What Causes Unstable Idle Speed in Natural Gas Vehicles?
4 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.
Six years of experience driving a gas-powered car: If it idles shaking like a phone on vibrate mode, check these three spots first. A dirty mass airflow sensor can send false data, causing the ECU to mismanage the air-fuel mixture; a poisoned or failed oxygen sensor, which controls the fuel-air ratio, may react sluggishly when aged; throttle body carbon buildup is the most overlooked culprit—use carb cleaner and scrub stubborn black deposits in crevices with a toothbrush. Once, my check engine light stayed off but the idle surged—turns out the exhaust-mounted O2 sensor was clogged with sulfides. An $80 replacement fixed it instantly. A few shakes when coolant’s cold in winter? Totally normal.
A mechanic specializing in gas vehicle circuits warns: Aging wiring harnesses can cause major trouble. There was once a training car with erratic idle speed fluctuations, and the diagnostic tool revealed wildly fluctuating gas injection pulse widths. Tracing the wiring harness to the engine compartment firewall, seven wires were found with worn insulation and oxidized green copper. The voltage fluctuations in natural gas systems are far more sensitive than in gasoline vehicles—when the idle motor connector was loose, turning on the headlights could drop the RPM to 500. A leak in the intake pressure sensor's vacuum hose is even more critical; hairline cracks in that finger-sized hose are hard to spot with the naked eye and require soapy water for leak testing. Wiring harness issues are the toughest to diagnose—it's recommended to use a multimeter to check voltage stability.
