What can cause engine speed to drop. Causes of a drop in engine speed at idle

Instability of idle speed is a very common malfunction of the fuel system of engines. internal combustion. By itself, the situation when the speed jumps does not have any harmful consequences, but, as a rule, if you sharply release the gas pedal when switching to neutral gear, the engine. With intensive road traffic, stopping in a traffic jam or at an intersection, this is fraught with an accident. Therefore, identifying the cause of the breakdown and eliminating it is a top priority.

Injection and carburetor internal combustion engines have almost the same causes of idle instability, but they are caused by defects or incorrect operation of completely different components, since the organization of the supply air-fuel mixture into the cylinders they differ significantly.

Common Causes of Idle Instability

Injection and carburetor engines have almost the same ignition system. The most common cause of failures is due to damaged spark plug wires. Their device is such that it is not always possible to check the serviceability on your own. Moreover, a removed and fixed wire can show the correct resistance values, and it can work unstably during vibration. Insulation breakdown can sometimes be seen in the dark by a bluish glow in damaged areas. The best way checks - installation of obviously good ignition wires. If the operation of the engine has changed in better side This means that the high-voltage wires need to be replaced. When the engine speed drops in wet weather, the spark plug wires are also most likely to blame.

In carburetor engines, the culprit can be.

RPMs drop when air is sucked in

Why does the turnover drop in this case? Sometimes engine speed jumps when air is sucked in after the carburetor or mass air flow sensor. V carburetor system supply, this causes a depletion of the working mixture, and in the injector, according to the readings of the air flow sensor, one amount enters the cylinders, but in fact a little more, which also depletes the working mixture.

To check, you need to use an aerosol for washing carburetors. It must be sprayed on suspicious joints while the motor is running. A change in the nature of the work (usually the speed increases sharply) will indicate the place of air leakage.

In engines equipped with an oxygen sensor (lambda probe), air leakage in the exhaust tract in the area up to the lambda probe can be a source of problems. He, determining the excess air after the combustion chamber, considers that the mixture is lean, and increases the fuel supply, as a result of which, accordingly, the engine speed of the car floats.

Malfunctions of the idling system of carburetor engines

In older units with a carburetor, the most common cause of failures lies in the clogging of the idle jets with unfiltered particles in the fuel or tar deposits, which also enter along with the fuel. in carburetors with solenoid valve idle, it is the valve that can cause the system to malfunction.

To check, you need to remove the power connector from the valve with the engine stopped and, after turning on the ignition, put it in place. The absence of a characteristic click will indicate a malfunction of the solenoid valve.

To flush the carburetor at home, you can use the flushing liquid in an aerosol can.

Malfunctions of the idling system of injection engines

In injection cars, the idle speed controller is most often the culprit for jumping idle speeds. This is a rod whose height is regulated by a stepper motor. Pollution of the rod with tar deposits from low-quality fuel is the most common. At the same time, the rod moves sharply, in jerks, in certain areas it can completely stall, while the idle speed of the engine floats. Deposits are washed off with carburetor flushing fluid.

Strictly speaking, almost all sensors can affect idle instability. Starting from the mass air flow sensor and ending with the lambda probe. Search defective element it is more convenient to produce using the diagnostic system. If the car is equipped with an on-board controller, then it will have an error code accepted in this device. By the code, you can determine which of these elements is faulty.

Checking the camshaft and crankshaft position sensors

Some sensors can be checked with a voltmeter. This is a mass air flow sensor, camshaft and crankshaft sensor. The last two are diagnosed by connecting a voltmeter to ground and a signal wire with the ignition on, but the engine not running. With slow rotation crankshaft the device will show periodic power surges. If the crankshaft sensor malfunctions, the engine will most likely not start, but with a non-working camshaft sensor (phase sensor), the engine will work. Only during this stroke, and when driving, dips or an arbitrary increase in speed are possible, since the power system switches from phased injection mode to simultaneous injection mode, and the speed jumps regardless of the driving mode.

Checking the mass fuel flow sensor

To check this element, a digital voltmeter with a measurement limit of 2 V is required. The voltmeter is connected to the signal terminals of the MAF with the ignition on. In particular, on the VAZ, these are terminals 1 and 3. The voltage value of a serviceable element should be in the range of 0.99–1.01 V. A voltage of more than 1.05 V indicates its failure. In this case, interruptions in the operation of the engine in all modes are possible.

The sensor in question is not repairable. The ingress of moisture on it when washing the sensitive element is guaranteed to disable it.

Floating speed with a faulty oxygen sensor

An oxygen sensor, or lambda probe, measures the oxygen content in the exhaust gases and, based on the data obtained, determines the quality parameters of the working mixture, giving a signal to deplete or enrich it. The device works normally only after it has warmed up to operating temperature- at least 300 degrees. Therefore, most of them are supplemented by an indirect heating system to speed up the entry into the operating mode. A heating failure or contamination of the working surfaces of the sensors with combustion products causes inaccuracies in the determination of the oxygen content, as a result of which the engine will be unstable.

temperature sensor

The electronic control unit (ECU) when starting a cold engine increases the idle speed, because at low temperatures the engine is unstable and it may stall. As the temperature rises, the ECU gradually reduces the speed and brings it to a minimum when the minimum operating temperature is reached. To measure it, a coolant temperature sensor is used, mounted on the cylinder block. It should not be confused with the temperature sensor mounted on the radiator, which is used to turn on the fan.

The temperature sensor is checked by measuring its resistance with an ohmmeter. A serviceable element at room temperature has a resistance of several tens of kilos, and when heated to 200 degrees (you can carefully use a lighter), it drops dozens of times. If the readings of the device do not change when the sensor is heated, then we can conclude that it is faulty.

Conclusion

We have considered several situations in which engine speeds float. To solve the problem, accurately establishing its cause, it is better to contact the specialists at the TO station.

Many motorists are wondering why idle speeds are falling. A drop in engine speed can happen for various reasons. Surely every motorist noticed this behavior of the engine, for example, when standing at a traffic light.

This article will look at the common causes of a drop in engine speed.

Why engine speed drops

Poor quality fuel

Very often, the cause of a drop or float in engine speed lies in poor fuel. If you refuel at different gas stations, then you will notice the difference in fuel quality. Everyone knows that in the countries of the former USSR, the owners of gas stations love to hack on gasoline. Because of this, the engine of your car suffers. What if there is gasoline? Bad quality, then there may be problems with the fuel system. Try to fill the car with another petrol station and compare: if there is no difference, then continue reading the article further.

Malfunction in the fuel supply system

Due to low-quality fuel, some elements of the fuel system may cease to function normally. Try to change fuel filters, this can help. If this does not help, then you will have to revise the fuel system.

The problem may still be in the uneven supply of fuel to the cylinders. In this case, you should go to the station Maintenance where a specialist will fix the problem. Do not try to do it yourself: by your actions you can disable the injector.

It is worth checking the pressure in the fuel rail: connect a pressure gauge, write down the results and compare them with the allowable results that should be in the owner's manual for your car.

The problem may also be fuel pump. It can pump fuel unevenly, and because of this, engine speed may drop.

Malfunction in the gas distribution mechanism

Over time, the operation of the gas distribution mechanism may be disrupted. You will have to re-mark the gas distribution mechanism. It can even go as far as cleaning the valves and intake manifold from soot. If the engine has not been repaired for a long time, quite a lot of carbon deposits can form in the manifold.

Malfunction in the air supply system

A problem with the mass air flow sensor can also be the cause of the RPM drop. In order to check it, connect the positive contact of the tester to the yellow wire that goes to the sensor, and the negative one to the battery. The voltage should be between 0.98 and 1.02 volts.

The malfunction may also lie in the oxygen sensor or in the engine temperature sensor, which calculate the number of revolutions when the engine warms up. It may also be a problem different compression in the engine cylinders. Due to uneven compression, the engine starts to run unevenly.

Many owners of used cars are familiar with such a problem as “floating” idle speed. Such a nuisance can occur both on very old cars that are equipped with a carburetor, and on more modern ones that already work on an injector. We’ll make a reservation right away that solving the problem on a carburetor engine is much cheaper and easier, but we’ll try to find out how things are with injection engines.

First you need to pay a little attention to the injector device. If we draw an analogy with a carburetor, then, speaking in simple terms, we can say that the carburetor has a completely mechanical control, i.e., the driver himself controls the suction, adjusting the force of air supply to the engine cylinders. In the injector, all "mechanical" obligations are assumed by the electronics. Of course, the electronics, or microcomputer, controls several installed sensors, and if the latter start to work incorrectly, or stop working altogether, problems with speed begin, since the control computer simply cannot determine what to do.

Possible causes of "floating" idle

As already mentioned, the most vulnerable link in the system electronic control are . One of the main ones is the idle speed sensor. As a rule, this sensor is mounted in the area of the control sensor. Checking the sensor is carried out using a multimeter. The procedure for checking the sensor is quite simple. It is necessary to measure the resistance between the contacts that are included in the sensor block. In this case, the ignition of the car must be turned off. Conventionally, the contacts are designated A, B, C, D. The resistance between the contacts should be in the range from 40 to 80 ohms. If during the measurement you received other readings, then the idle speed sensor needs to be replaced.

Another sensor that “mopes” quite often is the mass air flow sensor, or DMRV. To test the operation of the sensor, it is necessary to turn on the ignition, but do not start the engine. The voltage is checked with a multimeter already known to us. It is necessary to measure the voltage of the contacts to which the green and yellow wires are connected. The permissible value of the DMRV sensor is from 0.9 to 1.2 V. In addition, a sensor malfunction can also be determined by spark plugs. If they have black carbon deposits on them, this may also be a signal that it is necessary to replace the faulty sensor.

A less common cause of rough engine idle may be a malfunction in EGR system. This sensor is located in the intake and is responsible for the output. More precisely, the sensor is responsible for the removal of most of the exhaust gases, and a small number is re-injected into the engine cylinders to reduce the amount harmful emissions into the atmosphere, as well as to achieve complete combustion and energy efficiency of the fuel. It is recommended to periodically clean the sensor to keep it working properly.

Why do sensors fail?

There are 2 main reasons for sensor failure. Firstly - this low quality fuel: all kinds of additives that domestic gasoline contains, its low octane number not only clog sensors, but also cause electronics to malfunction. Secondly, the sensor may fail due to the excess of its service life, which is a completely natural reason. Installing a poor quality or defective sensor is a matter of course.

A little about replacing the sensor

If in the case of carbureted engine everything is simple, you can do without stands and do all the repairs at home, then with injectors the situation is much more complicated. Firstly, the sensors themselves, which are only a small link in the ICE chain, cost a lot of money, for which, for example, you can spend overhaul carburetor. Second, replace the sensor in garage conditions impossible, since after a lot of work of replacing and connecting it, it is likely that it will be necessary to “flash” the injector again in order to achieve a complete “recovery” of the car. Of course, such work is carried out with the help of computer diagnostic programs.

Fortunately, the sensors have a fairly solid service life and are capable of not causing problems to the car owner for about 150-200 thousand km. However, if you are the owner of a far from new car, with a rather impressive odometer figure, be prepared for the fact that problems may arise very soon.

A drop in engine speed is a rather unpleasant incident for your car. This is not only about inconvenience and questions about the health of the machine, but also about the operation of important mechanisms. The problem is that a sharp drop in turnover on Idling, when the car rolls in neutral, it can turn off the engine, and this already becomes dangerous for further movement. The driver may not notice such an occurrence, shift into second gear and release the clutch, causing heavy engine braking. The car can start “from the pusher” in this way, or it can simply stop, which will create emergency. When the car warms up, the drop in speed also adversely affects the driver's confidence. You have to constantly gas up with your foot or use other methods to keep the engine running. It is worth understanding the causes of the problem.

So, why do revs drop when warming up on cars with a carburetor? You can find many common causes, which unite the owners of both a certain car model and a specific type of carburetors. I must say that today cars with this type of fuel injection are no longer produced. This is not the best solution, since the carburetor is very sensitive to the quality and purity of the fuel, requires maintenance quite often and does not serve very well in different climatic conditions. This type of gasoline injection requires stability, but in our operating conditions it is simply impossible to provide such a luxury to a car. Therefore, various problems arise that can lead to significant discomfort and even danger of driving. Today we will look at the main causes of a sharp or undulating drop in rpm during warm-up on cars with carburetor fuel injection.

Perhaps it's not about technology, but about refueling?

As mentioned above, the carburetor is very sensitive to the quality of gasoline that you pour into the car's tank. If there are specks or impurities, the equipment will not be able to work normally. Dirty gasoline is cleaned by a filter for a certain time, but over time, such cleaning ceases to help. Fuel with impurities and inclusions can be a big problem, which affects not only the speed. Here are some important indicators of such a problem:

already a few kilometers after refueling low-quality gasoline you will feel changes in the dynamics of the car, fuel consumption will increase and stability will deteriorate;
engine idling speed can float, and this will indicate an unstable composition fuel mixture, at speed, small jerks and dips in the traction of the unit are also possible;
after the engine has cooled down, at the next start, warm-up may start unexpectedly with low speed, you will have to play with suction in unusual ranges for warming up;
it is also quite possible that after warming up to 30-40 degrees there will be a failure, which will need to be compensated by pulling the suction all the way or by gasping with the foot;
when warmed up to operating temperature, the engine will work relatively stably, but the speed will remain floating, these are the indicators of low-quality fuel in the car tank.

These criteria indicate that it is better not to go to your usual gas station anymore. Of course, the same problems are possible if the carburetor fails and needs service. But often these problems occur precisely when poor quality fuel low octane rating or with impurities. Therefore, the first step is to roll out gasoline on a warm engine and add good proven fuel to it.

It's time to service the carburetor - minor defects

The reason for the drop in speed during warm-up can also be small breakdowns in the carburetor system. These are torn membranes, displaced cable mounts, or a sticking damper actuator. Such troubles can be cured during the audit. this device. In the old days, every second car driver could independently sort out the carburetor, install a repair kit and drive on, listening to the happy rumbling of the unit. Today, the principle of repair is as follows:

you should go to a car shop or market to find a suitable repair kit for your type of carburetor, it is better to take original quality parts;
further, it is important to find a master who is well versed in carburetors and can help with troubleshooting problems in your device, there are fewer and fewer such masters in cities;
a specialist in a few tens of minutes will sort out the equipment, find the problem and install new gaskets, membranes, seals and other products from the repair kit;
then it is necessary to check the equipment to make sure there are no unpleasant effects, often this can only be done the next day on a cooled car;
the last step is regular visits to the master once every 2 years for maintenance and revision of the carburetor, this will keep the car in normal working condition for a long period.

Carburetor fuel injection has its advantages. For the maintenance and cleaning of the injector, you will have to pay a fortune. But repairing a carburetor, even with the replacement of certain parts, will not cost too much. But to get these benefits, you should find a master who is well acquainted with the features of your particular car model. Such a specialist will help to fix all the problems and continue to operate the machine without problems.

Condensation or dew point - is it possible?

On VAZ 2107 cars, a drop in engine speed very often occurs during warm-up, up to a complete stop of the unit. This problem is typical for all carburetors, except Solexes. The problem is that at temperatures from 0 to +5 degrees, condensate can collect in the carburetor chambers. This is a certain type of dew that forms during temperature changes. After starting the engine on a cold, the following process occurs:

first, an open suction starts an enriched mixture, which burns out without problems and does not cause noticeable changes in the operation of the power unit, which is very important for a normal start;
as the car warms up, the owner of the car lowers the suction, the mixture approaches the characteristics of the working one, and the entire engine system is already a little warmed up, and here the fun begins;
condensate or dew begins to get into this mixture and change its properties, in some models of devices this happens quite quickly and unpleasantly, causing the motor to stop;
the driver again pulls out the choke or presses the gas pedal, the mixture is enriched, the engine is running normally, but up to 60-70 degrees this process can be repeated endlessly;
after warming up to about the operating temperature, everything returns to normal, the speed returns to normal, the engine works well, so when you visit the station, the master will not find anything.

How to deal with such a situation, it is difficult even to imagine. the only in an efficient way there will be a replacement of the carburetor with Solex, but in these models there are often problems with starting the power unit in transient temperatures. So it is not easy to give any specific advice in this case. It is best to service the equipment on time and always keep the fuel equipment in good condition. good condition, this will help to obtain the proper reliability of the machine.

Are there any other reasons for the sharp drop in turnover?

There can be many reasons for such trouble. If you are repairing a car yourself, it is worth taking a slightly broader look at the trouble and trying to find the causes in other nodes. But moving to other features of the car is only worth it if you are convinced of high quality carburetor operation, normal fuel and other features indicated above. Here are some more ideas to test:

fuel filters - very often the filter elements get clogged, and the owners forget to change them on time, and this leads to serious problems with the vehicle;
thermostat - perhaps after a slight warm-up of the small circle, your thermostat opens and lets icy fluid into the engine abruptly, which leads to a drop in speed;
electronics - it is worth checking the ignition, the normal installation of the timing belt, the absence of problems with sensors and various electronic equipment on board your car;
turning on electricity consumers - perhaps some powerful device automatically turns on in your car, which puts a load on the unit, the speed inevitably drops;
valve system - in this case there will be no sharp drops and drops, but floating and unstable revolutions are quite possible, and under load they will turn into drops.

There can be many reasons for the instability of the engine. Sometimes the problem is that the generator stops producing normal voltage at a certain point, which affects the operation of the engine's electrical systems. The load on the motor may also be due to bad oil or internal breakdowns in the cylinder block or in the valve system. So in this case, you can dig for quite a long time, but it’s better to visit the service station and find the cause of the problem.

We suggest watching a video with a solution to one of the possible causes of this problem:

Summing up

There are dozens of breakdown options that lead to a sharp or smooth drop in speed on carbureted cars. But the issue is that the equipment turns out to be quite demanding to maintain, so you have to eliminate all the causes combined. If you always encounter such a problem, then this is the specifics of the carburetor installed in your car. Most likely, only replacing the device will help get rid of the trouble. If the trouble arose only a few times, it is worth trying to serve fuel system, replace the filter and install a new carburetor repair kit.

Cars with this type of injection are gradually giving way to injection systems. They are more reliable, more economical, last longer and do not bring such troubles as carburetors. Of course, in direct injection there are also many subtleties and features that should be remembered. But changing the carburetor to an injector is too labor-intensive and expensive. It is better to properly maintain your equipment and achieve it normal operation. Even with very good service, after 1-2 years you will again have to go to the service. Have you ever experienced a sharp drop in engine speed when warming up?

It is necessary that all systems of the power unit work properly. In this case, the engine should work normally both under load and at idle.

In practice, quite often, drivers are faced with a problem when, after releasing the gas, the engine speed does not fall or falls with a long delay. It is quite obvious that high idle indicates a problem and causes increased fuel consumption.

In this article, we will talk about why the engine speed does not fall, and also consider the main reasons why similar problems on and auto.

Read in this article

When you release the gas, the speed is increased or “freezes”: common malfunctions

Let's start with the fact that on many cars with an injector, the speed rises during warm-up. This is necessary in order to power unit worked steadily after a cold start.

However, after the temperature rises, the control unit lowers the idle speed, bringing them back to normal. On many cars with a carburetor, the driver independently increases the speed during warm-up, using the so-called "choke".

At the same time, after the engine is warmed up, normal idling is, on average, 650-950 rpm. If you press the gas and release the accelerator, the speed should increase, and then decrease again to the specified values.

Also, a situation often arises when the speed is slowly dropped or constantly kept at around 1.5 thousand rpm, 2 thousand revolutions, etc. Naturally, in such cases, the consumption increases and the internal combustion engine wears out more, which indicates the need for diagnostics.

So, let's start with common carburetor problems. Often the engine speed is not reset due to problems with the throttle. For example, when the driver presses on the gas, the flap must be opened wider so that the cylinders get more air for fuel combustion. After the gas pedal is released, the damper closes, the speed decreases.

If the damper does not close completely, an over-enriched mixture enters the cylinders, the speed is increased. The cause may be severe contamination of the throttle assembly or damage to the damper itself (deformation). First you need to clean the damper, carburetor cleaning fluid is suitable as a cleaner.

We also note that the damper does not close tightly even when the drive cable is worn out. In this case, the cable must be replaced. On the carbureted machines the engine speed does not drop often even if the gasket between the carburetor is out of order. It may also be the culprit intake manifold which is damaged.

The main task is to find the right ratio of fuel and air. Often a high level of fuel in the carburetor's float chamber also leads to increased RPM. The test should start with the needle valve.

Now let's move on to the injector. Please note that on many injection cars. As for malfunctions, the injection system itself is more complicated, that is, there are reasons high speed more than a carburetor.

As a rule, problems with revolutions can be caused by malfunctions of both mechanical elements and electronic components. In the list of main malfunctions, experts highlight malfunctions of the coolant temperature sensor, which is installed in.

In simple words, if the specified sensor gives an incorrect signal, the ECU considers that the engine is cold and activates the warm-up mode. In this case, the control unit raises the speed so that the power unit runs stably and reaches operating temperature faster.

Also, problems with speed can begin due to malfunctions and malfunctions (idle speed controller). It also happens that the throttle cable sticks and wedges. Another spring that closes throttle valve may stretch or be damaged.

Special attention should be paid to gaskets, since air leakage can lead to the fact that mixture formation is disturbed. This means that manifold gaskets, injector seals, etc. need to be inspected separately.

Floating revolutions: reasons

Note that in some cases, the speed does not just slowly fall or stay at the same level, but “float”. In this case, the engine may run unstable. first they fall, then they rise sharply and everything repeats. common cause such a phenomenon is the supply of excess air, which leads to “jumps” in revolutions at the twentieth.

Such problems arise in the event of a failure of the air supply sensor (), which allows the computer to calculate how much air has entered and how much fuel to supply to prepare the required mixture.

If failures occur, the control unit cannot prepare the “correct” mixture for XX mode, which causes speed jumps after releasing the gas pedal or when the engine is idling.

Summing up

As you can see, in order to determine exactly why the engine speed is not reset, in-depth diagnostics may be necessary in many cases. For carburetor engines often it is necessary to clean and adjust the carburetor itself, while the injector will require.

If the problem does not lie on the surface (the damper cable is soured, after washing or dry cleaning the carpet in the cabin is incorrectly placed, which presses the gas pedal, etc.), then it is better to deliver the car to the service.

The most difficult situation is when it involves the presence of a large number of sensors and actuators. In this case, even using diagnostic equipment does not always allow you to quickly and accurately determine the problem.

If diagnosis is difficult, it is best to deliver the car to a service that specializes in repair specific brand cars. As a rule, these are official dealer service stations, less often third-party organizations can be found.

Finally, we note that the timely detection of a problem allows you to save other components and assemblies. In other words, high idle, RPM float and jumps indicate that there is an air/fuel supply or carburetion problem. Ignoring such problems negatively affects the engine and its service life.