Why would the engine for a 94 Accord idle like the car is being reved up and down?

Answer 1

It happened once in my 1994 Honda Accord when the radiator fluid was frozen. The car was revving and jerking. I did not pay attention to it until the radiator started overheating. My 1993 behaves the same, until the engine gets really hot. I don't know why either. I hate giving vague answers, but in this case I must. This is called Idle Hunting. It is caused by an air leak somewhere between the throttle butterfly and the cylinder head. Honda's PGM-FI fuel injection system (carbureted cars don't idle hunt) relies on three sensors to decide how much fuel to inject--the Oxygen Sensor, which everyone calls the O2 Sensor; the Throttle Position Sensor, or TPS; and the Mass Airflow Sensor, or MAF. TPS tracks how close your foot is to the floor, MAF monitors the air coming in, and the O2 sensor decides how much of the air was used in combustion. The more air coming in, the more gas it gives you and the faster your car goes. If everything is working properly, at idle just a little air comes in, so the system feeds in just a little gas and your engine runs at a slow speed. If there's an air leak and more air comes in than is supposed to, the system gives you more gas. At about 2500rpm the computer thinks "wait a second! The throttle's closed. What am I doing giving this much gas?" and cuts off the fuel flow--which causes the RPM to drop. As soon as the RPM gets back to where it's supposed to be, the cycle begins again. It doesn't hunt when you're moving because it's supposed to run faster than idle then. At speeds above idle, the computer will look at the TPS, MAF and O2 sensor signals, note that the MAF and O2 sensors are saying one thing about the engine speed and the TPS is saying another, decide the TPS is lying and ignore it. It's got to: if TPS says "run 3000rpm" and MAF says "run 3300," supplying enough fuel to run 3000rpm will cause the engine to run very lean, which will destroy the engine if you do it long enough. Your foot doesn't care--it's not calibrated. You push down on the gas, the car moves, you're happy. Right about now you're asking yourself, "what if TPS says "run 2200" and MAF says "run 12,000"? Your car has two rev limiters in it--one on the fuel injection system, the other on the ignition. You can't redline the engine in these cars unless a major catastrophe, affecting MANY systems, has happened. But if, for some unknown reason, your car suddenly speeds up to 88mph in a school zone--and your flux capacitor, which you will NEED, very badly, if a cop happens to see you doing 88 in a school zone, quits working--get to the side of the road, right now, shut down and figure out what went wrong before you kill someone. Here's the vague part: Somewhere in that mass of air hoses, vacuum hoses et al is an air leak. Your job: Find it and fix it.