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Curing Rough Idle

Curing Rough Idle

Cars need maintenance, there’s no question about that. And when it comes to older cars and classics, well in many cases, they need even more maintenance, having at some point suffered from the ravages of neglect. And today many cars from the ’80s fall into this category. Too new to really be considered coveted and yet old enough that they’ve likely gone through several owners, they’re prime candidates for affordable wheels. Take 5-liter Mustangs. Today you can still net yourself a decent one for under $5,000, but chances are it needs some help. One of the most common problems is likely a rough running or poorly idling engine. The purpose of this article is to highlight some of these common problems and how to fix them, so your mighty “Five-Oh” runs just as good, if not better than it did during the Reagan/Bush Sr. years.

Idle Air Control

Perhaps one of the biggest issues, not to mention the most common problem to a stumbling SEFI 302 Mustang engine concerns the Idle Air Control valve; that governs the car’s engine speed when it’s stationary. Mounted on the throttle body spacer on 1986-1993 Mustangs and the back of the intake on 1994-1995 cars, it incorporates a valve that opens and closes, a bit like a flapper device to control said engine speed. The IAC is linked to the car’s EEC-IV engine computer via an electrical connector and the computer is able to monitor, through its other sensors, how much to open and close the valve, depending on outside temperature and engine temperature when the car is at idle. However, as the years go by and the car is driven more and more miles, carbon deposits, a by-product of poor quality fuel that’s burned and then reburned through the car’s emissions control system, can settle on the valve, clogging it up. As a result it cannot open or close properly, which affects the engine speed, causing idle surge. By disconnecting the negative battery cable, then the IAC from the harness and removing the throttle body and unscrewing the IAC from the T-B, you can give it a good cleaning by delicately spraying carburetor cleaner into the valve. If the IAC is the problem, then once it is cleaned and restored, the car should run much better.

Throttle body

However, in most cases, if the IAC is clogged then there’s a good chance the throttle body is too. On 5-liter cars the throttle cable, via the gas pedal, directly opens and closes a plate inside the throttle body – the further it opens the greater the air volume and engine speed. If the IAC is clogged and the throttle body can’t close all the way, chances are the engine will be revving and stumbling at idle, because the valve can’t control the idle speed and the throttle body will be sticking. So every time you hit the gas with the car in gear, it will stumble and in some cases stall out. On 1986-1995 5-liter Mustangs, the throttle body is secured with just four bolts and sealed with a gasket. It’s easily removable for cleaning. On 1986-1991 models you can use general carburetor cleaner to get rid of the deposits, on 1992 and later models, which use a slightly different stock throttle body with ceramic coating, make sure you use a throttle body/carb cleaner that’s formulated for use on coated T-Bs, since some traditional carb cleaners, because of the solvents contained within them, can damage the inside surface of the throttle body, which will cause further problems.

PCV Valve

If you’ve addressed the above problems and the car is still having trouble idling or running properly, it’s time to look at the emissions controls. The first point is the Positive Crankcase Ventilation valve, which funnels unburned hydrocarbons (exhaust fumes), back into the intake manifold where they can be burned off again, reducing emissions. After about 40,000 miles, the valve can become clogged, but it’s cheap and easy to remove and replace. It’s located on the back of the engine, on the right side below the upper intake plenum.

Thick Film Module

First introduced on Mustangs for 1984, Ford’s Thick Film Ignition system is a staple part of every SEFI 5-Liter. However, it can be problematic. An electronic module, mounted on the distributor shaft is linked to the computer via a wire extension that links up to the main harness and is responsible for controlling the spark on these engines. However, like most modules, it likes to operate in a certain temperature range (up to around 275 degrees F), beyond that heat affects its ability to operate and it can simply cut out. So, all of a sudden, you can be driving along the road and the car stalls out and then won’t start. However, depending on how much heat damage has been done, sometimes the engine will start again once it’s cooled down. Ford tried to get around the problem by fitting a rubber boot over the distributor to keep temperatures down, but it was barely adequate in most cases. If your Mustang experiences this problem, i.e. it will start when cold, but then dies when warmer, replace the TFI module with a new Ford Motorcraft unit (aftermarket versions generally aren’t as durable). Some Ford enthusiasts joke that they always keep two TFI modules handy, just in case one goes bad!

TFI Coil

The actual coil for the TFI system can also cause problems. Over time, due to heat and age, it isn’t able to step up the voltage from the battery, to provide adequate spark to fire the engine. Each time the coil operates it more than doubles the voltage in the car’s electrical system in order to provide adequate voltage to crank over that big V-8. However when this happens, energy is depleted and much like a battery, it will eventually fail. In these cars the coil is mounted on the left side inner fender and can be easily replaced. Also check the top of the coil where it meets the coil/distributor wire, on cars that have been sitting for extended periods, this can corrode, affecting the coil’s operation. Use a wire brush to clean the rust off the top of it. Installing a higher capacity coil is often a good suggestion, especially if you plan on modifying the car (if it isn’t already). Good options are available from MSD, Accel and Mallory.

Ignition Wires/Distributor

Plug wires are designed to transport the energy generated by the coil, to the spark plugs, in essence, they’re electricity conductors. Like most ignition parts, heat is a big enemy of spark plug wires. Over time, heat causes the composition of the wires to break down, which affects their ability to efficiently conduct energy from the coil, since electricity is always looking for the path of least resistance. As a result this can cause arcing, which can make the car hard to start, or causes it to repeatedly stall, especially when the engine is cold. As they age, plug wires also give off greater Electromagnetic Interference, which causes radiation and affects operation of sensors like the MAP, TPS and even the hotwire MAF sensor, causing further driveability problems. Replacing your plug wires is cheap insurance, but remember, if the car is street driven, high output inductive wires, which are able to provide a longer spark for greater cranking power and smoother running, are a good suggestion. Another good idea is to check the underside of the distributor cap and the condition of the fittings. Original OE Ford Motorcraft caps use brass fittings, but over time these can corrode, increasing EMI and resulting in an engine that is both hard to start and runs poorly. Additionally the cap itself can crack, allowing moisture to enter the distributor, shorting out the electrical current, which will cause similar problems. The rotor arm is also a common failure point, since the tips can corrode, increasing electrical resistance and making the car harder to start. It’s easier to replace the arm than trying to clean it.


One other common area concerns the spark plugs. Like any other part of the Mustang’s ignition system, the electrodes on the plugs wear over time and need replacing. The more times energy is expended in firing the engine, the more heat passes through the electrode. Over time the plug gap tends to increase, which along with carbon deposits from burning the fuel, makes it harder for the plugs to fire the engine, since the electrical resistance is now much greater. On cars equipped with electronic fuel injection, like our 5-liter Mustang, old or worn plugs; will often cause the computer to add extra fuel to start the engine when cold. This not only fouls the plugs (by coating them in gasoline), but can also cause problems with the car’s emissions control system, specifically the Oxygen sensors and catalytic converters. What results is an engine that runs rough and frequently stalls. Fouled plugs can be identified by a thick black coating around the base of the electrode, a result from too much fuel being dumped into the combustion chamber. A spark plug with normal wear, should display a slight grey, or red/brown coating at the base. Even if you’ve just bought the car, it’s a good idea to change the plugs anyway. On 5-liter engines, replacement plugs should be gapped at .45 of an inch when installed on a relatively stock engine.

Catalytic converters

Another problem on these cars, concerning rough running can be attributed to the car’s H-pipe system. From the factory, 5-liter Mustangs came with four catalytic converters, two pre-cats, mounted up stream and two main ones. An air injection system, that runs all the way from the smog pump, to just ahead of the main converters is also incorporated, as are two Oxygen sensors that relay the amount of O2 in the exhaust system and send voltage signals back to the EEC-IV processor so it can adjust the air/fuel mixture accordingly. The two pre-cats were largely incorporated to warm up the sensors as quickly as possible in order, to achieve the optimum air fuel ratio (13:1-14:1) and hence reduce both fuel consumption and emissions. However, over time the converters can become clogged, often due to extensive stop/start driving, lack of use, poor quality fuel or infrequent engine tune-ups. Clogged converters increase the exhaust backpressure, which causes the engine to run poorly and frequently stall. Eventually the internals of the converters can collapse entirely. You’ll likely notice this by a rattle, especially when the car is at idle. Sometimes the oxygen sensors can become coated in carbon, which, much like the MAF sensors, doesn’t allow them to accurately monitor the level of oxygen in the exhaust, which causes the computer to add too much or too little fuel. If that’s the case, you’re best option is to replace them.

Because it’s been 15 years since the last 5-liter Mustang was produced, the supply of new, factory replacement H-pipes have become very limited in recent years. Thus a good option, if your factory H-pipe is tired, is to invest in a replacement H-pipe with dual high-flow converters, which further reduces backpressure, even over a stock system in good order. These are widely available through the aftermarket, with Bassani, BBK and MAC Products offering some good quality options.

Fuel system

Yet another possible cause of rough idling and stalling can be related to the fuel system, specifically the electric tank-mounted pump. This uses a pick up device mounted in the tank to draw fuel and pump it to the rails and injectors via a main line and approximately 45 psi, controlled by a pressure regulator. The excess fuel is then returned back to the tank via a secondary line. The pump’s factory flow capacity is rated at approximately 88 liters per hour, but over time, this can drop, resulting in erratic fuel delivery and a rough running engine. Because the fuel in the tank also cools the pump, if a past owner has run the car dry on numerous occasions, the pump is more likely to overheat and fail. Generally the pumps start to deteriorate every 10 to 12 years. So, if your Mustang is proving hard to start and the idle noticeably dips and soars, regardless of whether the engine is hot or cold, check the fuel pump. Larger capacity pumps, 110 and 190 lph units are a good option on stock, or mildly modified 302s. Removing the pump on these cars simply requires disconnecting the electrical feed at the back of the car, unfastening the two tank straps, dropping the tank and pulling out the pump assembly. Make sure you use a brass instrument to undo the lock ring (to prevent any spark).

Another aspect that can affect fuel delivery is a clogged filter. On 5-liter cars the filter is encased in a metal bracket in front of the gas tank. Remove the filter and give it a good shake. If you hear rattling, it needs replacement.

Although generally not a common problem by any means, sometimes leaking fuel injectors can also cause idling issues. If you smell fuel in the engine bay while the car is running, it might be a crack in one of the injectors or a bad O-ring. It can also be the fuel rail coupling. On early SEFI 5-liter cars, the rail coupling was a notorious weak spot and fuel could puddle onto the intake manifold and exhaust, sometimes even causing fires. These were subject of a recall, so if you can, run your car’s VIN to see if was subject to a recall and the problem has been addressed. Believe it or not, there are still some cars out there that haven’t received this fix.

EGR Valve

Crucial to the proper operation of the 5-liter’s emissions control systems, the Exhaust Gas Recirculation Valve, mounted on the back of the throttle body spacer (1986-1993 Mustangs) or intake (1994-1995), funnels unburned hydrocarbons from the exhaust manifolds back into the combustion chambers to burn them off and reduce emissions. The valve is linked to the EEC-IV computer by a harness connector with a sensor to monitor exhaust gas flow (Delta Pressure Feedback), and various vacuum lines that run to the intake manifold, oil breather and plumbing from the smog pump to the back of the lower intake that incorporates a regulator and down into the air injection system that links up with catalytic converters (on 1994-1995 Mustangs there’s a pipe directly from the valve to the right side exhaust header). If there’s a problem with the EGR system, the first place to look, should be the vacuum lines. Bearing in mind that most of these cars are approaching 20-plus years in age, if the lines haven’t been replaced, chances are they’re brittle and at least one of them is cracked. When the engine is running, increase the idle speed and listen for a hissing sound coming from the back of the intake manifold. If you hear it, chances are you’ve got a vacuum leak.

If you have one or have access to one, you can also use a scan tool to check actual operation of the EGR valve itself. If the scan tool detects an error code, then it’s likely that the DFP sensor has gone bad and will need to be replaced. Also inspect the tubing that runs from the exhaust manifold to the EGR as sometimes this can get clogged, restricting flow, which will cause a driveability issue. You’ll be able to identify this by running the engine and then disconnecting the tube from the EGR. If you can’t feel any vacuum pressure and the engine idle doesn’t change, there’s a clog somewhere in the system, as normally a leak in the EGR system, with exhaust gas still recirculating, will cause the engine to run rough. However, in most cases the EGR rarely fails, so if the car is running poorly, check the other problem areas mentioned in this article first.



While it might not be an issue directly relating to the metering on your car that’s causing the idling or stumbling problem, it’s a good idea to have a basic understanding of what system your car runs, as both the Speed Density and Mass Air systems on these cars can have their own set of issues.


When Ford decided to install sequential fuel injection on the 1986 Mustang 302 V-8 it made enough changes to almost qualify it as a brand new engine. Air was fed in through a square-shaped filter housing on the right side of the engine bay, up through a tube into the throttle body, mounted in front of a two-piece intake plenum. Just behind the throttle body was an idle air control valve and throttle position sensor. Mounted behind the intake plenum on the firewall was a Manifold Absolute Pressure sensor, which monitored engine speed and air density entering the plenum and sent the readings to the car’s Electronic Engine Control (EEC-IV) computer, which along with data from the oxygen sensors, manifold pressure and throttle position sensor, would then consult its own internal look up tables and select the amount of fuel to deliver. In these Mustangs, eight individual injectors operating in sequence, squirted fuel into the combustion chambers under high pressure (around 45 psi). This system was called Speed Density, because it used the engine’s turning speed and airflow, to determine the amount of fuel required to maximize power, fuel economy and emissions (generally between 13:1 to 14:1 parts air relative to fuel).

However, the problem with the Speed Density system used in these cars; was that it was rather basic. The computer was designed to operate the engine based on a set of fairly narrow pre-determined parameters, i.e. factory power and torque. Now while it’s able to fairly accurately measure the airflow and calculate fuel delivery based on those readings, it doesn’t take much to confuse the computer. In addition there are no cold or hot wire sensors placed directly into the air intake stream, so the computer cannot precisely measure the volumetric efficiency of air entering the engine, nor the temperature of that air. As a result, it can be more accurately described as assuming the required fuel load rather than precisely calculating it, which means even the smallest changes outside its regular operating parameters can cause driveability problems.


Unlike a Speed Density setup, Mass Airflow incorporates two wires mounted directly in the engine’s air intake path. Using voltage, one wire measures the ambient temperature of the air (cold wire), the other (hot wire) is designed to maintain a temperature that is constantly 200 degrees F hotter than the cold wire and read the airflow rate. Because the rushing of the air into the engine cools both wires, it triggers voltage from the wires back to the engine computer. From this information, the EEC-IV processor is then able to accurately measure the air and add the required amount of fuel based solely on the engine’s volumetric efficiency (airflow). If you’ve ever driven a stock MAF equipped 5-liter Mustang (1988 and newer California, 1989 and newer everywhere else), you’ll have also noticed it idles a lot smoother than a Speed Density car. Also, because the MAF system doesn’t rely on the manifold absolute pressure sensor, (it becomes a Barometric sensor on these cars) to help maintain the correct air/fuel ratio and the computer MAF equipped 5.0s tend to suffer less from erratic idling.



If you own or are looking at buying a Speed Density equipped Mustang (1986-1987, 1988 (non California) and it’s running rough, one specific problem area concerns the vacuum line running from the Manifold Absolute Pressure sensor to the intake. Considering that these cars are well over 20 years, old, chances are, if the hoses haven’t been replaced they’ll be brittle. Even the smallest leak can cause the engine idle to surge and even the car to stall, since the computer can’t accurately read the airflow from this sensor to deliver the required amount of fuel. If the engine is running, open the throttle and listen for a hissing sound at the back of the manifold, if you hear it, the hoses need to be replaced.

On MAF cars, a specific idling issue can often be traced to dirty sensor tips in the MAF unit (the hot and cold wires). If the car is equipped with a dirty or an aftermarket high flow filter that’s coated in oil, some of that residue will eventually find its way up the inlet track and coat the wires. Dust and all other manner of objects can then attach to the oil or residue, which prevents the wires from accurately measuring the airflow, so the computer can’t read the VE correctly and can’t deliver the precise amount of fuel. Generally this problem seems to affect the 1994-1995 5-liter cars, more than the earlier ones. To fix the problem, simply remove the MAF unit and use a small dose of carb cleaner and delicate wire brush to clean the tips of the sensor, before reinstalling it back on the car.

Acknowledgements: Special thanks to Joe Da Silva and Rob Cohelo at Da Silva Racing www.dasilvaracing.com.



This neglected 1989 Mustang was purchased for less than $3,000. It had numerous problems, including a rough running engine.


One of the most common issues causing a poor idle, is a clogged idle air control valve. On 5-liter Mustangs; it’s located by the throttle body on the right side of the engine by upper intake manifold, illustrated to good effect on this modified car.


Here’s a sample of common components that can often be the cause of a poor idling or rough running engine – from right to left (clockwise) Mass-Air Meter housing, fuel rail, distributor, injectors, MAF meter, fuel pump, TFI module and ignition coil.


A very common issue, even if the car is relatively well maintained can be vacuum hoses. Considering the age of most 5-liter Mustangs, it’s a good chance that one or more of the hoses has cracks, allowing vacuum air to escape, which will cause rough running.


Here the EGR hoses are inspected. The EGR is major part of the car’s emissions control system and spent exhaust fumes are funneled through the valve, back into the combustion chamber where they’re reburned to lower exhaust emissions. If the gas can’t circulate properly, vacuum pressure is lost and the engine stumbles.


Another problem; can be a worn distributor cap or rotor. Generally, the Motorcraft distributors and related parts are very reliable, but worn contacts can cause arcing and electromagnetic interference, which can make the engine hard to start and even damage other sensors. Replacing the cap and rotor is cheap insurance.


On 5-liter cars equipped with Speed Density reading, air density is monitored by the Manifold Absolute Pressure sensor; mounted on the firewall. Vacuum lines run from the sensor to the intake manifold, allowing it to read the air density. If a hose splits or cracks, again vacuum is lost, the sensor can’t read the airflow and the EEC-IV computer, because of its very basic programming can’t deliver the correct ratio of fuel to air, causing idle surge, stumbling and loss of power under load.


A very common problem on 5-liter Mustangs and many Ford vehicles dating from the mid ’80s through the early ’90s; concerns the Thick Film Ignition Module on the distributor that controls the spark. It often overheats, causing the engine to cut out as it warms up. If that happens, replace it immediately.


This picture shows the BP sensor on a Mass Air equipped 5-liter Mustang. Because these cars use a mass air meter to measure airflow coming into the engine, the MAP function becomes redundant. Instead the sensor is used to measure outside Barometric Pressure and the vacuum line to the intake is removed and the hole plugged.


Here’s a factory Mass Air Sensor, as seen installed on a 1989 Mustang GT. This offers a far more accurate method of measuring airflow than Speed Density and also results in a generally smoother idle. However, it can become clogged, especially if an oil-coated aftermarket air filter is installed, as seen here.


With the sensor unit removed from the car’s snorkel assembly, we unscrew the MAF unit from the housing.


The MAF uses two wires, a “cold” wire, to measure the air temperature and a “hot” wire that’s maintained temperature 200 degrees F hotter than the “cold” wire to measure the rate of flow via voltage. Not only is the system more accurate than Speed Density, it’s far more accepting of performance modifications. However the wires are very delicate, and should be cleaned with a tiny wire brush, if they’re coated in debris.


If you’ve checked the IAC, throttle body and distributor, it’s also a good idea to pull the spark plugs and examine the electrodes, especially if you’ve just bought the car or it’s been sitting a while.


On this particular Mustang, the plugs were showing signs of corrosion, meaning they’d been in a car, which had been sitting around for a very long time.


Clogged catalytic converters can also be another problem affecting driveability. Sequentially injected 5-liter Mustangs came with this four-cat H-pipe system from the factory. A clogged converter will increase exhaust backpressure to the point that the car will frequently stall, because the gas can’t escape.


Two oxygen sensors are mounted close to the front pre-converters to monitor oxygen content in the exhaust and use voltage to tell the computer to adjust the air/fuel mixture to minimize emissions. Over time, they can become clogged in carbon particles, which results in too rich a mixture and rough running.


Shown for illustration purposes is a typical late ’80s/early ’90s converter, containing a honeycomb catalyst. The material inside the converter can break down, clogging it up and causing the car to run rich. You can often tell if you hear a rattling sound coming from the exhaust upon start up.


Generally, the Exhaust Gas Recirculation on these cars is very reliable, but sometimes a bad sensor or clogged vacuum tubing can cause the engine to stumble and run rough, both at idle and when the car is in motion.


Scan tools and timing lights can come in very handy when troubleshooting idle problems. Many 5-liter Mustangs run advanced timing, but sometimes a loose distributor can cause rough running. Using a timing light, you can determine how much advance spark the distributor is running. If it’s being done in conjunction with old plugs and poor quality fuel, it can be recipe for disaster, because the more aggressive spark and low octane fuel can cause flameouts in the combustion chamber.


Stock fuel pumps in 5-liter Mustangs tend to fail about every 12 to 15 years. If the car is hard to start, or shows a loss of power and erratic idle whether the engine is hot or cold, chances are it could be a defective pump. Direct replacements with a higher flow rate, like this Carter 110 liters per hour, are a good substitution for the stocker.


If you can, take the Mustang to a shop and get them to put it on a chassis dynometer. This will be able to show you the power and torque curves, plus the air/fuel ratio right through the rev range, helping to pinpoint exactly what’s causing a driveability issue – knowing your car’s actual power and torque numbers are a side benefit.



BBK (Exhaust H-pipes)

Federal Mogul (Fuel pumps)
Ford Racing Parts (Ignition parts)

MAC Performance Products (Exhaust H-pipes)
(951) 296-3077

Motorcraft (Ford OE replacement sensors, ignition, emissions equipment)

MSD (Ignition parts)
(915) 855-7123

Published Dec 7th, 2015

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