In this article, we'll discuss engine idle issues, surging and "hunting" problems. First let's cover the "basics" in terms of the possible causes of engine surging/hunting, where engine speed suddenly (or slowly) changes.
A wide variety of issues can prompt an engine surge. The problem may involve an annoying change (up/down) of engine speed while cruising at a steady pace, a low drop (or even cut-out) at idle or when approaching a stop, a wild high engine speed of several thousand rpm followed by a drop to near zero, etc.
• Transmission not locking up (or slipping)
• Vacuum leak
• EGR valve stuck open
• Dirty/sticking IAC (idle air control valve) (P0505)
• Engine temperature sensor
• Knock sensor
• Clogged EGR ports
• Variable valve timing solenoid clogged/sticking
• Air in cooling system
• MAF connector loose/intermittent connection/faulty MAF
• Fuel system pressure issue
• Clogged fuel filter
• Plugged/restricted exhaust
• Crankshaft position sensor (P0336)
• Power steering pressure sensor circuit
• A/C compressor cycling on/off due to low refrigerant
• Faulty TPS
• Clogged/restricted catalytic converter
• Ignition system
• High pressure oil system (diesel)
These are areas to suspect first if initial diagnosis is unclear:
• Fuel pump control circuit
• Spark plug(s)
• Ignition system
• Transmission lock-up issue
• ECM power circuit
• A/C signal circuit
• ECM power circuit
• A/C signal circuit
• Fuel injector(s)
ENGINE SPEED “HUNTING”
• Fuel pump control circuit
• Spark plug(s)
• Ignition system
Crankshaft position sensor (CKP)
A faulty CKP issue should be accompanied by a P0336 code. The CKP is (usually) a two-wire sensor (signal and ground). The sensor features a permanent magnet (or three-wire hall-effect sensor, featuring ground, voltage and signal) mounted in the engine block that aligns with a toothed reluctor wheel that is attached to the crankshaft.
As the crank rotates, the reluctor wheel passing by the magnet generates an AC signal that the ECU uses to identify engine speed. Depending on the engine design and model, the number of reluctor wheel teeth may vary. Keep in mind that even within the same engine family (the GM LS engines are an example), the tooth count will vary, and the tooth-count must be matched to the programming in the ECU.
In combination with the camshaft position sensor (CMP), the CKP signals are used by the ECU to manage fuel injection and spark delivery.
A faulty CKP (or CKP circuit) can easily cause an intermittent misfire, which naturally results in what the driver may perceive as a surging effect. A bad CKP can also cause a no-start and intermittent stalling issues.
The root cause may involve a faulty CKP, or an issue with the reluctor wheel (damaged or missing teeth or metallic contaminants on the teeth), a dislodged reluctor wheel or, obviously, a short or open in the wiring harness. Most reluctor wheels are press-fit to the crankshaft. Although rare, if the wheel loosens and moves out of position (out of phase or crooked), and if this problem is suspected or confirmed, this must be addressed immediately, since a loose reluctor wheel can not only cause misfiring issues, but if severe enough, can also create mechanical damage (block interference or possible piston skirt damage).
If, on the rare occasion that a reluctor wheel must be replaced or repositioned, do not attempt this without the specialty tool required to properly register the wheel in-time with the crankshaft. This service is best performed by an engine builder who is familiar with the specific engine type at hand. If the problem is mechanically severe enough, the easy way out would be to replace the crankshaft with a new crank that already features an installed reluctor wheel.
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.
In order to isolate the issue, clear any codes, then disconnect the IAC and start the engine. If the P0505 code does not re-appear, the IAC is faulty. If the code does re-set after disconnecting the IAC and running the engine, chances are there’s a wiring problem, so check for shorts along the harness all the way to the ECU. Also check for continuity on the IAC wires.
As one example of tracing an idle control system malfunction, following is Toyota’s description of a P0505 code on a 2008 Yaris equipped with the 1NZ-FE engine:
The idling speed is controlled by the ETCS (Electronic Throttle Control System). The ETCS is comprised of the throttle body, throttle actuator (which opens the throttle valve), the TPS (Throttle Position Sensor, which detects the opening angle of the throttle valve), the APP (Accelerator Pedal Position sensor), which detects the accelerator pedal position and the ECM, which controls the ETCS.
Based on the target idling speed, the ECM controls the throttle actuator to provide the proper throttle valve opening angle.
The ECM monitors the idling speed and idling air flow volume to conduct Idle Speed Control (ISC). The ECM determines that the ISC system is malfunctioning if the following conditions apply:
• The learned idling air flow volume remains at the maximum or minimum volume five times or more during a drive cycle.
• After driving at 6.25 mph or more, the actual engine idling speed varies from the target idling speed by between 100 and 200 rpm, five times or more during a drive cycle.
• If the actual idling speed varies from the target idling speed by more than 200 rpm five times or more during a drive cycle, the ECM illuminates the MIL and sets the DTC.
NOTE: The following conditions may also set DTC P0505:
• The floor carpet may overlap slightly onto the accelerator pedal, causing the pedal to be slightly depressed (causing the throttle valve position to be slightly open).
• The accelerator pedal may not be fully released.
A malfunctioning MAF sensor can cause a wild up/down engine idle speed (say, 0-2,000 rpm). This might be caused by a loose harness connection or by a damaged or contaminated sensing wire.
The MAF sensor measures the amount of air flowing through the throttle valve. The ECM uses this information to determine fuel injection time and to provide the necessary air-fuel ratio. Inside the MAF meter is a heated platinum wire which is exposed to the flow of intake air. By applying a specific current to the wire, the ECM maintains a specific reference temperature at the wire. The incoming air cools the wire (and an internal thermistor), affecting their resistance. To maintain a constant current value, the ECM varies the voltage applied to the MAF. This voltage level is proportional to the airflow through the sensor. As a result, the ECM uses this to calculate the intake air volume.
The circuit is constructed so that the platinum hot wire and the temperature sensor provide a bridge circuit. The power transistor is controlled so that the potentials of A and B remain equal to maintain the predetermined wire temperature.
If there is a defect in the sensor (or an open or short in the MAF circuit), the voltage level deviates from the normal operating range. The ECM interprets this as a malfunction in the MAF meter and sets the DTC.
P0101: This indicates high voltage (engine speed less than 2,000 rpm, coolant temperature 158 degrees F or higher and voltage output of the MAF is more than 2.2V); or low voltage (engine speed is more than 3,000 rpm and MAF voltage output is less than 0.93 V).
P0102: MAF circuit low input (less than 0.2 V). Detection is based on an open in the MAF circuit for three seconds or more. The MAF may be faulty or simply disconnected or the MAF may be dirty/contaminated (NOTE: If the engine is equipped with an aftermarket “oiled” air filter, it is possible that an over-oiled filter may be causing this).
P0103: MAF circuit high input (more than 4.9 V). This usually indicates a short in the meter circuit. The MAF may be disconnected or damaged.
P0104: The MAF circuit is incomplete (poor connection, wiring broken or frayed or poor connector connection. This might also indicate an intake air leak.
Diesel engines (we’ll cite Ford’s 7.3L, 6.0L, etc., as examples) typically feature a dedicated high-pressure oil system that operates the fuel injectors. The high pressure side typically runs at about 500 psi at idle, 1,200 psi at about 3,300 rpm and about 3,600 psi under full-load acceleration.
This system involves a high pressure oil pump and an IPR (Injection Pressure Regulator). Sticking (or wear) problems with the high pressure control regulator can cause engine surging (most commonly noticeable at lower rpm and at idle), as well as intermittent engine shut-off during low speed braking and/or when approaching a final stop.
If the engine cuts out during a stop, with the transmission placed in neutral or park, the engine fires up again, but dies again when approaching a final stop. Other symptoms can include intermittent difficult starting, a slight stumble when the accelerator pedal is nailed while the engine is turning around 1,000-1,500 rpm and/or annoyingly extended cold-cranking in freezing temperature. Granted, various injector issues could cause some of these problems, but if a customer’s truck enters the shop with the surging/intermittent shut-off issues, definitely inspect the high pressure oil system.
I’ve owned several Ford F350s with the 7.3L Navistar turbo diesel, and have experienced this identical set of issues (most notable on my 2003 model). When these drivability problems began to appear, the “off-the-cuff” reaction was to replace the fuel filter, suspecting that it was moisture-contaminated.
While it’s imperative to regularly replace diesel fuel filters anyway (especially in cold climates), if the first few filter changes don’t solve the glitch, suspect the high pressure oil control valve or regulator. The high pressure oil system runs at very high pressure, and any interruption in pressure flow will cause the ECM to attempt fuel enrichment changes.
NOTE: While it’s certainly easier to replace a sticking control valve, the high pressure control valve is usually rebuildable (basically just disassemble, clean and reassemble). Also be sure to check the high pressure oil rail and its connections for external leakage (which will not only make an oil mess at the rear of the intake manifold, but will cause pressure drops).
It’s also important to remind customers that only specified engine oil should be used in diesel applications, in part because of the special anti-foaming additives in the oils, critical for maintaining an adequate and constant pressure to the injectors to prevent aeration and sub-par injector spray patterns (these anti-foaming agents can break down in the 3,000- to 5,000-mile range). Citing the Ford examples, several oils are appropriate and should carry an API rating of CF-4/SH or CG-4/SH or higher. One example is Shell Rotella-T 15W40. ●