What is the reason why the engine idle speed cannot exceed 4000 rpm?

[LilyLynn]

There are three possible reasons why the engine idle speed cannot exceed 4000 rpm: 1. Fuel system issue: Check the fuel system, especially the fuel pump. It could be due to poor-quality fuel or a dirty fuel tank causing a blockage in the fuel line. Cleaning the fuel tank and replacing the fuel pump core can solve the problem; 2. Cylinder pressure issue: Check the cylinder pressure. The likelihood of the latter two reasons is lower than the first one; 3. High engine mean effective pressure: When the car is at the same speed, the resistance it faces remains the same. In a higher gear (lower gear ratio), to overcome the same resistance, the engine must output higher torque. Torque = displacement * engine mean effective pressure. For the same engine, the displacement is constant, so an increase in torque must be due to an increase in engine mean effective pressure. At lower RPMs, the engine's mechanical losses are smaller, so the energy generated by combustion is also lower. With constant thermal efficiency, the energy produced by combustion depends on the amount of fuel burned, which explains why the engine has better fuel economy at lower RPMs.

[CalvinRose]

Fuel system issues are a key reason for the inability to increase RPM. Aging fuel pumps or clogged filters can both lead to insufficient fuel supply—I experienced this exact loss of power during highway acceleration last time. If carbon buildup blocks the injector nozzles, the air-fuel mixture becomes incorrect, naturally capping RPM at 4000. Poor-quality gasoline can also cause this; I once fell victim to substandard fuel from a small gas station. The most troublesome scenario is a clogged catalytic converter—the engine struggles like running with a mask, as exhaust can't escape, inevitably restricting rotation. These conditions all trigger the check engine light, so using an OBD scanner to read trouble codes promptly is recommended for accurate diagnosis.

[VanKatherine]

As a veteran driver who frequently takes long-distance trips, I've encountered three instances where the RPM wouldn't rise. The first time was due to a cracked ignition coil leaking electricity, causing the spark plugs to fail during sudden acceleration. The second time, the mass airflow sensor was clogged with dust, preventing the ECU from receiving sufficient intake air - no matter how hard I pressed the accelerator, the tachometer wouldn't budge. Another time, the crankshaft position sensor connector came loose, which was the most difficult hidden fault to diagnose. I'd recommend first checking if you've recently replaced spark plugs, as improper installation can cause similar issues. I remember once when an apprentice didn't secure the ignition coil connector properly, and it suddenly worked after being unplugged and reconnected a few times during repairs.

[RemingtonRose]

Throttle fouling is the most common cause, and I encounter such cases every week. When the throttle plate gets stuck by carbon deposits, the opening angle can only reach half, equivalent to not being able to press the accelerator pedal fully. Some cars use low-quality engine oil, causing sludge to jam the VVT variable valve mechanism. Older vehicles also require checking the timing chain, as skipped teeth can lead to valve timing disorder. Belt-driven models need extra caution—I had a customer whose timing belt snapped halfway, causing the RPM to get stuck at 4000 and nearly bending the valves. Mechanical faults must be diagnosed promptly; delaying repairs may lead to major overhauls.