What could make your 91 Honda Prelude idle irregular until it gets warm?

Answer 1

ECT

Check engine codes. It sounds like you ECT sensor is out of range. Engine coolant temp

New Answer

I am guessing it is fuel injected.When your car is warming up, it adjusts the fuel/oxygen ratio, as needed. These cars should not be left idling to warm up, you should actually get in and drive it.

NEW ANSWER

The word "irregular" has a pretty broad meaning. I assume you're saying the RPM's vary alot instead of being steady. Small variations of a few hundred RPM's probably have to do with "regular" maintenance items such as spark plugs, wires, air filter, vacuum line leak, or fuel filter. Larger RPM movements are indicative of dirty or defective sensors, coil defects, or electrical shorting. The engine is most sensitive to part problems or neglected maintenance when the engine is cold. Recommend: Clean and regap the plugs. Measure the resistance (Ohms) of each of the plug wires. Each wire has a different "healthy" range depending on how long the wire is. The correct values are usually available in the Haynes Manual in the first two pages of Chapter 1 or Chapter 2. Replace the PCV valve (easy and inexpensive). Check the air filter and replace if in use over 18 months or 15,000 miles. Go to Auto Zone and use their vehicle code-readers (for free) to read your engine's computer codes. I wouldn't be surprised if a code appeared regarding the oxygen sensors. If the Ox codes do show up, then the reason for the initial rough idle may be due to the "wrong signal" being sent to the onboard computer (OBD II system). The computer is trying to compensate for what it thinks are signals telling it that the engine is running too lean...and then to rich. This causes the engine speed to oscillate back-and-forth until the Ox sensors heat up and start reading correctly. The Ox sensors are heated electrically before the engine exhaust reaches its usual high temperatures in order to help the Ox sensor to "smell" the exhaust for too much or too little fuel. Write in another question if you want to give a more exact description of the problem. Also, one more thing, warming up a car too much can be bad, but warming up the catalytic converter and bringing your engine temperature up off of the bottom of the temperature gauge scale is OK. The computer and sensors are designed to adjust various items to help the engine run smoothly. A VERY cold engine should not be driven away immediately unless you're being chased by a band of gypsies -- then it's OK to haul off while the engine is cold. The forces placed on the engine are more damaging when the oil is cold and thick, leaving parts unprotected and available for premature wear. Modern engines with proper coolant are designed to operate at an elevated level, not cold. Otherwise, they'd have to put four more radiators in the engine and air condition the engine to keep the temperature down. It's preferable to warm the engine for 30 seconds to 90 seconds to allow the oil to warm up and help the hydraulic lifters operate at optimum. The engine will last longer and save you the fuel wasted when you drive a cold engine. Z-man

REAL AWNSER

Actually I own a 90 prelude and I am a certified mechanic. I would disregard the above awnser (no offense). First off your car is not obd II. If your cars idle is surging up and down from around 1000-2000 rpm or so you should start off by checking your coolant. I know this sounds weird but there is a sensor in your throttle body that needs to be warmed up by the coolant as it heats up. When your cooling system is empty this idle problem always happens. Should you find that your coolant is full, the idle air control valve and boost valve are suspect.