The situation when a properly working cold car suddenly turns into a “vegetable” after warming up is a classic problem for many owners of both old and modern cars. Loss of traction When you're in a hot engine, you often have unstable idling speeds, jerks when you accelerate, or even stop the engine completely if you let the gas pedal go, and it's not just discomfort, it's a signal that the combustion or control system has undergone critical changes under the influence of temperature.
The physics of the process is simple: when heated, the linear dimensions of parts, the viscosity of technical fluids and the electrical resistance of conductors change. Internal combustion engine It is a complex system where the gaps between the piston and the cylinder, the valves and the saddles are designed to operate at operating temperatures. If these gaps are broken due to wear or defects, the motor stops holding the engine. compressionBut most often, the fault is electronics, which get the wrong data from the sensors and impoverish the fuel-air mixture.
In this article, we will take a closer look at the main nodes responsible for power loss and help you localize the malfunction without visiting an expensive service. Understanding the nature of the problem will save time and money, since the symptoms may be similar, but the problem is not very easy to understand. treatment - totally different.
The effect of temperature expansion on the engine
The thermal expansion of metals is a fundamental physical law that engineers take into account when designing, but which becomes the enemy when knots wear out. When the motor reaches an operating temperature (usually 85-95 ° C), aluminum pistons increase in volume. If the gaps between the piston rings and the cylinder walls exceed the permissible limits due to the output, crankcase gases break out. compression.
On a cold engine, the gaps are still covered by oil film and smaller pistons, so the compression is normal. But once the temperature rises, the engine "takes apart": the pressure in the cylinders drops, and the fuel-air mixture burns inefficiently. A large mileage, where the production of cylinder walls is already significant.
⚠️ Warning: If the engine stalls hot immediately after stopping and starts poorly until it cools, this may indicate a steam plug in the fuel system or a critical thermal expansion of the timing valves.
In addition, thermal expansion affects gaps in the valve mechanism. In engines that require manual adjustment of thermal gaps, their absence or excessive importance leads to the fact that the hot valve simply stops closing tightly or, conversely, does not open completely. power-specifications and the cylinder capacity.
Heat gaps in numbers
Valve gaps are usually one-hundredths of a millimeter (e.g., 0.20–0.25 mm for intake) and a 100 degree change in temperature can change the size of the valve rod by several microns, which becomes critical when accumulated wear.
Fuel system problems and steam traffic jam
One of the most common causes of power loss is the formation of a steam stopper in the fuel line. Gasoline or diesel fuel, passing through the ramp and injectors, is heated by the running engine. If the temperature rises above the boiling point of the fuel at the current pressure, it begins to evaporate. The resulting gas bubbles occupy the volume, displacing the liquid fuel, and The fuel pump cannot supply the required amount of fuel to the cylinders.
The mixture becomes too poor, causing detonation, thrust dips and overheating, especially in hot weather or when using fuel with a low octane number, which is more prone to evaporation. injector Fuel supply is faced with this less often than carburetor car owners, but the problem is relevant for everyone.
- 🔥 The high temperature in the under-hood space contributes to the rapid heating of the fuel ramp.
- ⛽ Low quality fuel with volatile fractions boils faster.
- 📉 A faulty fuel pressure regulator can make things worse without returning the excess to the tank.
- 🌡️ Absence or damage of the thermal insulation screen between the engine and the fuel line.
To diagnose this phenomenon, it is often enough to let the engine cool down, and if after cooling the car starts and goes normally, and after 20 minutes of operation the symptoms recur, look for the source of local overheating of the fuel. heat-insulating or replacing a fuel filter that can also be jammed with paraffins (for diesel).
Failure of the ignition system on the hot
The electrical resistance of the parts of the ignition system also depends on the temperature. The ignition coil, the ignition module or the switch can have internal microcracks or insulation defects. In cold, these defects do not appear, but when heated, the material expands, the cracks increase, and breakdown Or interturn.
The result is a slight spark or no spark at all on certain cylinders, the engine starts to "trottle", its power drops, its fuel consumption increases, and the exhaust pipe smells of unburned gasoline, especially old coils and high-voltage wires, the insulation of which has lost elasticity.
You can diagnose the problem visually at night: with the engine running, you can sometimes see the "dancing" of sparks under the hood. A more reliable method is to replace suspicious elements with knowingly functional ones. spark-plugIf the gap on the electrodes is too large, it will be more difficult to break the spark on the hot one due to the decrease in the density of the mixture and the growth of resistance.
| Element of the system | Symptoms of hot. | Method of verification |
|---|---|---|
| Ignition coil | Trotting, ignition failures | Resistance measurement, replacement |
| Hall's sensor. | Unstable idling | Oscilloscope, multimeter |
| Switch | Loss of spark during heating | Heated with a hair dryer, replaced |
| High-voltage wires | Electric shocks, glow | Visual inspection in the dark |
Sensors and electronic control (ECU)
The modern car is driven by an electronic control unit (ECU), which builds a fuel card based on the readings of the sensors. Crackshaft position sensor (CRPS)This element is often made using magnetic materials, whose properties change when heated, and if the sensor floats at temperature, the ECU receives the wrong signal about the position of the pistons and delivers a spark or fuel in the wrong time.
The second important element is Mass air flow sensor (MADR)When heated, its thermocell can give an understated reading, which makes the ECU think less air is coming in and reduces the fuel supply. The engine goes into emergency mode, losing up to 50% of power. Also worth mentioning is the knock sensor: if it malfunctions and "hears" a non-existent detonation on the hot engine, the ECU will aggressively shift the angle of ignition ahead, killing the speaker.
☑️ Sensor checks
And the car's brains, too, can dry out or swell over time, losing capacity as the temperature rises, causing the controller to fail, and in such cases, only soldering of the capacitors or replacing the control unit helps.
⚠️ Warning: When diagnosing sensors, be sure to check the state of the engine mass. Poor contact of the sub-conductor wire when heated increases resistance, distorting the readings of all sensors.
Problems with the exhaust system and catalyst
A clogged catalytic converter is a scourge of modern, eco-friendly cars. Over time, the catalyst cells break down or melt, creating a powerful resistance to exhaust gases. A cold engine can still blow through this plug, but as the revs and temperature increase, the pressure in the exhaust manifold increases so much that the cylinders cannot effectively cleanse the combustion products.
It's a strangulation effect, where fresh mixture can't get into the cylinder because it's full of exhaust gases, and the power drops catastrophically, the car accelerates with difficulty, and the top speed is 60 to 80 km/h. The diagnosis is simple: you need to twist the upper lambda probe or disconnect the receiving pipe. catalyst Replacement or removal.
Similar symptoms can be caused by a jammed valve in the exhaust tract (if it is provided by the design) or a heavily overgrown soot EGR system. The exhaust gas recirculation valve can be wedged in the open position, releasing hot gases back into the intake. This reduces the density of incoming air and causes overheating.
To check the catalyst without disassembly, connect the pressure gauge to the place of installation of the lambda umbrella. If at revolutions of 2500 rpm the pressure exceeds 0.3 kgf / cm2, the catalyst is clogged.
Mechanical malfunctions and compression
If the electronics and the fuel are fine, the iron is left. The burnout of the valve is one of the most unpleasant reasons. The thin edge of the exhaust valve can burn out due to poor mixture or detonation. The cold valve can still grind and hold the compression, but when heated, the metal deforms, and the gases begin to break into the collector. This is accompanied by slamming into the silencer and loss of power.
And we also have oil caps and piston rings, which when you heat it up, the oil liquefies, and if the rings are stuck or the oil caps are hollowed out, the oil starts to enter the combustion chamber, and it burns up, and it creates a soda that can increase the temperature even further and cause the ignition of the heat, when the engine continues to work after the ignition is turned off.
To accurately diagnose the mechanics, you need to measure the compression on the hot engine and, preferably, conduct an endoscopy of the cylinders, which will allow you to see the condition of the piston group and valves without disassembling the engine.
Loss of power on hot is most often caused by a complex of reasons: thermal expansion of parts aggravates the existing malfunctions of the electronics and fuel system.
Why does the car stop hot after stopping?
This is most often due to overheating of the gas pump or the formation of a steam plug in the fuel ramp, and also the possibility of overheating of the ignition module or DPCV, which lose their performance at high temperatures.
Can antifreeze get into cylinders when heated?
Yes, if the gasket is broken or there is a micro-crack in the head of the block, and when heated, the metal expands, the crack opens, and the antifreeze enters the cylinder, causing tripling and loss of power. White smoke from the exhaust is a clear sign.
How does bad gasoline affect the operation of a hot engine?
A low octane number causes detonation, and the ECU, as it detects the detonation, leads the ignition to the later corners, which dramatically reduces power and raises the exhaust temperature, exacerbating overheating.
What to do if the power loss occurs on the road?
Stop, let the engine cool down. Check the fluids, the pipes, if you can, blow out the system or replace the fuel filter. Don't try to overheat, it will lead to major repairs.
Could there be a reason for the air filter?
Yes, a heavily polluted air filter creates a vacuum in the intake manifold. On a hot one, when the air density is already lower, the lack of oxygen becomes critical to the mixture, and the engine "chokes."