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Fuel Injection System Servicing

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There are a few methods that can be utilized to clean fuel injectors however each method is dependent upon what you wish to accomplish.

Preventive Maintenance

Most fuel suppliers add fuel injector cleaning detergents (additives) to their fuels to help reduce the buildup of deposits in gasoline engines. Commonly used deposit-control additives include polysibutylamine, polyisbutylene succinimide and polyisobutylene phenylamine, but these same additives can also build up on intake valve stems causing them to stick. To prevent this from happening, additional additives called "fluidizers" must also be added to the fuel, but over time, these can contribute to the formation of combustion chamber deposits that raise compression and the engine's octane requirements.

One of the best additives is polyetheramine. It keeps injectors, valves and combustion chambers clean without the help of any additional fluidizers, but it costs more than twice as much as the other commonly used additives and is more likely to be found in the more expensive premium fuels.

How much additive does it take to provide an adequate level of protection? Industry sources say the recommended level is about 1,000 parts per million (ppm) of dispersant-detergent in the fuel, which costs the gasoline supplier less than a penny a gallon. Even so, as much as 85% of the gasoline that's being sold contains only one-tenth of the recommended dosage, or only 100 ppm of additive. Consequently, using inexpensive gas contributes to the formation of injector deposits.

Periodically adding a bottle of fuel injector cleaner to a tank of gas is good preventive maintenance, especially if you do a lot of slow speed driving, but often they are ineffective in cleaning deposits that have already set in. Never add more than the instructions suggest. The detergent agents in the cleaner are very concentrated and too much and or too frequently can cause rubber parts in the fuel system to fail.

If you do a lot of slow speed driving periodically take the vehicle out on the highway and accelerate (safely) hard several times. The increased fuel flow can help wash away light deposits.

Injector Deposit Origination & Formation

Where do the deposits come from? Mostly from the fuel itself - gasoline is a mixture of many different hydrocarbons, including oilfins, which are heavy, waxy compounds. The heavier the hydrocarbon, the more energy it yields when it burns. When the engine is shut off, the injectors undergo heat soak. Fuel residue in the injector nozzles evaporates, leaving the waxy oilfins behind. Because the engine is off, there is no cooling air flow through the ports and no fuel flow through the injectors to wash it away, so heat bakes the oilfins into hard varnish deposits. Over time, these deposits can build up and clog the injectors. On four-cylinder engines, the #2 and #3 injectors are in the hottest location and tend to clog up faster than the end injectors on cylinders #1 and #4. The hotter the location, the more vulnerable the injector is to clogging from heat soak.

Driveability Issues

The injectors probably need to be cleaned, if an engine is experiencing any of the classic symptoms of dirty injectors, such as lean misfire, rough idle, hesitation and stumbling on light acceleration, a loss of power, poor fuel economy, and higher hydrocarbon and carbon monoxide emissions

It doesn't take much of a restriction in an injector to lean out the fuel mixture. A restriction of only 8% to 10% in a single fuel injector can be enough to cause a misfire. In turbocharged engines, dirty injectors can have a dangerous leaning effect that may lead to engine-damaging detonation. When the engine is under boost and higher rpm's it needs all the fuel the injectors can deliver. If the injectors are dirty and can't keep up with the engine's demands, the fuel mixture will lean out, causing detonation to occur.

Fuel Injection Systems

TBI - throttle body injection; one or two injectors spray fuel into the air as it heads to the cylinders.

MPI - multiple-point injection; uses one fuel injector for each cylinder. Sprays fuel into the intake manifold, firing at the intake valves. These are smoother than TBI systems, and have more power and better mileage.

SMPI - sequential multiple-point injection; the injector only fires when the fuel can go straight through the valve and into the cylinder, instead of splashing onto a closed valve similar to multiple-point injection systems. SMPI systems are the most efficient, generating the most power and gas mileage.

PT Cruiser 2.4L Engine SMPI System

The PT's 2.4L engine utilizes a Sequential Multi-Port Electronic Fuel Injection system. The SMPI system is computer regulated and provides precise air/fuel ratios for all driving conditions. SMPI systems generally offer smoother performance, more power and improved fuel economy when compared with other fuel injection systems. The PCM operates the fuel injection system, and various sensors and switches provide the inputs necessary for the PCM to correctly operate these systems. The PCM can adapt its programming to meet changing operating conditions.

Fuel is injected into the intake port above the intake valve in precise metered amounts through electrically operated injectors. The PCM fires the injectors in a specific sequence. Under most operating conditions, the PCM maintains an air fuel ratio of 14.7 parts air to 1 part fuel by constantly adjusting injector pulse width.

The fuel injectors are 12v electrical solenoids. The injector contains a pintle (valve) that closes off an orifice at the nozzle end. When electric current is supplied to the injector, the armature and needle move a short distance against a spring, allowing fuel to flow out the orifice. Because the fuel is under high pressure, a fine spray is developed in the shape of a hollow cone or two streams. The spraying action atomizes the fuel, adding it to the air entering the combustion chamber.

In some instances, deposits on the tips of the injectors can change the desired fuel spray pattern, which causes the fuel to take longer to evaporate in the cylinder. When the fuel doesn't burn properly driveability problems can occur.

Service access includes a fuel test port and fuel supply tube quick disconnect fitting on the fuel rail , however we recommend that you have a service manual on hand since the fuel system must be depressurized (OE Fuel System Pressure Specs 400 kpa 34 kpa (58 psi 5 psi) before servicing. See the Fuel System Delivery guide in the Pit area for the fuel pressure release procedure.

Fuel Injection System Servicing

Although there are many different brands of fuel injection cleaning systems available, technicians report good results with MotorVac by Sun and also by BG. Although the MotorVac system is probably more thorough, the BG cleaning system also gets good results.

The benefits realized by injector cleaning obviously will vary depending on the condition of the injectors prior to cleaning and how badly they were clogged. Injectors that are really dirty should show more of a noticeable improvement in performance than ones that have only a light accumulation of deposits. Either way, performance, fuel economy and emissions should all be better after a cleaning.

Most high-mileage engines as well as engines that are used mostly for short trip stop-and-go driving are the most likely prospects for injector cleaning. Although DC does not include a service interval for cleaning the injectors, some experts recommend cleaning them every 25,000 to 30,000 miles to keep them flowing at peak efficiency.

The best injector cleaning is done at service shops. To check injector condition, a technician connects a fuel pressure gauge, momentarily energizes the fuel pump, and then operates each injector. The drop in pressure for each injector is measured. If they are all close or equal, everything is good. If there is a difference, the injectors need cleaning. Fuel injectors can be cleaned either on or off the vehicle, or when necessary replaced all together.

On-Vehicle Cleaning

One popular "On-Vehicle" cleaning method includes forcing injector cleaner directly through the injectors at a concentrated rate. This service can be performed by most repair shops and is usually only done when there is an injector-related driveability problem with the vehicle. The technician connects the injector cleaner and its supply tank directly to the fuel lines at the engine. All fuel return lines to the car's fuel tank are blocked so the cleaner goes only through the injectors. With this type of cleaning, the concentration of the cleaner is very strong, so injectors are cleaned quickly, usually in less than 20 minutes. It also has the advantage of cleaning some carbon off the backsides of the engine's intake valves. Carbon on the back of the valves soaks up or blocks fuel delivery into the cylinders, and rough idles or hesitations during acceleration result. Some experts recommend replacing the spark plugs after doing an on-vehicle injection cleaning. The residue that's loosened and washed away by the solvent may increase the risk of plug fouling.

There are some limitations with on-vehicle injector cleaning. Badly clogged injectors may not pass enough solvent during a normal cleaning cycle to be thoroughly cleaned. Some baked-on deposits can be very difficult to remove, which may require the technician to prolong or repeat the cleaning process. They may also have to conduct some additional tests to confirm that the injectors responded well enough to the cleaning treatment. A test drive may be needed to see if the driveability symptoms have been eliminated, or they may have to check emissions to make sure HC and CO levels are back to normal. A power balance test is another way to confirm engine performance and check for weak cylinders (there should be less than a 10% power variation between cylinders). An injector pressure drop test will tell them if the injectors are flowing evenly or not.

If this method of cleaning injectors doesn't work, there are two choices: replace the faulty injectors or have them removed and cleaned off the vehicle. Injectors that are really dirty may not respond well to on-car cleaning. You may have to use a more powerful solvent and/or longer cycle time to loosen the baked-on deposits. That's where off-vehicle injector cleaning equipment really pays off.

Off-Vehicle Cleaning

"Off-Vehicle" cleaning is done on equipment which is very expensive therefore very few repair shops have this equipment. Instead, they send out injectors requiring cleaning to a local specialist. The injectors are mounted on a machine and pulsed electrically while cleaning fluid is forced through the injector backwards. Reverse flushing the injectors provides an added measure of cleaning.

After the cleaning process, the injectors are typically mounted on a test manifold and energized to spray solvent into clear graduated cylinders. By comparing the volume of fuel delivered, it's easy to see if all the injectors are flowing evenly. As a rule, there should be less than 5% to 7% variation between injectors. If an injector isn't passing as much liquid as its companions, they can subject it to more cleaning. And, if it fails to respond to additional cleaning, there's no guesswork about which injector needs to be replaced. Flow-testing also allows you to compare the actual flow rate of each injector to OE specifications. If the flow is within specifications, you know the injector should perform properly when it is reinstalled back in the engine.

As you can see each method has its advantages and disadvantages. Not sure which method to use - check with your local DC dealer or repair shop to determine which method is most appropriate for your vehicle and or issue.

Additional Components to Clean

Another component that also may need to be cleaned to remove fuel varnish is the throttle body. Fuel vapor rising up through the intake manifold can accumulate and vaporize around the throttle plate and air bypass circuits, causing a change in the idle air/fuel mixture. Sometimes you can see the deposits, and sometimes you can't. Either way, cleaning the throttle body and intake tract also may be necessary to fully restore engine performance, idle quality and emissions. See the Throttle Body cleaning guide in the Pit area for additional information.


Fuel Injection System Servicing




Fuel Tank Additives

1. Easy to use.

2. Inexpensive. Under $5.

3. Used sparingly, good for preventive maintenance.

1. Possible harm to injectors, seals, O2 sensors, and catalytic converters due to the harsh chemicals.

2. Can't identify leaking injectors, weak springs, a poor spray pattern, or any other problems.

3. No way of accurately knowing if any or all the injectors have been cleaned or how well each one is operating.

On Vehicle Cleaning

1. Uses a higher concentration of chemicals, which results in a more effective cleaning when dealing with a driveability issue caused by dirty fuel injector. Good for cleaning the tops of intake valves.

1. The higher concentration of chemicals may attack rubber and plastic components in the fuel pump, regulator and fuel lines. Greater risk of harm to injectors and seals.

2. May have to perform additional tests to confirm that the injectors were thoroughly cleaned. Additional cleaning may be necessary.

3. No parts are usually replaced.

4. More expensive, about $50 - $150.

Off Vehicle Cleaning

1. Usually works better than on vehicle cleaning.

2. Each injector is usually tested for coil resistance, leaks, spray pattern, and flow rate.

3. Many shops conduct side-by-side testing for an accurate comparison.

4. Ultrasonic cleaning solution is safe and biodegradable. The cleaning is primarily done by the ultrasonic energy and not by the solution itself.

5. New parts are installed such as necessary.

1. Injectors have to be removed from the vehicle and taken in or sent to an injector shop.

2. A little more expensive, about $35-$50 per injector.

3. The injectors may still need to be replaced anyway.

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