When an engine is cold, many malfunctions can cause low RPMs, even after the engine has warmed up. These issues can occur regardless of the type of fuel or air-fuel mixture formation.
The most common problems include
- leakage of air from the intake manifold due to mechanical damage or loose connections,
- fuel delivery issues caused by a faulty fuel pump,
- worn or clogged injectors, or a clogged fuel filter,
- a drop in compression in the combustion chamber due to worn parts of the CPG,
- damage or incomplete closing of the valves,
- failure in the ignition system due to incorrect adjustment, failure of spark plugs, coils, high-voltage wires, and sensors.
To determine the cause of low RPMs when the engine is cold, it is necessary to eliminate possible causes one by one.
When starting a cold engine, the crankshaft rotation speed is usually 2-3 times higher than when idling after warming up. This ensures stable operation of the engine and reduces the time it takes to reach operating temperature.
Low RPMs when the engine is warm are usually a sign of an air-fuel mixture problem.
What is the usual idle RPM when starting an engine?
The crankshaft speed after starting a cold petrol engine is typically set at 1200-1500 RPM using a mechanical or electronic idle speed controller, or independently.
For diesel engines, the warm-up RPM is typically around 1100. With automatic control, the speed will gradually or abruptly decrease as the engine warms up until it reaches 600-800 RPM.
However, exceptions are possible and the exact number of engine revolutions during cold start and at idle depends on the design of the power system, the settings of the electronic control unit (ECU), and the temperature of the coolant.
In models that meet EURO-5 and EURO-6 emissions standards, the electronic control unit often does not raise the cold speed above 1100 RPM to limit emissions.
In some cars, the crankshaft speed immediately after starting the engine is 800-1000 RPM and then increases. This is to reduce the load on the engine at low temperatures when it is most vulnerable.
The number of revolutions of the engine during a cold start and the time for them to decrease to idle also change depending on the temperature.
In cold weather, it takes longer to warm up the coolant, so the crankshaft speed may not drop to the idle speed for more than 5 minutes. In summer, it stabilizes in 2-3 minutes.
Therefore, engine speeds less than 1200 RPM or their slight fluctuation immediately after starting are not always a sign of problems.
You should only be concerned when the crankshaft speed does not rise above 600 RPM when cold, drops below 400, or does not stabilize as it warms up, and the engine stumbles or stalls at startup.
Warm-up speed ensures stable operation of a cold engine at idle, efficient supply of more viscous oil, warming up the engine and coolant to operating temperatures.
In models with catalytic converters, quick heating of their surface is additionally provided.
Excessively low warm-up speeds and the malfunctions that caused them can lead to more severe malfunctions. And at lower temperatures, the engine may simply not start.
In modern cars with an ECU, the crankshaft speed during warm-up is set automatically depending on the temperature of the coolant.
In most cases, excessively low engine speeds when cold are a sign of a lean air-fuel mixture. This can be from an excess of air in it or interruptions in the supply of fuel,