Rundown on Idle Speed

Jan. 1, 2020
Remember how some folks used to complain about cold fast-idle settings that seemed too high? Some people wouldn't have even noticed ...

A quick study in idle speed control, and one good fix for a problem that likes to boomerang.

Remember how some folks used to complain about cold fast-idle settings that seemed too high? Some people wouldn't have even noticed the problem if there hadn't been a tachometer to watch.

For some customers, things got even worse in the early days of fuel injection. One customer complained repeatedly that his 1986 2.3-liter Ranger was idling too fast after morning starts. We couldn't get him to understand that the Engine Controller was making the decision, and we couldn't perform any adjustments to change it. He came back one day with a big smile on his face and told me that he had removed his Idle Air Control valve, installed some thin washers on the bolts so that they were sandwiched between the valve and the gasket, and that his cold fast-idle problem was gone.

This do-it-yourselfer had given the engine enough of an air leak past the IAC gasket that the engine was idling at a speed that Ford refers to as the "dead band." The dead band is the point where the PCM settles into a pattern of not using idle speed control at all because the engine is already idling at or above the PCM's target speed.

"If Ford wants to use my idea to solve their problem, they can pay me for it," he told me. I assured him that no automaker would be interested in a modification that allows the ingestion of unfiltered air into an otherwise healthy engine. I always wondered how many miles he got out of that truck before he wiped out the piston rings.

Idle strategies: Something old, something new

In spite of the fact that idle strategy is as old as engines, understanding it can be pretty dicey on today's vehicles. It doesn't take much of a problem - or perceived problem - to generate customer concerns, especially on vehicles equipped with a tachometer.

For years, automakers utilized vacuum or electric throttle kickers to compensate for engine load at idle. And just about every automaker once used those old dashpots to catch the throttle lever when the accelerator is released to let the engine return slowly to idle. Without the dashpot function, frequent stalling can occur.

In the early days of GM's Computer Command Control, most GM vehicles had one of those electric idle speed control motors mounted on the side of the carburetor interacting with the throttle lever, but some GM passenger cars were equipped with a purely mechanical device called an Idle Load Compensator. This little item was so simple I wondered why it hadn't caught on years earlier.

Increased idle loads lower engine vacuum, which allows the internal spring to extend the plunger against the throttle lever, opening the throttle plate ever so slightly to raise idle speed. As idle speed increases, so does manifold vacuum, which in turn acts against spring pressure to reverse the opening. A properly adjusted Idle Load Compensator automatically finds the proper idle speed as A/C or power steering loads come and go. Ford used a similar item on carbureted Escorts until Electronic Central Fuel Injection replaced the Escort carburetor in 1987. Mitsubishi adopted an electric unit on some of their platforms that had an integrated Motor Position Sensor. Some Japanese automakers remained convinced for years that wax pellet technology was still the best thing going for cold fast-idle control.

Motors, switches and pintles

Electronic Fuel Injection (EFI) became the order of the day on most vehicles by the late '80s. The TBI and EFI systems used by GM and Chrysler favored those compact little PCM-driven, four-pin stepper motors we're all so familiar with. These motors can be operated through a hundred or so 'steps' to move a pintle in an air passage machined into the throttle body to regulate airflow around the throttle plate, thus raising and lowering idle speed.

Until multipoint fuel injection took over in the mid-'80s, Ford's CFI-equipped V6 and four-cylinder engines used a Carter-built Idle Speed Control (ISC) similar in operation to the old externally mounted GM-style idle speed control motor. Like the GM ISC, the Carter-built unit contains a nifty little DC motor and an extendable plunger sitting on an Idle Tracking Switch that opens and closes to inform the Engine Control-ler when the throttle is released.

Always check the continuity of the idle tracking switches on these Ford units when you're chasing an oddball idle concern or a tip-in stumble; this input is very important to the PCM for fuel and timing control.

You can find the terminals with your ohmmeter. The Idle Tracking Switch should have less than 5 ohms without pressure on the tip of the plunger and no continuity with mild pressure applied. These units also act in such a way as to close, then open the throttle at shutdown to handle the anti-diesel and "no-touch starting," by holding the throttle slightly open until the engine starts. They also perform the dashpot function when the throttle is released suddenly, raise idle speed when A/C or power steering loads are added, and so on.

Beginning in 1985, Ford multipoint EFI uses an Idle Air Control (IAC) valve, which allows air to bypass the throttle plates. Unlike the stepper motors still used by GM and Chrysler, the Ford units only have two terminals and one winding. These units regulate IAC plunger position using a duty cycle percentage, readable as a scan tool PID.

Adaptive learning strategies brought a whole new dimension to the world of idle speed control - not to mention fuel management, transmission shift timing and a host of other operations that were once purely mechanical.

Decisions, decisions

One of the inputs utilized by Ford PCMs for the ever-necessary dashpot function is Vehicle Speed input. As long as the vehicle is rolling, even if the throttle has been released, the idle speed is carefully and deliberately decreased at a controlled rate to prevent potential engine stalls. Almost all vehicles use this strategy at one level or another, and on some vehicles it's more noticeable. An idle speed control problem can result from a ragged Vehicle Speed Signal, but that doesn't happen a lot. Some plastic aftermarket Engine Coolant Temp (ECT) sensors tend to fail and cause screwball idle, starting and fuel mixture concerns. There also are a lot of problems across the board with throttle plates sticking shut because of the buildup of PCV sludge, even on low-mileage vehicles. This same concern infects the two-wire pintle-type Idle Air Control valves and can cause hard cold-starts and warm-engine stalling.

On another note, some customers who haven't owned their cars very long might complain about elevated idle speeds during coast-down maneuvers, particularly on manual shift vehicles. Some manual transmission-equipped Ford Escorts have a noticeable speed-related idle dashpot strategy of this type, but unless a customer is particularly finicky, he or she might not complain about it.

The problem is that when the customer does notice it, there isn't much you can do to change it, especially if the strategy is built into the PCM. It helps to be familiar with the product. Comparing the vehicle in question to a similarly equipped unit is helpful, but this may not always be possible. It's wise to gather as much information as you can from cars that are operating correctly so you can compare what you know to be right on a particular vehicle to what a customer perceives as wrong.

The wrinkle comes when you have to determine whether what the customer is feeling is PCM strategy or if there actually is a problem that needs to be addressed. A knowledgeable scan tool junkie knows idle speed concerns can be generated by a Throttle Position Sensor signal that reflects a "P/T" or "Part Throttle" reading when the accelerator has been released. Remember the old Idle Tracking Switch function? The PCM still needs to know when you've released the throttle and when you've got your foot in it. To decide where Closed Throttle actually is, most of today's PCMs read and store Closed Throttle Angle voltage in the RAM when the ignition is switched on so as to determine when the throttle is at idle. That's why it's so important for a customer to keep his foot off the gas pedal when starting the engine.

One good fix

This brings us to the vehicle in question. Some customers who own a 1998 to 2000 Ranger with a manual transmission and a 2.5-liter engine might complain of an extremely high idle (3,000-plus) with the clutch depressed. The effect doesn't settle down until they've coasted to a stop. The 3.0L V6 Rangers of this vintage have a less pronounced normal dashpot strategy that some customers complain about, but it's handled by the PCM. Don't spend a lot of time trying to fix it if everything else checks out okay. A quick look at scan tool throttle angle readings (both voltage and mode), rpm, IAC and Vehicle Speed (VSS) will tell the story.

My cohort and I have seen several four-cylinder '98 to 2000 Rangers with high-idle coast-down problems like this. The TPS voltage will hang about 0.07 volts above base TP voltage, fooling the PCM into thinking the driver is still holding the throttle off idle. As vehicle speed decreases to zero with the throttle released, the IAC percentage will decrease and the rpm will drop to almost normal, even though the TP Mode PID will still be reading Part Throttle (P/T) when it should be reading Closed Throttle (C/T). If fiddling with the TP sensor wires lowers the voltage or changes the mode, then you probably have a connection problem at the sensor. After pinching and twisting the wires at the sensor, the concern might be hard to duplicate for awhile.

The cause of the poor connection isn't visible with the naked eye, even with the terminal removed, but it's usually at the point where the terminal is crimped around the copper wire. Ford released TSB 98-26-8 with instructions to reflash the PCM, but that endeavor proved to be futile on all the vehicles we tried it on.

In January 2000, Ford released TSB 00-03-05 that outlines a procedure for replacing the TP sensor pigtail on '98 to 2000 Rangers and Super High Output (SHO) Tauruses. While this is a viable option, it would seem that the crimps on the new pigtail might be just as prone to give trouble later on as the original. This is especially so because Ford packages the new pigtail with butt splice connectors, which means additional connections in each wire.

A conscientious driveability guy will always be more prone to use solder and heat shrink than butt splices or Scotchlocks, particularly because oxidation and the resulting voltage drop can wreak havoc with sensor signal readings. If you use the Ford pigtail and the butt splice connectors that come with it, make sure you apply heat to the shrinkable insulating sleeve on the butt splice and shrink it tightly to the wire so the connection is sealed against air and moisture.

Mass Airflow meter-equipped Tempos ('92 to '94) exhibited a similar problem with the MAF sensor connector terminals; these were related symptoms that generally ranged from stalling to MIL illumination. We found that soldering the MAF terminal crimps proved to be a simple, yet permanent repair. Ford's fix for the Tempo MAF terminals included a pigtail as well.

If the TP sensor pigtail is unavailable and if you have the skill to delatch the terminals from the connector shell (there are tools available for this), try soldering the crimp on each wire connector. One important note is to make sure the connector is in an upright position or the solder might work its way toward the business end of the terminal. This could prevent the end from reconnecting to its male counterpart. Soldering the crimped connectors has permanently repaired every 1998 to 2000 four-cylinder Ranger we've encountered with this problem.

About the Author

Richard McCuistian

Richard McCuistian is an ASE certified Master Auto Technician and was a professional mechanic for more than 25 years, followed by 18 years as an automotive instructor at LBW Community College in Opp, AL. Richard is now retired from teaching and still works as a freelance writer for Motor Age and various Automotive Training groups.

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