Q: At idle, on an intermittent basis, the rpm repeatedly surges and returns to normal

asked by on May 29, 2016

While idling, my rpm will surge from about 1100 to 1700 over the time frame of a second or two then return to 1100 over that same time frame, then repeat. If I put a load on the engine by turning on the airconditioning compressor, the surging will often stop. This is an intermittent problem but seems to occur at least 50% of the time and seems to be worse after the engine has warmed up. Thank you for any input.

My car has 225000 miles.
My car has a manual transmission.

Car idle is rough Inspection $94.99 - $114.99 Get a Quote

There a few things that can cause this situation on your car. The most common have to do with the throttle body and the idle air control system.

The throttle body is the where your your throttle valve is housed. The throttle valve is connected to the gas pedal via a cable. When the throttle body gets gummed up with carbon, it can cause a variety of issues. It should be thoroughly clean with some carburetor cleaner and a rag. Use the carburetor cleaner sparingly. The build up of carbon can have adverse affects on idle quality.

The most likely cause of your problem is the idle air control valve (IAC). The IAC is the most common part that fails on your particular car and is often gummed up with carbon as well. It has a couple mounting locations that I can’t be exact about unless the car were in front of me. Most times, it is round cylinder with two hoses attached to it and an electrical connector. It is either by the throttle body or mounted somewhere on the intake manifold. One hose connects to the intake manifold and the other will be connected to the hose that connects your air cleaner to the motor. Often times, these valves can be cleaned, but most often they will need to be replaced.

The other possible problem is the idle speed control unit that is mounted in the dash above the glove box. This doesn’t fail as often as the IAC, but does fail at times. This component unique to BMW. Most vehicles use the PCM (Powertrain Control Module) to control idle.

There may be other issues causing this, such as intake manifold leaks that can occur in many places. I do believe your particular problem, based on your detailed description, has something to do with the IAC system.

If you want to have the faulty component identified in person, consider YourMechanic. They will be able to properly diagnose your rough engine idle and determine exactly what needs repair.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing


Experienced Mechanic?

Earn up to $70/hr

What others are asking

Why Is My AC Blowing Warm Air?
There are quite a few reasons an A/C system (https://www.yourmechanic.com/services/ac-is-not-working-inspection) will cease to provide you with cold air. Air conditioning systems are one of the more complex systems on a passenger vehicle. Without getting into to all the complexities and...
How do I display the average miles per gallon?
The Base Message Center keeps track of all of the functions your truck performs. This message center will tell you all about how your vehicle is performing. The Average Miles Per Gallon feature can show your truck's fuel economy. This...
Car weak when starting
If the vehicle sounds weak when you turn the key, as in it cranks slowly, or struggles to turn over, then it is most likely a battery or starter issue. I would have the battery load tested to make sure...
car stall s and the trac off come on
Hi there. Most of the time the trac off will come on when there is a powertrain problem. This can be voltage related or it may have a crankshaft position sensor that is failing among other parts. I usually start...
Can a strut fail by just using a car hoist?
Hi, thanks for writing in. In my 35 plus years of servicing vehicles, I have never seen a strut go bad when I lifted a vehicle on a lift, however this does not mean it is not possible. When the...
p0073
Hi there. The Power Control Module (PCM) uses information that it receives from the ambient air temperature sensor to regulate the levels of the ambient air temperature and intake air temperature. If the ambient air temperature reaches a level above...
Idles but when driving it acts like its out of fuel.
Check all of the battery ground cables to the car on the battery, engine, and chassis. There is a bad ground somewhere that is causing the ECM to not work properly. This would cause the fuel pump to not work...
I have a 2007 Infiniti G35 sedan, when pressing the pedal softly I get a slight delay in acceleration, and sometimes a small jerk
Hi there. Typically when an engine "bogs down", it is caused by a fuel or air restriction issue. Common components that can cause this are mass air flow sensors, MAP sensor, EGR valve, fuel injectors being dirty or clogged or...
Popping/clunking sound from right passenger side front. Front suspension was inspected over the weekend
Hi there. In most cases, when you have a problem like this, it's caused by a loose or damaged strut, strut mount, stabilizer bar bushings or CV joints being worn out. The stabilizer bar bushing specifically is usually the part...

Related articles

Top 10 Factors to Consider When Buying a Luxury Car
The The idea of owning a luxury car is appealing. The glossy paint, swoopy lines, and prestigious badges speak of the owner’s success in life, while the interior opulence offers scented leather surfaces, high-tech telematic devices, and all the creature-comfort...
A Buyer’s Guide to the 2012 BMW 3-Series
The The 2012 BMW 3-Series has been wildly popular in the US since it was first introduced in 1975. This series of luxury sedans sets itself apart from other luxury models with its superior performance and driving dynamics. No matter...
5 Essential Things to Know About Luxury Vehicles
There’s There’s something to be said about the looks and offerings of luxury vehicles, but appearances can always be deceiving. If you’re considering buying a luxury car, there are a few things you need to know before you make your...