rough idle and low rpms at a stop light, but drives fine

I have a 2006 jeep grand Cherokee. Drives great but sometimes at a red light the rpms drop to about 750 then lower and shutters then jumps back up. It only does this when im at a red light.

My car has 86000 miles.
My car has an automatic transmission.

Experienced mechanics share their insights in answering this question :
Hello. This is very common on this particular vehicle. Typically it is caused by a dirty or failing throttle body. I have also had a few of these with a sticking thermostat which causes the engine to run cooler than it should and makes the car run like this.

I typically start by cleaning the throttle body first. If that does not take care of it I would use a scan tool to check the coolant temperature and the throttle sensors. If you are thinking of having this done, have a certified mechanic inspect your rough idle and low rpm’s to make an accurate fix.

How to Identify and Fix Common car Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced mechanics :

The most common reason a Jeep Cherokee is shaking while stationary and idling is because of a bad spark plug or coil/coil wire. The most common reason a Jeep Cherokee is shaking while driving down the road is because of suspension and tire issues.
The most common reasons a Jeep Grand Cherokee has rough idle are a vacuum leak, an issue with the spark plugs, or a problem with the ignition coil.
Typically, the idle speed will be set at a value in the range of 600 to 800 RPM, but the exact value will depend on the number of engine cylinders and whether the vehicle is equipped with a manual or automatic transmission.
Dirty fuel injectors may cause the engine to run lean which will in turn, cause hesitation when accelerating.
One of the main reasons why your vehicle could be vibrating at idle could be due to worn out spark plugs. When a vehicle`s spark plug is faulty, it can interfere with the air-fuel ratio leading to improperly firing cylinders, resulting in a misfire while idling. Replacing your spark plugs will solve this problem.
Along with faulty spark plugs, a build-up of dirt and grime is the likely cause of cars stuttering when idle. A clogged filter or fuel injection system can starve your engine of fuel or oxygen, causing your car to judder and shake.
Oxygen (O2) sensor

An oxygen sensor monitors and analyzes the amount of oxygen in a vehicle`s exhaust system after combustion. A dirty, damaged or malfunctioning O2 sensor (or sensors) affects the air/fuel mixture, causing the engine to idle rough.

Dirty/Faulty Fuel Injectors – Fuel injection systems play an essential role in starting your vehicle. The system injects fuel and air into the cylinders. If clogged or gummed up, the injectors don`t provide the right mix of fuel and air, resulting in a rough idle or slow acceleration.
Typically, engines idle at about 600-800 rpm. Older engines may go up to 1200 rpm or more while warming up. But if the engine is revving up while idling, even after it`s sufficiently warmed up, you might have a problem. High idle speeds waste fuel, cause extra wear and tear on your engine, and can be unsafe as well.
For regular cars, idle speed is typically between 600 and 1,000 rpm, just enough to keep the engine`s ancillary systems going, but not enough to move the car forward much.
Any unusual shaking or vibrating forces coming from the engine is cause for concern. It could be something as simple as old spark plugs producing an uneven power delivery, it could be something serious like worn or broken engine mounts, or it could be even more serious in the case of internal engine damage.
An engine that is hesitating to accelerate is most likely dealing with a fuel/air mixture that is too lean. Engine`s that are running inefficiently will begin to show signs like hesitation which will only become worse over time.
Worn out spark plugs are one of the most common causes of engine shaking. If your spark plugs are dirty or worn out they will be unable to fire properly and create the spark your engine needs to ignite the fuel. This causes your engine to misfire. Replacing your spark plugs is typically inexpensive.
How to fix it? Cleaning or replacing the fuel filter and adding a fuel injector cleaning treatment to the fuel tank should take care of clogged fuel injectors. If the rough idle persists, it`s time to take your car to an auto repair shop.
The most common causes of the issue you are having is faulty spark plugs and faulty ignition coils. If a spark plug is the cause of your misfire, it is recommended that all the spark plugs be replaced at once for best maintenance practices.
A rough idling engine can be caused by spark plugs or spark plug wires. Spark plugs use the electrical current received from ignition coils to ignite the air/fuel mixture within the combustion chamber. A plug that is damaged or installed incorrectly can result in fuel being burned at an inconsistent rate.
A PCV valve that`s stuck open will allow too much air to enter the intake manifold, leading to a leaner idling fuel-air mixture. Stuck closed leads to a build up of pressure in the crank case which creates a variety of oil leaks! And this in turn can lead to a range of issues, including a rough idle.
Yes. While the damage may seem small initially, the longer you drive with a bad oxygen sensor, the worse the damage will become. Eventually, you may experience rough idling, poor acceleration, engine misfires, an illuminated check engine light, and failed emission tests.
If you start to experience engine misfires, stalling, or rough idling when the car is stopped, it can also be a warning sign of a failing TPS.
Poor Idle. A malfunctioning fuel pump can cause the vehicle to stutter while idling and not run correctly. If your engine RPMs fluctuate at idle or the engine idles too low and almost dies, the problem can be the fuel pump.
A clogged fuel or air filter can make your engine sputter and your acceleration lag because the engine is not getting enough fuel or air, respectively. The combustion chamber needs the correct amount of air and fuel in order to generate combustion efficiently.
500rpm is a bit low to me . Most cars I`ve dealt with it`s around 850rpm . If the idle is set to low when you come to a stop from running at a faster speed ( especially quick ones ) the idle speed may not be high enough to compensate to keep the engine running .
What RPM should a car drive at when accelerating? When accelerating your vehicle, it is best to stay within a 2000-2500 RPM range. This will give you enough power to reach an acceptable speed level while also ensuring the engine stays within safe operating limits.
If your RPM starts fluctuating, there might be issues with the engine`s combustion process, a communication gap between the IAC and the ECU, or it can be a worn-out spark plug.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

My ’98 expedition drives fine for a while, then after a few hours, when sitting still, it idles rough then shuts off.
ANSWER : After some testing done to verify the EGR valve flow to see if the valve is intermittently sticking open causing the stall. This can be done using a scanner with the capability to open and close the EGR valve and monitor the DPFE sensor at the same time. There is a technical service bulletin 05-4-17 to replace the EGR valve if the valve sticks open. The computer system should be retested to verify the proper operation of the EGR valve and DPFE sensor. Once the EGR valve is replaced and system is verified the EGR system is working correctly without the EGR sticking open anymore then have a certified technician, like one from YourMechanic, scan the computer to clear any codes. Once codes are cleared a road test should be done on the car to verify the repairs.

idle is low and engine light flashes when I drive.
ANSWER : Hello, and thank you for writing in. The vehicle may be having many types of problems. With a lack of power and a ticking sound, we must look at the level of the oil and the drive belt. Both can cause similar symptoms and noises. If your oil is low, then you have a leak and you need to turn your attention to finding it. If the belt is broken, frayed, loose, or a pulley is bad, then you will need to replace the belt and take any further corrective action. If these components check out, then you should turn your attention to the charging system and make sure the battery and alternator are working correctly. If necessary you can then resort to testing for proper fuel pressure, spark power, and look into the air intake system. For more diagnostic help, contact our service department to schedule an appointment.

My ’98 expedition, it drives fine for a while, then after a few hours, when sitting still, it idles rough then shuts off.
ANSWER : After some testing done to verify the EGR valve flow to see if the valve is intermittently sticking open causing the stall. This can be done using a scanner with the capability to open and close the EGR valve and monitor the DPFE sensor at the same time. There is a technical service bulletin 05-4-17 to replace the EGR valve if the valve sticks open. The computer system should be retested to verify the proper operation of the EGR valve and DPFE sensor. Once the EGR valve is replaced and system is verified the EGR system is working correctly without the EGR sticking open anymore then scan the computer to clear any codes. Once codes are cleared then the car should be road tested to verify repairs. If you’d like to get this fixed, have a certified technician, such as one from YourMechanic, perform the necessary tests and properly diagnose your Check Engine Light firsthand.

I have 97 nissan altima, It stalls when I stop at stop lights and idle, the RPM vibrates and falls down and the lights come on, it
ANSWER : Hi there. Usually when you have a problem like this, it is caused by fuel system components that are clogged with excessive carbon build-up or deposits. This creates a restriction in the efficient flow of fuel and can cause the vehicle to stall. It’s also possible that emissions system components like EGR valve, mass air flow sensors or O2 sensors are faulty. I would recommend having a professional mechanic come to your location and complete a car starts the dies inspection, so they can pinpoint the source of your issues and recommend the right repairs.

rough idle and low rpms at a stop light, but drives fine
ANSWER : Hello. This is very common on this particular vehicle. Typically it is caused by a dirty or failing throttle body. I have also had a few of these with a sticking thermostat which causes the engine to run cooler than it should and makes the car run like this.

I typically start by cleaning the throttle body first. If that does not take care of it I would use a scan tool to check the coolant temperature and the throttle sensors. If you are thinking of having this done, have a certified mechanic inspect your rough idle and low rpm’s to make an accurate fix.

Rough idle when in drive..but idle is fine when in nuetral or park
ANSWER : Hi there. Check for vacuum leaks on the engine. A small leak will cause the engine to stumble when the rpm’s are low, like in gear on automatic transmissions. Look around the intake manifold for the gaskets being broken. Check all of the vacuum lines for any signs of leakage with loose ends or cracks in the tubing. If you need further assistance with your engine running rough only at a stopped position with the transmission in drive, then seek out a professional, such as one from Your Mechanic, to help you.

Low coolant light comes on when AC, defrost or heater is on, but only if the car is in gear and stopped, what’s wrong with my car?
ANSWER : First, you want to make sure your cooling system is full. Check the radiator not the coolant jug. And check it when it is cold. If it needs coolant, add some. Of course, I don’t feel this problem is an actual low coolant condition. I would expect to find an electrical failure in the instrument cluster or from a component or wiring harness that is connected to the instrument cluster.

As a technician, I have access to reports from technicians across the country. I could not find any reports of this problem, so diagnosing this will require an experienced tech as this is not a common problem. In addition, this happens at intermittent times making it difficult to recreate. The most challenging part of this diagnosis will be recreating the symptom. I suspect it has something to do with the brake switch circuitry because it only seems to happen when you have your foot on the brake. I would begin my diagnosis there with a wiring diagram and test light in hand. The brake switch, instrument cluster, and environmental controls all interact with each other. Something is crossed either because of a failed module or damaged wiring in a harness.

Aside from making sure the coolant is full, this will not likely leave you on the side of the road. If you take it in to be diagnosed, be prepared for them to spend more time than usual to figure it out. Since it isn’t a key problem, you may need to live with it until it completely fails making it much easier to diagnose. Either way, I do recommend having a certified mechanic diagnose the warning light to make sure there isn’t anything serious going on.

Good luck.

Why does my Echo rough idles at a red light or stop sign?
ANSWER : Hello,
What you are describing is a common sign of a dirty or faulty idle control valve. The idle air control valve is a small valve on the engine’s intake system that reads the air intake as it comes into the motor. This is controlled by the car’s ECM which uses this information to make adjustments to the air/fuel ratio depending on various inputs such as outside air temperature, intake air temperature, load and various other things. As you accelerate, your car’s engine is receiving a much higher dose of fuel than when at idle and conversely, when you let off the gas pedal, there is a sudden change in this fuel supply as a result of your foot letting off the pedal. When this happens, the job of the idle air control valve is to bring this deceleration down to a slow and smooth idle rather than suddenly cutting off the fuel supply causing the motor to die. When the idle air control valve is not working properly, this cause a disruption in this process resulting in the engine not being able to idle properly. I would recommend having a professional come to your location to diagnose and inspect your vehicle.