Experienced drivers always carefully monitor the nature of the engine’s work. And if deviations are noticeable, for example, floating engine rpm at idle, this may be the first sign of impending problems. And you should not ignore these symptoms. It is better to deal with the causes before they lead to more serious malfunctions.
How to understand that the idle speed is unstable
Most often, the engine idles at 750-900 rpm. The easiest way to tell if the idle speed is jumping is to check the tachometer – if you have one on your dashboard. With an analog device, the arrow showing the engine speed will jerk noticeably.
If the tachometer is digital, unstable revolutions can be noticed by constantly changing figures on the screen. In this case, the range of changing values can be from 500 to 2000 rpm.
However, floating engine speeds at idle can be easily detected by ear. A serviceable machine usually makes a monotonous sound when running, but when the engine speed is floating at idle, the sound is constantly changing, as if someone is pressing and releasing the gas pedal. Sometimes there is also a vibration: it can be felt on the body or noticed by the vibrations of the engine itself under the hood.
What factors lead to instability of operation
The main reasons that the engine speed jumps without load are as follows:
- failure of one of the main engine systems involved in its working process: fuel, ignition, intake or gas distribution;
- suction of excess air into the engine, which violates the composition of the fuel-air mixture. Most often this happens because of a violation of the tightness of certain elements of the intake system;
- low compression in the cylinders, which can be caused by problems with both the block head and the cylinder-piston group.
The main causes of unstable idle speed
Injector problems
One of the systems that is definitely worth checking if the revs are floating is the injector. The injectors of the fuel system are often contaminated with deposits, due to which both the volume and the shape of the atomized fuel torch are disturbed. This is a reason to wash injectors.
And if as a result of diagnostics it turns out that the injector is out of order, it will be necessary to replace it to eliminate floating engine speed at idle.
Low fuel pressure
There can be several reasons for low fuel pressure:
- failure of the fuel pump;
- faults in the operation of the pressure regulator in the fuel ramp;
- strong contamination of fuel filters (as a rule, there are two of them – coarse and fine).
Filters are easy to replace, but the other elements will have to be checked in the service and repaired if necessary.
Spark plugs malfunction
If the revs float, the cause may be a poor condition of the spark plugs, on which the quality of ignition of the mixture depends most of all. Moreover, unstable idling can be a consequence of missing spark just one of the plugs. What plugs are better to buy for replacement and how to select them correctly, we have already told you.
Disturbance of tightness of the intake system
Unauthorized extra air intake violates the composition of the fuel-air mixture, as a consequence – idle speeds are walking.
Leakiness can occur in different parts of the intake system. But most often problems occur with the following elements:
- collector gaskets, which over time shrivel up;
- hose and hose connections, such as plastic tees, connectors, etc. – they can crack;
- vacuum hoses, such as those to the brake booster, which crack over time;
- collector, if it is plastic – with age, microcracks may appear in its walls.
Tightness is restored if possible, but if this is not possible, simply replace the failed elements.
Mass air flow sensor
Floating engine speed at idle is a consequence of a malfunction of the mass air flow sensor (MAF), which is installed after the air filter. The sensor detects the volume of air passing through it and sends the information to the ECU, on the basis of which the fuel injection is adjusted.
Sensors have different designs, but most often they are unsuitable for repair. If it is the fault of the DMRV that the engine rpm is jumping at idle, it must be replaced.
Throttle valve
The throttle valve becomes covered with deposits over time and may stick or not close completely. As a result, more air than necessary enters the combustion chamber. The problem can be eliminated by cleaning the unit with special chemistry.
The idle speed regulator
The idle speed regulator is designed to maintain the speed without load when the throttle is closed. And with its malfunction, unstable idle speeds are a logical result.
The causes of malfunctions in the regulator can be different. If they consist in the loss of mobility of the valve due to contamination, unstable idling can be eliminated by its cleaning and lubrication of the stem. And in the case of a malfunction of the electrical part, the node will have to be replaced.
Carter ventilation valve
Failure of this element leads to the fact that the pressure inside the system increases, crankcase gases in excessive quantities get into the combustion chamber and violate the composition of the mixture. The crankcase gas ventilation valve is often disassembled and repairable. If the problem is caused by accumulation of deposits, in most cases it is sufficient to simply flush the valve. But in case of wear or breakage of elements, the valve will have to be replaced with a new one.
Low compression in the cylinders
Another reason why the idle speed “jumps” is too low compression. The main reasons for its loss are:
- Lodging of piston rings due to caking. Sometimes with this problem can cope with specialized chemistry, but it may also require reassembly of the engine.
- Wear or scoring in the cylinders, making ineffective even serviceable piston rings. The issue is solved by repair or replacement of the cylinder block.
- Destruction of the cylinder-head gasket, which must be replaced as soon as possible.
- Irregular valve clearances, which can be corrected by adjusting them, or valve destruction (cracking), requiring its replacement.
Totals
- Determine the floating engine speed at idle can be by tachometer or by ear.
- Instable engine operation may be to blame for malfunctions of the ignition system, intake, gas distribution, fuel supply, as well as sucking excess air and low compression.
- Causes of floating idle speeds are usually related to injectors, fuel pressure, spark plugs, intake system leaks, DMRV, throttle plate or idle regulator.
- The problem is not as harmless as it may seem to an inexperienced car owner. Unstable engine speed can be a symptom of serious malfunctions.