Honda 1985 to 1996 with Port Fuel Injection-Surging Idle

In order to maintain a stable idle on a cold engine, Honda uses a device called a Fast Idle Valve. This valve allows additional air to bypass the throttle plate when the throttle is closed. The Fast Idle Valve contains a thermo-wax element. This element expands when it is heated and pushes a plunger closed against its seat, shutting off any additional air to bypass the throttle plate when the engine is fully warmed up. When the plunger is not seated, extra air is metered past the closed throttle plate to allow a fast idle (refer to diagram #1). A common failure of this component is its inability to close completely. The symptom that occurs when this happens is a surging idle as opposed to a high steady idle when the engine is at operating temperature and the throttle is closed. Honda uses a deceleration fuel cut function. This function de-energizes or cuts off the fuel injectors when engine RPM is high and the throttle is closed. RPM is determined through distributor input, and throttle position is determined by the throttle position sensor. In this type of situation, such as coming off a highway exit ramp, fuel is not required, so the ECM cuts off the injectors. What creates the surging idle is what we will explain next. If a vacuum leak occurs on a MAP sensor equipped model like these types of Hondas, engine RPM will be higher than normal.

Since the ECM does not have the ability to detect vacuum leak, it is programmed to think that since the engine RPM is higher than normal, and the throttle is closed, the vehicle must be decelerating. At this point the ECM will shut off the fuel injectors until the engine RPM drops below the expected value. Once the injectors are energized, engine RPM will go up and the process will repeat itself until the problem is corrected.

So, if you run across a Honda exhibiting this symptom, here are few checks to isolate the problem. Coolant level-check coolant level on a cold engine when the system is not pressurized. A low coolant level will not heat the thermo-wax element properly, preventing it from fully closing the Fast Idle Valve. If the coolant level is OK, next, allow the engine to reach operating temperature. At this point, disconnect the throttle position sensor. When you do this, the engine RPM should not only be higher than normal but it should now be steady.

When the TPS is disconnected, the decel fuel cut function is no longer active. To check the fast idle valve (refer to diagram #2) remove the two screws at the top of the valve. The plunger which actually restricts or blocks air flow will now be visible. Check to see if the plunger is fully closed. When you push down on the plunger, engine RPM will increase because you are allowing more air into the engine.

If you can feel or hear air being drawn past the valve, you can be sure that the valve is not working properly. If the plunger is fully closed, then it is necessary to locate the source of the extra air that is getting into the engine, such as vacuum leaks, faulty PCV valve, or throttle plates binding and not fully closing.