Solving Automotive Idle Problems
The first thing we should talk about is the engine’s idle circuit. I’m going to cover a fuel-injected engine here. Carbureted engine idle problems are handled differently. Since there are so many different configurations used depending on make and model, I’m only going to speak in general terms.
One of the prime suspects here is the idle air control valve, or IAC. This is an electronic motor that varies an opening in the intake that allows air to bypass the throttle plate when it’s closed. The amount of air needed to maintain your idle is calculated by the computer. The computer dictates an IAC positioning based on inputs from the engine’s sensors, usually:
- Coolant temp
- Air temp
- MAP sensor readings
- BARO sensor readings
- O2 sensor readings when in closed loop
- Air fuel sensors or AF sensors (similar to O2 sensors but a little fancier)
- MAF sensors if equipped
- TPS sensor
- Load sensors such as the type for power steering pressure
- AC clutch activation
Those are just the ones I can think of off the top of my head. I have a reason for listing all of these sensors. If you have a check engine light on for any for the above sensors, repair that fault first, then recheck for your idle problem.
I’ve had many people come to me and say that they’ve cleaned their IAC valve and still have an idle problem. Personally, I haven’t had much luck cleaning IAC valves. I recommend replacement instead of cleaning if you find yours to be bad. I’m not saying don’t clean them, because you might just get lucky, but if cleaning doesn’t work, then you might end up having to replace it.
Video Title: Solving Automotive Idle Problems – EricTheCarGuy Video Description: In this Article, Solving Automotive Idle Problems, Eric looks at what can be the cause of Solving Automotive Idle Problems with your vehicle .Thumbnail: http://www.ericthecarguy.com/images/faq_buttons/Large_FAQ_Images/Idle-Issues-icon-large.png
I DO NOT RECOMMEND REMOVAL OF THE THROTTLE BODY TO CLEAN IT, EVER. Especially if you have a DBW (Drive By Wire) throttle. People come to me all the time saying that they’ve done this and an almost equal number report problems worse than what they had before the cleaning. Heed this warning. You don’t need to remove a throttle body to clean it. If you have a DBW throttle body, DO NOT MOVE THE THROTTLE PLATE MANUALLY TO CLEAN IT, especially with the key in the ON position. This will confuse the computer and it will have to be reset in order for the engine to start and run again. In fact, this is the same thing that might happen if you remove the throttle body to clean it. Once you find yourself in this position, you’re going to have to find someone with the proper scan tool to relearn your throttle position.
To avoid this, have a buddy in the vehicle with the key on. Have them depress the gas pedal all the way to open the throttle plate so you can gain access to do your cleaning. This will likely prevent you from upsetting the throttle position and causing the condition mentioned above. Honestly, the only time I recommend cleaning a throttle is when it’s sticking. I’ve personally never seen cleaning a throttle body correct an idle issue. I think it makes you feel better more than the car. Don’t get me wrong, clean parts are usually working parts, but in the case of the throttle, I only recommend cleaning the throttle body if you notice your gas pedal sticking or hanging up when you use it.
Your intake gets dirty because of one main reason: the PCV (positive crankcase ventilation) system. Actually, it’s a worn engine, but the PCV system delivers the gunk from that worn engine. The PCV system helps vent crankcase pressure and reduces emissions. Whenever you have moving parts in an enclosed space, like pistons moving inside an engine block, you need to vent the pressure that inevitably builds up.
Modern engines use the PCV system. Before the PCV system, engines used an oil draft tube that vented crankcase pressure to the atmosphere. The PCV system vents crankcase pressure and fumes back into the intake manifold to be re-burned. The PCV system also uses engine vacuum to help aid in the removal of this pressure and the fumes created inside the engine. The addition of engine vacuum is what makes it a positive system instead of a passive system like on engines with oil draft tubes.
As the engine wears, cylinder wall clearances get larger, piston rings don’t seal as well, and there is more blow-by during engine operation. Blow-by is the combustion gases that leak past the piston rings; it’s what is mainly responsible for the buildup of gunk in your intake. The more worn your engine, the more blow-by you have. The more blow-by you have, the more gunk buildup in the intake and throttle body assembly.
In summary, you can clean the throttle body all you like, but the fact is, if your engine is worn, your intake and throttle body will continue to get dirty. Don’t remove the throttle body to clean it; you don’t need to. Cleaning your throttle body likely won’t do anything to solve an idle problem. It might make you feel better, but it might do little to solve an idle problem. If you have a DBW system, I would recommend leaving it alone, as you could cause more problems than you solve with a cleaning. Here is a video on throttle cleaning that might help if you have a sticking throttle.
As stated, mixture problems are the main cause of idle issues. As a first step, I always recommend looking for vacuum leaks, as this is probably the most common cause of idle problems. Here’s a video showing one method of finding vacuum leaks.
Some of you might be a little nervous about using carburetor cleaner, and I understand. Know that you can also use water in a spray bottle. It might not be as effective, but at least you don’t have to worry about catching something on fire. Another method is to add smoke to the intake with the engine off and look for where it escapes. They make machines to do this, and I’ve also seen people use a cigar to produce the smoke.
It doesn’t matter what method you use, as long as it’s effective at finding a vacuum leak. Once you’ve found the vacuum leak, repair it and recheck for your idle problem.
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.
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.
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.