Tracing poor engine idle

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Tracing poor engine idle

Engine idle issues can obviously involve a wide range of potential faults, involving ignition, fuel, vacuum and mechanical problems. In this article, we’ll discuss a variety of potential sources that result in poor engine idle characteristics. (See the list at right for potential causes.).

We’ve included both general tips as well as a few case-study examples of how “mystery” idle problems were addressed in specific cases. Our thanks to both Craig Truglia and Greg Montero for their invaluable insights.

A case study

2001 Mitsubishi Montero 3.0l rough idle

By Craig Truglia, Owner & Service Manager, Car Clinic Inc., Mahopac, N.Y., and Associate at Technician Service Training

This was a tough one. A 2001 Mitsubishi Montero equipped with the 3.0L engine came into the shop with a mysterious idle problem. The engine ran really rough at idle when it was cold. It had a non-specific P0300 random misfire code. It had a DIYer as its owner, so we also had to consider all of the potential areas that he had tampered with. We knew we were in for a fun one, but had no idea how many things we would have to chase down in order to get it running normally again.

In the beginning, nothing appeared that suspicious. A DIYer, who otherwise only brought us inspections, had a problem that he could not fix himself. He told us that it ran really rough when it was cold. The only code it has is a P0300 random misfire and it never had new spark plugs. Because he had to remove the intake manifold to do the tune-up, he gave the car to us.

When we pulled out the first plug we saw that it was fouled with gasoline. The customer demanded the car back ASAP, so we were not given the time to ponder why the spark plug was wet. We returned him his car and hoped for the best.

A few days later the customer complained that the car was doing “the same thing.” We expected to get the car back, but nothing came of it.

Then, a couple weeks later, unannounced a green Montero was parked behind our shop that morning. We started it up and this time it was running horrifically. Only scatological terminology would really explain how the quality of its idle at this point.

In diagnosis, generally we are always looking to trace down the root cause of everything, what the Eagles’ Don Henley may call “the heart of the matter.” We presumed the vehicle either had an ignition, fuel or mechanical problem. What are the chances it’s more than one, right?

First, we wanted to see if the engine was not getting proper fuel. We checked fuel pressure (41 psi) and volume (0.3, a little low). Then, we wanted to make sure that the plugs were getting proper kV and all its ignition coils had good waveforms on our labscope. To be honest, I don’t know what a good Mitsubishi coil should look like. But I do know that most coils have similar waveforms, so I played the “comparison game.” I simply checked what all six coils looked like on my eight-channel labscope and they all looked the same. After not finding any obvious fuel or spark-related problems, we did a relative compression test to rule out an engine mechanical issue.

Personally, we find that on some weird cars (Mitsubishis, Subarus, and even Hondas) relative compression with a labscope is not an effective test. This vehicle tested really well, so we were sort of dumfounded. Obviously, something was wrong, this car was idling really bad.

Sometimes I feel like auto repair is like an arms race. It was time to pull out the big guns. So, we used the ACE Misfire Detective and it reported misfires and a relative compression issue (this particular tool makes measurements at the tailpipe). Now, we had good reason to suspect the engine had a mechanical issue. We put a pressure transducer into number 1 cylinder to check compression and timing.

We hit the nail right on the head! The pressure transducer revealed that the timing was really bad. The Automotive Test Solutions EScope makes it really easy to interpret.

Read a pressure transducer waveform just like the following: At 0 degrees top dead center the BANG or Power Stroke is occurring. The line is up high because the cylinder is at its most compressed when the spark event occurs. At 180 degrees bottom dead center the exhaust valve opens as long as engine timing is correct. When at 360 degrees TDC the piston begins going down and the cylinder decompresses (the SUCK) begins. At 540 degrees BDC the SUCK completes and the SQUEEZE begins. At 720 degrees TDC the process repeats.

If the waveform shifts in either direction left or right, timing may be advanced (left) or retarded (right). Other irregularities reflect that the car is not sucking, squeezing, banging or blowing correctly.


The timing belt had to be a few teeth retarded, so we sold the guy on a timing belt thinking we finally got to the heart of the matter.

When we took it all apart we saw that the belt was loose and the tensioner was bad, so we knew we did the right thing. We put everything back together and were happy until we started it up. The Montero was running exactly as it did before! We made sure to recheck timing, but as you can see it was indeed repaired. Something else was indeed wrong.

We had two places left to look. While we ruled out a dramatic fuel delivery problem, we had not yet ruled out an issue with the fuel injection. Furthermore, cylinder compression was suspect because of the ACE Misfire Detective’s measurements from before. To check both of these things, we had to take the intake manifold off all over again!

First, we checked compression the old-fashioned way and all the cylinders passed the test. Now it was time to test the fuel injectors. Because we couldn’t do a fuel injector balance test or any other testing with the intake manifold off, we had to bench test them.

Luckily we had a Launch X-Sonic Clean at our disposal. We found that the injectors were all way off. It took hours of cleaning using the machine, but we were able to get the fuel injectors to test really close to one another.

We put it all back together and the car actually ran well for once. No more rough idling, no more P0300, no more anything!

The moral of the story is that sometimes Don Henley is wrong: Sometimes there is not a heart of the matter, no one cause of everything. Cars can have several major problems that make it really hard to trace a root cause. What you need to do is divide and conquer: rule out fuel, ignition, and timing related causes; and test using the right tools.

Mitsubishi idle speed motor types

By Greg Montero, AAM, Identifix Chrysler & Mitsubishi Team Leader, Certified ASE + L1

Editor’s note: It’s sheer coincidence that the following also involves idle control on Mitsubishi engines. Mitsubishi has used a few different types of idle speed control systems over the years. This article will give an overview of each type.

Servo motor type with motor position sensor (MPS)

This is a motor that uses two wires and is a direct current (DC) reversible motor. The polarity is reversed by the powertrain control module (PCM) to operate the motor. This particular motor is unique in that it physically opens and closes the throttle linkage. In addition to the servo motor, there is an attached motor position sensor (MPS). The MPS basically works like a throttle position sensor (TPS) in that it is an analog voltage sensor. The MPS should normally have a voltage of around 0.9 to 1.0 volts when the engine is idling. As the servo motor physically opens the throttle linkage, the MPS voltage should increase.

This motor can be tested using a 9-volt battery. Reversing the leads on the 9-volt battery will drive the motor in each direction. Mitsubishi recommends against using 12 volts as it is possible to cause the motor to lock up with the higher voltage, resulting in a false indication of a failed motor. When testing this motor it is critical that it is calibrated to the MPS. In other words, with the motor retracted all the way, the MPS voltage should be around 0.5 volts. As the motor is extended out, the voltage should increase smoothly and be around 4.0 volts. The PCM relies on this voltage to ‘know’ where the motor is positioned along with the inputs from the TPS and the idle contact switch.

Air bypass reversible DC motor

This motor is very similar to the servo motor in that the PCM controls two wires and reverses the polarity to increase or decrease the idle speed.

The difference with this style motor is that, as the motor is operated, it is opening and closing an air bypass opening, not opening the throttle linkage. This style motor also uses a position sensor but instead of an analog sensor, it uses two Hall Effect sensors that provide the PCM feedback.

When the key is first turned to the “on” position, the PCM drives the motor all the way to the closed position. The PCM recognizes this as zero steps and begins counting up from there as the motor is opened. The sensors produce a 5-volt square wave signal. The steps can be read using a compatible scan tool. Like the older version, this motor can also be tested using a 9-volt battery.


Idle air control (IAC) stepper motor

The stepper motor is a DC-operated motor. It contains a permanent magnet rotor with two sets of windings placed around it. There are a total of six wires to this motor. Two of the wires have battery voltage with the key in the ‘on’ position from the multiport fuel injection (MFI) relay. The remaining four wires are the ground controlled circuits to the PCM. The PCM pulses each of the four windings to make the motor move. The end of the motor shaft contains a pintle. The pintle moves in and out, controlling an air bypass passage.

This motor can be tested two ways: 1) Test the resistance of each winding. Each winding normally has a resistance of 28 to 32 ohms at 70 degrees Fahrenheit. Quite often this test will show a failed winding. Be aware that this motor is prone to fail due to a shorted winding. When this happens, the PCM can be damaged. 2) Bench test the motor for pintle movement by applying battery power to the correct pins and then using a pair of jumper wires to the correct pins to control the ground circuit. A scan tool can also be used to view the steps to determine how far open or closed the motor is.

Linear solenoid idle air control (LSIAC)

This style uses a two-wire, air-bypass solenoid controlled by the PCM. The PCM duty cycles the solenoid between 10% to 90% at 1.5 kHz to 2.5 kHz. This particular valve is very fast to respond, capable of going fully closed to fully open in 20 milliseconds (mS). The PCM supplies both the power and the ground. The power lead is normally duty cycled. The ground lead is then monitored for current flow. The current increases with an increase in valve opening.

In addition to the PCM-controlled motors, there are two types of purely mechanical devices that provide additional air into the engine when cold. The two types are: Fast Idle Air Valve (FIAC) and the Flow Limited Control System (FLICS).

The FIAC contains a thermo-wax element and an air valve, similar to an IAC pintle. The thermo-wax valve is contracted when cold, allowing the pintle to move to the most open position. As the coolant passing over the thermo-wax element heats up, the wax expands pushing the valve closed. After a calibrated engine temperature is reached, the valve is completely closed.

The FLICS valve is similar to the FIAC in that it is coolant controlled but instead of using a thermo-wax element it uses a bimetal spring. The theory is the same — when the engine coolant is cold, the valve is open all the way. As the engine coolant warms up, the bimetal spring expands and closes off the valve to reduce the air flow going into the engine.

Hopefully, this information will be useful for those Mitsubishi vehicles that come into your shop.

Engine vacuum issues

Commonly, intake vacuum at idle should be steady between about 16 and 22 inches. Although a too-low reading can indicate a vacuum leak, insufficient vacuum value could also be caused by exhaust system restriction, such as a plugged catalytic converter (too much back pressure). If the vacuum reading tends to constantly jump high/low, this could indicate poor valve sealing (worn/damaged seats or possibly worn valve guides).

Rough idle: If the vacuum leak is large enough, the engine may have trouble idling at all (stalling out). In addition to checking for vacuum leaks, consider the PCV valve and the EGR valve. If the PCV valve is incorrect (wrong valve for the application), or loose-fitting, this can be a contributing factor. If the EGR valve is sticky and hangs-up at idle, this can mask itself as a large vacuum leak. PCV and/or EGR problems can cause the engine to experience lean misfire, which should be revealed with excessive HC exhaust output values.

Excessively high idle speed: If a small vacuum leak is present, the engine’s control module will attempt to maintain normal idle speed and compensate by closing down the throttle body air bypass. If a larger leak is present, the ECM may not be able to compensate for the extra air, resulting in a high-idle speed. Common leak sources include any of the engine’s vacuum fittings, throttle body gaskets, intake manifold gasket, hoses or vacuum-operated accessories, such as the EVAP purge valve or power brake booster. Other potential sources can include damaged or ill-fitting fuel injector O-rings or a worn throttle shaft.


EGR checks

During EGR valve opening, a small amount of exhaust gas is allowed to be sent back into the intake manifold. This action is intended to lower combustion temperature and to slightly dilute the air/fuel mixture. The intended result is to lower NOx (nitrides of oxygen). If the EGR valve sticks (not closing or leaking due to carbon buildup), a constant stream of exhaust gas can enter the intake stream, resulting in a lean condition. This can initiate a random-misfire DTC.

Whether the EGR valve is designed to operate by vacuum or by electronic control, the valve should remain closed when the engine is cold and idling. As the engine reaches full operating temperature, or when the engine is placed under a load, the valve should open. Note that some EGR valves are designed to open when exhaust back pressure reaches a specified level.

If you observe the EGR valve (where this is possible) with the engine running, apply a few momentary throttle bursts while watching the EGR valve’s valve stem. If it doesn’t move at all, either the valve is stuck or it’s control has a problem (vacuum line or ECM control, depending on design). If the EGR is vacuum operated, try to operate the engine at a fast idle. With a hand vacuum pump connected, apply vacuum directly to the valve. If this results in a momentary drop in engine speed, the EGR valve is likely not the problem.

Finding vacuum leaks

While visually checking for vacuum leaks and listening for telltale whistling noise is useful, the use of a smoke machine is invaluable in locating vacuum leaks.

This type of leak detector injects artificial smoke into the intake manifold. The smoke will travel throughout the system, and (if a leak is present) will exit at the leak source, making it relatively easy to detect the problem area. This is easier (and safer) than using propane or applying compressed air (while spraying the suspect areas with soapy water to look for bubbles). Propane is flammable, and using pressurized air (if not carefully regulated) can damage plastic intake manifolds.

Using propane

When used in conjunction with an infrared exhaust analyzer, propane can be very effective. Since CO (carbon monoxide) readings will normally drop when a vacuum leak is present (and HC readings tend to jump around), propane can be directed at suspected vacuum leak areas. A vacuum leak can be detected (while applying propane) if CO rises and HC drops a bit. By the way, a fluctuating HC reading can point to not only a potential vacuum leak, but can be caused by an excessively lean fuel mixture as well. If the HC values tend to bounce around, try applying propane directly into the throttle body (this will momentarily richen the mixture). If this has no effect, the problem is likely a vacuum leak. If the HC reading stabilizes and the engine suddenly runs smoothly, this indicates a lean idle problem.

Check cylinder compression

Perform a cranking compression test (spark plug removed, ignition disabled and throttle held wide-open). Note the reading on your compression gauge and compare this to the OE specs (generally speaking, cranking compression should be in the range of about 140 to 160 psi in each cylinder). A low cranking pressure can indicate one of a number of mechanical problems, including a leaking cylinder head gasket, worn or damaged piston rings, insufficient valve seating (burned valve, worn seat, bent valve, weak/broken valve spring or worn camshaft lobes). You can take this test a bit further to isolate the problem by squirting a few drops of engine oil into the spark plug port (this will temporarily help to fill any piston ring voids). Immediately perform a compression test again on that cylinder. If the pressure readings are suddenly higher than before, this indicates that the rings are not doing their job (worn rings, cracked rings, worn cylinder wall). If the readings are the same as before, pressure is being lost in other areas, such as the head gasket, valves, springs or camshaft lobes.

Basic ignition checks

Naturally, random misfire caused by a loss of spark or a weak spark may simply be due to fouled spark plugs, excessive plug gap, faulty spark plug wire, weak ignition coil, cracks or carbon tracks in an ignition coil or rotor.

If, upon spark plug inspection, you find carbon fouling or evidence of engine oil, this is an indication that the valve guides/seals are worn. Replacing spark plugs will only help in the short term. The oil seepage issue must be addressed to correct the problem.

If the spark plugs appear to be in good condition and are not fouled, inspect the plug wires and their boots for damage or improper connection to the spark plugs. Also check resistance of each plug wire. You’ll need to know the proper resistance is specified for the wires, but generally speaking, if resistance exceeds approximately 50,000 ohms per foot, the wire is junk. When dealing with coil-on-plug (COP) ignition systems, check the coil for cracking and/or carbon-tracking.

Also, while any manufacturer’s direct-replacement spark plug may be listed for a particular engine make/model, the reality is that certain engines simply don’t like certain-brand spark plugs. Check with a specialist for the make/model of vehicle and try to get a recommendation for the best-suited brand of spark plugs. Another dark reality is that not all plugs are created equal in terms of quality and reliability. While the majority of brand-name plugs out there are of high quality, there are a few that are hit and miss (some good, some bad). If in doubt, stick with a brand that you’ve had good experience with. Simply selecting the wrong choice (a poor quality spark plug) can create idle and drivability problems. NOTE: In a distributorless ignition system where a coil shares cylinders, a misfire in paired cylinders indicates a faulty coil.


Fuel injection checks

If the fuel injectors don’t deliver the required amount of fuel, random misfires can occur, which creates a lean air/fuel mixture. This can be caused by dirty injectors (resulting from bad fuel and lots of stop/go driving, or by a faulty fuel pump, pressure regulator or a restricted fuel filter.

With the ignition on (engine not started), use a voltmeter to determine if each injector is receiving voltage, and confirm that each injector solenoid clicks when the injector circuit is grounded. If voltage delivery is poor, chances are that the injector(s) is plugged. Depending on the severity of residue, either clean or replace.

With the use of an injector test kit (which includes a fuel pressure gauge), you can perform a drop test. With the gauge connected to the fuel rail, you can measure fuel pressure drop at each injector as each injector is selectively energized. Any injector that shows a healthy pressure drop as compared to other injectors is likely clogged. A cheap and easy repair attempt can be made by dumping a can of fuel injector cleaner concentrate into the fuel tank. It’s worth trying as opposed to removing and cleaning the injector(s). If it doesn’t clean the injectors properly, well, you were planning to yank them anyway.

Remember to consider the basics: Today’s fuel seems to have more problems than in years past, due to either water contamination or excessive alcohol “blends.” Although the fuel industry won’t admit to it, it’s not uncommon for smaller fuel retailers to have their tanks only partially filled, to avoid spending larger amounts of cash in one shot. This can result in increased surface area in the tanks to sweat and contribute to increased water contamination. Bad fuel can easily cause lean fuel mixtures and can be responsible for random or consistent misfires.

Fuel trim check

Using your scan tool, check short-term fuel trim and long-term fuel trim values at engine idle. An accepted range is +/ – 8. If the values are higher (+10 or more) for both short-term and long-term fuel trim, a lean condition exists. Increase engine speed and hold speed for about one minute at approximately 1800-2000 rpm. If short-term trim drops back to a normal range, this indicates the likelihood of a vacuum leak at idle. If short-term trim does not change, this indicates the likelihood of insufficient fuel delivery (clogged injectors, weak pump, faulty pressure regulator, restricted fuel filter).

For those diesel fans among you — Yet another idle variable: Today’s lousy diesel fuel

Any customer who owns a diesel-powered vehicle may complain about engine surging, poor idle and hard-starts, most commonly caused by incredibly contaminated diesel fuel, which is becoming a bigger problem than in years past. The quality of today’s diesel fuel has not advanced. In fact, it’s deteriorated. Today’s diesel fuel contains more asphaltene solids and water than the fuel of just a few years ago.

Asphaltenes are components of asphalt that are generally insoluble and are present to some extent in all diesel fuel. These black, tarry asphaltene are hard and brittle, and are made up of long molecules. Fuel with a high percentage of asphaltene will drastically affect the engine performance by reducing the ability of the fuel to burn completely. Asphaltenes also shorten the life of a fuel filter.

Water is the greatest concern because it is the most common form of contaminant. Water may be introduced into the fuel supply during fueling when warm, because moisture laden air condenses on the cold metal walls of fuel storage tanks or from poor housekeeping practices. The effects of water in diesel fuel can be serious. Water can cause a tip to blow off an injector, or reduce the lubricity of the fuel which can cause seizure of close tolerance assemblies such as plungers

It’s important to understand that most fuel contains some amount of water as a result of either condensation, the pipeline, the storage facility or the transporter.

Water displaces the diesel fuel. When the fuel is displaced wear occurs because lubrication is now absent.

Water that enters the combustion chamber results in even more serious damage. When it comes in contact with the heat of the combustion chamber it immediately turns to steam and often explodes the tip of the injector. Water causes corrosion of tanks, lines, injectors and greatly reduces combustibility.

It is estimated that eight out of every ten diesel engine failures have been directly related to poor quality and contaminated fuel. The build-up of contaminates in the fuel systems and storage tanks can quickly clog filters, thus resulting in engine shut down, fuel pump wear and diesel engine damage.

For diesel customers, it’s important to regularly add a diesel fuel purifier additive (available at most auto parts stores). An alternative is to install an aftermarket fuel purifier in-line unit. An example is the diesel purifier offered by Dieselcraft Fluid Engineering (reportedly removes 99.997% of the visible water and over 95% of the natural contaminates found in diesel fuel). See their web page at

Diesel high-pressure injector operation

Citing a 2002 Ford 7.3L turbo diesel engine as merely one example (available in F-250 and F-350 Super Duty trucks), while the fuel injector timing and duration is controlled electronically, physical operation of the injectors is handled hydraulically, using an external high pressure oil pump that feeds from the engine’s oil supply. A high pressure oil pump control valve (injection pressure regulator) regulates oil pressure from the pump to the injectors.

If this control valve sticks, due to wear, internal burrs or foreign material, engine operation will suffer. Typically, you’ll notice the problem under low-RPM operation, such as during braking up to and at the point of final stopping (where the engine suddenly shuts down, but can be re-started easily while in neutral or park), cylinder misfiring during idle and low-RPM/slow-to-moderate-speed cruising engine surging/hunting. These problems (which are commonly intermittent in nature) are often misdiagnosed as requiring a fuel filter replacement.

In many cases, simply shutting off the engine and restarting (coincidentally after the filter change) can possibly re-set the actuator piston in the control valve and make it operational again. The engine seems to run fine, which mistakenly confirms the fuel filter as the culprit. However, if the control valve is problematic, the drivability issues can come and go with no rhyme or reason. The control valve (on the Ford Navistar 7.3L diesel) is located on the upper right side of the engine (inboard of the left valve cover). The valve runs about $200 and labor time is about an hour.

Other diesel-related poor-idle problems (with any brand of diesel) may be caused by fuel system contamination, where algae has accumulated within the fuel tank, eventually clogging the injectors. This can result in poor idle, engine surging or complete engine shut-off and no-starts, depending on the severity of the contamination.


Potential causes of poor idle quality: Various examples of causes. This list is certainly not all-inclusive

• Vacuum leaks

• Timing belt (jumped timing)


• EGR (sticking)


• Fuel filter clogged/restrictive

• High pressure injector pump control valve (diesel)

• Throttle body (dirty, sticking, worn throttle shaft, etc.)

• Fuel injector seal unseated (allowing air to pass)

• Cracked distributor cap

• Faulty spark plug wire(s)

• Faulty spark plug(s)

• Mis-installed distributor (out of time)

• Worn/faulty camshaft

• Bent valve(s)

• Bent pushrod(s)

• Faulty (broken or weak) valve spring(s)

• Faulty (damaged/loose) rocker arm(s)

• Mis-adjusted valve clearance

• Water or other contaminants in fuel (gas or diesel)

• Weak ignition coil(s)

• Dirty/sticking fuel injector(s)

• Lean fuel mixture

• Worn distributor shaft/gear

• Worn/damaged camshaft distributor drive gear

• Worn/damaged piston ring(s)

• Coolant contamination (via block or head crack or porosity, or failed head gasket)

NOTE: DTC P0300 — A random misfire code can be set on OBD-II onboard diagnostics systems when multiple misfires occur randomly in multiple cylinders. The cause is most commonly a vacuum leak in the intake manifold, throttle body or vacuum plumbing, a defective Exhaust Gas Recirculation (EGR) valve that is leaking exhaust into the intake manifold. Another real possibility involves defective fuel. Less common causes may include faulty spark plug wires (or wire connections), worn or fouled spark plugs, a weak ignition coil, contaminated fuel injectors, low fuel pressure, or weak valve springs. If a misfire occurs in only one or two cylinders, a cylinder-specific misfire code should appear rather than a random misfire code.






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