on all spark plugs, this could indicate another problem; most likely, your engine is flooded. My point is that if you find one cylinder that is different from the rest, look to that cylinder for the cause of the problem.

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