Power steering pressure sensor
An example of this is seen in the early Honda Odyssey. The sensor wire (for the power steering pressure) tends to corrode or break loose. The ECU gets confused, and may drive the engine speed up and down (trying to compensate for engine load during power steering operation).
Air in coolant
In order for an engine coolant temperature sensor to provide correct values to the ECU, the sensor must be immersed in coolant at all times. If air pockets (circulating through the system) hit the sensor, the intermittent hot air/coolant exposure to the sensor can cause values to fluctuate, and the ECU receives alternating/intermittent temperature values. In turn, the ECU attempts to manage fuel and spark to adapt to the changing values. Make sure that the cooling system is full, and bleed air from the system as needed.
If the throttle shaft is worn, consider that the TPS (throttle position sensor) is positioned at the far end of the shaft. Any deviation in throttle shaft position (due to shaft wear) will create uneven signals to be generated by the TPS reacting to this deviation. If closed TPS voltage changes, the ECU may assume that your foot is on the throttle, causing a richer fuel mixture to be delivered. This can also be caused by a faulty TPS, or even a poor ground.
With the engine off, check TPS voltage with the accelerator pedal relaxed. Then work the pedal a few times and see if the voltage changes. If you continue to obtain different readings with the throttle released, suspect a sticking throttle shaft or faulty TPS.
As another example, Honda engines (I believe 1996 and later) feature a FITV (Fast Idle Thermo Valve) that’s located directly under the throttle body. This valve is prone to sticking. If the plunger is backed out too far, this can affect a vacuum leak, causing a high idle after the engine has warmed to operating temperature. This can also result in a pulsating/fluctuating engine speed. While most repair manuals recommend replacing this valve assembly, it can be removed and cleaned. It’s not uncommon for an IAC to be blamed for an FITV issue.
Idle air control valve (IAC)
Most OEMs refer to this as an IAC, while other names are used by some such as AIS (Automatic Idle Speed) and ISC (Idle Speed Control).
A problem with the IAC should throw a P0505 code. A failed/problematic IAC can cause engine stalling when off-throttle, and/or excessively high engine rpm, particularly at idle.
The IAC, signaled by the ECU, controls the throttle opening at idle. When the engine idle speed is either above or below the programmed range, the ECU prompts the ISC to either increase or decrease bypass airflow.
Naturally, as the IAC prompts the throttle plate to open, engine speed increases. The IAC features a plunger mechanism that may be stuck/sticking. Depending on the model, the IAC may be cleaned to eliminate open/closed sticking problems. It’s not uncommon to find the IAC solenoid plunger fully extended. This may be an indication that the ECU recognizes an air leak and is trying to lower idle speed by closing the idle air bypass circuit.
The IAC valve is prone to carbon buildup. Note that some IAC valves also feature a vacuum hose that connects the IAC valve to the intake manifold. If the hose is cracked or damaged, the engine will react as though the IAC valve is faulty. Also note that Toyota and Lexus vehicles may feature a non-motorized, magnetic IAC valve, requiring periodic cleaning of the IAC air inlet.
Sign up for a FREE subscription to Auto Service Professional magazineSubscribe