Solving Automotive Idle Problems
The Idle Circuit
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
- RPM
- 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
Cleaning Throttle Bodies
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.
Look for Vacuum Leaks
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.
Too Much Fuel, Fuel Injector Problems
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.
19 thoughts on “Solving Automotive Idle Problems”
RJ
February 13, 2024 at 6:18 am
Hello Mr car guy i have a 92 honda civic that when it warms up it will idle like its about to die ive replaced the fuel filter the fuel pump i checked the iacv i replaced the spark plug wires about 2 years ago along with the spark plugs and it just recently started acting up where should i check next??? Please i would really appreciate your help
Wes Crews
October 27, 2023 at 11:34 pm
Hello,
Vehicle in question: 2002 Honda Odyssey. Problem: originally no start condition.
Remedy as per your video – repaired solder joint in main relay .Vehicle now runs but has a bouncing idle.
Checkers power steering pressure switch wiring.. all ok.
Would a defective IAC valve cause this issue ? If not ? What else?
I’m stone walled. BTW… no DTC codes present.
Would sincerely appreciate any advice or pointers.
Absolutely love the videos.. they have been a tremendous help.
Hope to hear back.
Wes in MS
Ron Kanna
May 22, 2023 at 3:14 pm
1995 Accord 2.2l VTEC
My idle surges from 1000 to 2500 constantly until I unplug my IAC valve. I have cleaned my egr ports and tried a new IAC valve but I still have the same problem? Something is telling the valve to stay wide open when it is plugged in??? Could a temperature sensor be causing it?
The only thing I can think is that I put a new aftermarket IACV on and it is also faulty?? I find it hard to buy a $300 part when that might not be the problem??!! I also replaced the IAT sensor, the temperature sensor and put a different throttle body on, I have bled the cooling system several times and it STILL SURGES!! Sometimes I can hold the rpm’s at about 4000 for several seconds and then it will idle down but only until I give it more throttle then it starts surging again???
James
March 25, 2022 at 3:36 pm
2002 accord V6 j-series sat for a year. New alternator, battery, fuel, pump, spark plugs, etc. idle surge issue and when it happens the TCS light comes on as well.. any ideas? IACV probably
John
January 26, 2022 at 11:56 pm
I’m having a problem with a 2002 Acura TL type-S. I have codes P0505 and P1519, saying there’s a problem with the IACV. The car is surging at idle, from 1k to 2k RPM. I cleaned the throttle body and the IACV valve and did not help. Replaced the IACV and did the idle relearn procedure and still doing the same. Replace the FICV that right below the IAC and improved a little. It now starts at high RPMS, then starts to surge for s few minutes, then stays between 1450-1550 RPMS. Only does it in Park and Neutral. I’ve checked electrical connections and everything seems fine. Have checked for vacuum leaks or EGR problems and didn’t find anything out of the ordinary. Have replaced and bled the cooling system in the process also. Any suggestions on what to check?
Gage
March 26, 2023 at 1:25 pm
I had have the same problem which left me very clueless on what to do after changing my MAP Sensor along with IAC Valve I proceeded the idle relearn procedure and still nothing untill when I just started to hit my throttle body and the idle have just went back down to regular idle which was at 845 for mines. That’s when I’ve notice my throttle body is going bad. I had changed my throttle body, done the relearn procedure drove for 10 miles and bam it was fixed. Before It would just throw P0505 code. Hope this helps
Kenneth DeLome
October 4, 2021 at 11:30 pm
ok so my daughter has a 06 scion s/a it has been running as it should and now she is having a issue under a load and at a momentary idle im somewhat savy with motors but mostly chevys with that said i go with my gut and want to start at the tank…. thoughts …. this problem just started about 2-3 days back when she was running late to work and ran low on fuel on her way after work.
Oscar Talamayan
September 10, 2021 at 5:13 pm
Own a 2010 Camry LE 2.5L 4cy. Issues are :
Car ENGINE TURNS OFF
1. while making sharp nor slight tuns
2. Engine turns off suddenly for a block straight driving.
3. When car Stalls it can turn engine on again.
4. Car Stalls only when driving
5. When parked, No lights in dashboard. Normal idling. Don’t Stalls and rpm at steady 6,000
Please Help
Zack lowe
July 20, 2021 at 7:31 pm
Having a problem with my 2000 Honda Civic ex when it’s in park and ac is on the car sounds like it wants to stall out and the idel tends to jump. when the ac is off the idle is fine it idles at 500-1000 rpm could it be my (IAC) ?
ron
February 12, 2021 at 3:22 pm
i am currently having this issue. 2003 honda crv 2.4 UK manufactory model. things ive tried, 1)purchased the $300 oem iac , 2) iac relearn procedure 2sec on then 3k rev for 10 without any load, 3) removed the ecu/pcm fuse for 5 min then replace and tried the relearn again, 4) check for vacuum leaks, 5)replaced the pcv incase this was the issue. i initially removed the old iac cleaned it and installed with no luck, this is why i bought a oem replacement. One thing i’ve noticed, when i erase the p0505 code with obd scanner, the crv idles perfect until driven and the check engine light comes back on. hopefully you may have some insight. thank you in advanced
Paul Petty
December 22, 2020 at 12:21 pm
I have a Nissan Altima that won’t will crank but won’t stay running,if I hold the throttle down it will run but I’d I release the throttle it dies…please helpppp
Leon Witherspoon
September 4, 2020 at 8:41 am
Hey hi everyone, I owned an 95 Honda Civic EJ1 D16y8 block with Z6 head. My problem is that when I started the car it idles fine but once at operating temperature it idles down and shut off. Any help would be appreciated.
Shaquille Smith
August 29, 2020 at 4:03 pm
My 2000 honda accord is just surging i change the water pump now i put everything back together start one crank but the idle just keep reviving ive clean the idle control valve an intake an throttle body with carb cleaner but it still surge up an down from 1500 to 2000 rpm can u help
Uzi
July 30, 2020 at 3:48 am
Hi ERIC hope you will be doing fine…. i am having a issue with my idle with AC on it drops badly while AC and stays low idle with AC on can you plz help me out…. plz plz plz
Donald
July 24, 2020 at 7:45 am
Hello! I have a 1993 Honda Civic EG6. I have an idle problem on it. the engine is sucking. Anyone have this experience and how to solve it.
Juneju gacutan
July 13, 2020 at 2:34 pm
Hi! I have a honda civic lxi 1998 model automatic. My oroblem is that when i turn-on the AC, it will shut down. Kindly advise. Thanks in advance
Sam
June 25, 2020 at 9:47 pm
Short and sweet 1993 Honda Accord CB seven. No modifications EX model. I am having the idle issue that is all so common where it fluctuates after it warms up a bit. I have a replaced the idler units in the front of the intake manifold and cleaned throttle body. check for water leaks and air leaks.With no avail. I have purged my water system. I have a video of me running the car and then putting my finger over the top hole in the throttlebody on the side closest to the firewall and idle drops and stays perfect. Does this mean I need to replace the throttle idle sensor?
Billy
June 7, 2020 at 3:48 pm
I have a 2005 Silverado 1500 5.3l V8, recently when I am driving and I come to a stop light and im just sitting there in Drive, my battery gauge will move back and fourth and will have a slight rough idle to it when only the gauge is moving, the battery is brand new, what could it be? I know I have stuff plugged in to my 12v ports that I use, the battery gauge doesn’t drop below 14
Dylan
March 13, 2020 at 4:47 am
Hi, I was changed my mini cooper s r56 oil filter housing gasket and oil feed line and return line. But I foud the turbo is not like before have power, especial I cannot hear the dump valve sound and wastegate looklike not working. the code is P112B, p0340 and p2188. when I was changed before only have p2188.