Is it normal for your 1989 prelude Si to have a hi and low idle at start-up?

Answer 1

If the engine has cooled down, then yes, the high idle occurs first and then as the engine becomes more efficient at burning fuel, the engine slows down. The onboard computer gets information from the two oxygen sensors located in the exhaust. The sensors report to the computer how well the engine burns the fuel and how well the catalytic converter "cleans" what is burned. When the engine is started, the engine is burning less efficiently at lower RPM's. The computer can tell this from the data it receives from the oxygen sensors (and to a lessor extent, the other sensors as well), so the computer is programmed to add more fuel because while cold, the engine runs more efficiently at higher RPM's until the engine is warmed up. This fast idle is about 1750 RPM's and will drop to about 750 RPM's when the engine is burning fuel efficiently (and cleanly). There is some debate about whether a vehicle should be warmed up before driving. Some say the vehicle's computer compensates for the inefficiency of a cold engine, and therefore, can be driven immediately. Further, this idea of driving immediately is thought to be good for the engine by heating it up more gradually and therefore avoiding the problems of damage due to sudden temperature changes. However, there is a line of thinking that the engine was designed to operate at a "warmed" temperature range with consistency, performance, and long life. Indeed, the smog equipment operates better at higher temperatures, and that makes the engine run more efficiently, using less fuel, and costing less money. The amount of waste of a cold engine is 80% higher than with an engine that is warmed up. Furthermore, an engine that is running at high RPM causes the transmission stress when engaged, not to mention the brakes. And the transmission is VERY EXPENSIVE to repair. Since the engine was designed to run at warmer temperatures, I would recommend warming up the engine for about 2 - 3 minutes before engaging the transmission. I do not recommend letting the engine get "hot" though, as this can cause problems when done repeatedly. If the engine was warmed up and shut down for a short time, upon restart, the RPM's probably will not rise more than the normal 750 or 800 RPM's unless subjected to subzero temperatures. Most likely, the normal temperature for your engine, when warmed up, is just before the half-way point (about midway). Bottom-line recommendation, on a cold day (