The engine “sausages” - why the speed fluctuates and how to solve the problem

Floating engine speed at idle is a problem that many car owners encounter. It manifests itself in the form of unstable engine operation. The unit may stop for no reason, and fuel consumption increases.


There are many malfunctions that cause this problem: from air leaks to sensor malfunctions. In this article we will look at the most common of them, and also describe diagnostic and repair methods.

How to determine idle instability?

This can be done in two ways - by ear and using a tachometer. In the first case, instead of the usual operation, the internal combustion engine produces sounds of different frequencies, moving from high to low tones. It may seem that at this time someone is pressing and releasing the gas pedal. In some cases, the unit begins to vibrate or even stall. Sometimes error p0016 occurs.

As for the tachometer method, each car has a set idle speed standard, but the general range is from 750 to 1000 rpm. If the sensor indicator (numbers on the panel) goes outside the norm, for example, fluctuates between 700 and 2000 rpm, and these changes are accompanied by sounds uncharacteristic of normal engine operation, then the idle speed is unstable. The problem is that not all older cars have tachometers, so sometimes you have to diagnose by sound.

Why do the revs fluctuate?

Unstable speed is an unpleasant phenomenon that often occurs in used cars, both with injection and carburetor power systems. This problem can affect not only domestic models, such as Lada Priora, Kalina, UAZ Patriot, but also “Germans”, “Japanese”, “Koreans”, etc.

Common reasons that can cause a malfunction:

✅ Malfunctions in the engine power system can manifest themselves in the form of contamination or breakdown of injectors, fuel filter or breakdown of the fuel pump. These problems lead to uneven fuel supply
✅ Throttle malfunctions. Its wear or clogging causes unstable operation of the internal combustion engine.
✅ Malfunctions in the ignition system. Due to wear on the spark plugs or faulty high-voltage cables, the motor also becomes unstable. Failure of one or more coils may cause misfire.
✅ Irregularities in the intake system, such as leaks from defective vacuum pipes, create an imbalance in the air-fuel ratio
✅ Malfunctions in the motor control system. The reason may be the ECU itself or the software.

Sometimes the problem lies in the MAF or TPS regulators. If the sensor readings differ from the normalized ones, the operation of the internal combustion engine is disrupted. The cause may be a dirty air filter. Unstable operation of the motor is sometimes due to a restriction in the air flow that passes through it. In some cases, the problem is a problem with the injection system. Floating speed may be caused by a dirty catalyst.

Spark plug

They are responsible for igniting the combustible mixture, so the stable functioning of the internal combustion engine depends on them. When worn, dirty or malfunctioning, not only instability at idle speed occurs, but also serious vibrations.

First of all, you need to check the electrodes. Over time, they wear out, causing the gap between them to increase, which leads to disruption of sparking. This causes unstable ignition of the combustible mixture and uneven operation of the engine at idle. The service life of SZ is limited to 20-30 thousand km, while spark plugs with an iridium electrode can last up to 100 thousand km.


Contamination of parts, be it carbon deposits, oil deposits or other forms of defects, can become an obstacle to the formation of a spark. Using the wrong type or size of SZ will cause the mixture to burn incorrectly, causing RPM fluctuations. Damage to the high voltage wire that connects the spark plug to the coil can reduce spark quality, resulting in poor ignition efficiency.

The main reasons why there may be traces of oil deposits on the SZ:

✅ Worn or damaged piston rings. If they malfunction, engine fluid enters the combustion chamber and settles on the NW
✅ Defect of valve stem seals. Their failure also leads to oil entering the combustion chamber.
✅ Malfunction of valves and their guides. Their failure causes a leak in the seal.
✅ Using oil that is not suitable for the engine. Due to the consumption of lubricant by the unit, carbon deposits appear on the SZ.

Problems in the operation of the crankcase ventilation system also lead to unstable operation of the internal combustion engine. When the EGR valve fails, high pressure builds up in it, causing lubricant to be forced through the caps and piston rings.


The problem can also be caused by flooding the SZ with fuel. This occurs as a result of air filter contamination, ignition malfunctions, and IAC problems. On carburetor engines, the problem is sometimes caused by incorrect settings of the fuel injection system.

Solving the problem yourself

If your engine speed starts to fluctuate, first of all you need to check the idle speed sensor. This is a control element in the intake system that regulates the amount of air entering when the damper is closed. If the regulator is clogged or worn, it can cause the stem to stick.

The vacuum tubes connected to it wear out over time. Their damage causes a loss of pressure in the system. The operation of the regulator is also influenced by the antifreeze temperature controller and the TPS. Its performance may be impaired as a result of nutritional problems. Try cleaning the contacts, check the connections, vacuum pipes, or replace the IAC.


The engine speed may begin to fluctuate due to a leak in the intake system. The ECU only registers the volume of air that flows through the controllers in the manifold. If there is leakage from the outside in other places, the unit cannot detect the actual amount of oxygen and is not able to properly adjust the engine, which leads to unstable speed. The composition of the mixture becomes unbalanced.

Suction is possible through vacuum tubes. If they are worn and cracked, they will need to be replaced. Air can also be sucked in through microcracks in the intake manifold, worn or damaged hoses and pipelines, or the throttle valve seal. Over time, all gaskets and hoses age, crack and require replacement.


Even one faulty injector will cause floating engine speed. You can try to fix the problem using special additives added to the gas tank. But such measures turn out to be effective in the early stages of car operation. As a preventative measure, it is recommended to carry out cleaning every 10-20 thousand km. mileage A complete solution requires replacing the injectors.

The problem of floating speed may be a clogged throttle valve, dirty or broken MAF (mass air flow sensor), low compression in the cylinders, etc. We looked at the most common faults that you can fix yourself. In case of serious problems, for example, a decrease in fuel pressure in the system, it is recommended to contact specialists.