What Causes Unstable Idle Speed in Natural Gas Vehicles?
2 Answers
Unstable idle speed in natural gas vehicles may be due to uneven combustion of natural gas. Specific causes: Clogged idle air jets, malfunctioning idle control devices, weak spark in individual spark plugs, poor valve sealing, intake manifold leaks, incorrect ignition timing (too early or too late), or improper idle speed adjustment. Solutions: First, adjust the idle speed. If the issue persists after adjustment, check if the idle jets and idle air jets are clogged. If clogged, clean them with gasoline or acetone and blow through with compressed air. If not clogged, stabilize the engine at a certain RPM and listen for air leaks at the intake manifold or carburetor gaskets. If leaks are found, tighten screws or adjust gasket thickness to resolve.
As an experienced mechanic who has seen many natural gas vehicles, the most common cause of unstable idle in gas-powered cars is icing or clogging in the pressure regulator. Last week, a taxi had this exact issue—the tachometer was jumping erratically like a dance during idle. Upon inspection, we found the gas filter hadn't been replaced in two years, and impurities had blocked half of the regulator's inlet. Since natural gas absorbs heat during pressure reduction, icing in the pipelines can also lead to unstable gas supply. Additionally, carbon buildup in the fuel rail nozzles, resembling coffee stains, can clog the injectors, causing uneven gas injection each time. In such cases, the entire fuel rail needs to be disassembled and cleaned, followed by recalibration of the gas ECU data. While at it, it's good practice to replace the spark plugs—natural gas vehicles require a 0.2mm smaller spark gap than gasoline cars for sufficient ignition power.
