Throttle blade/bore deposits *are commonly overlooked *causes of driveability complaints and mysterious trouble codes, experts said. In most cases, no fault codes are present even though the engine runs poorly. The complaints often include: stalling-especially during deceleration or periods of high electrical or air conditioning load; rough or erratic idling; and off-idle hesitation.
These conditions are so common, many technicians now include a visual throttle blade/bore inspection with every engine diagnosis and periodic maintenance service.
In a good engine, some combustion gases always slip past or ``blow by'' the piston rings into the crankcase. These gases, which are mostly unburned fuel, are called blow-by. The engine purges blow-by through the positive crankcase ventilation system and burns it.
Meanwhile, blow-by vapors carry oil droplets into the intake system. The combination of oil, unburned fuel and other contaminants in the blow-by accumulate on the throttle blade and throttle bore, inside the idle air bypass passage and on vacuum ports in the throttle housing near the throttle blade. Initially, engine heat bakes the contaminants into a gummy residue, then into a varnish technicians often call ``coke.'' Eventually, these deposits may turn into carbon.
Remember that some engine and intake system designs are more prone to throttle deposit problems than others. What's more, poor maintenance increases the chances of deposits occurring on any fuel-injected engine. This happens because inadequate maintenance usually increases the amount of blow-by an engine generates.
Throttle deposits affect performance in five major ways. First, varnish, gum and carbon deposits may alter and restrict normal low-speed air flow into the intake.
Second, these deposits may cause a serious air leak by preventing the throttle blade from closing when the driver takes his foot off the gas pedal. Then the engine control computer tries adjusting idle speed to compensate, causing symptoms such as rough idling, hesitation and stalling.
Although the resulting idle speed variations are usually large enough to annoy the driver, they may not be large enough to trigger diagnostic fault codes in the engine control computer.
When the throttle blade closes normally, the engine computer expects to see an appropriate ``closed-throttle'' voltage signal from the throttle position sensor (TPS) and/or a throttle switch. Throttle deposits that hold the throttle blade open confuse the computer's decision-making process further by disrupting or eliminating the closed-throttle signal.
Third, deposits may cause stalling by restricting the idle air bypass passage that routes air around the throttle blade. Typically, an engine control computer adjusts idle speed by regulating the volume of air flowing through the bypass channel. An electric valve, usually called an idle air valve, controls air flow through the passage.
A restricted idle air bypass could trigger a trouble code by forcing the computer to open the idle air valve more than and longer than normal to prevent the engine from stalling.
Fourth, coke/carbon deposits may hamper vacuum-operated components by clogging vacuum ports near the throttle blade or inside the intake plenum. Restricted vacuum ports can affect devices such as the power brake booster, distributor vacuum advance diaphragm and the transmission vacuum modulator.
Fifth, deposits can dampen throttle response by insulating the air charge temperature (ACT) sensor. The ACT sensor, which often threads directly into the intake plenum, senses the temperature of air entering the engine and cues the computer to tailor the air/fuel mixture accordingly. (Note that the ACT sensor's influence on mixture varies from system to system.)
For example, when the driver floors the gas pedal, the engine inhales a slug of cooler, denser air. Inputs from several sensors-
including the ACT-should cue the computer to richen the mixture. Driveability specialists reported that a badly coked ACT sensor cannot detect changes in intake air temperature.
Overall, visual inspection is still the most reliable, accurate way to identify harmful throttle deposits. On more-advanced engine control systems, scan tool readouts of throttle angle, air flow etc. may point to the problem.
But many service-age vehicles afflicted with throttle deposits lack the sophisticated data stream (information output) that provides such information. Or, a specialized, vehicle-specific scan tool is required to access this test data.
Typically, it takes a minute or two to remove the intake air duct and visually inspect the throttle assembly and intake plenum with a small flashlight, a compact, flexible inspection light or a borescope. Driveability specialists urged
technicians to inspect and clean the throttle assembly before taking other diagnostic steps. A thorough cleaning eliminates the throttle-
related symptoms described earlier.
``Usually, looking over the throttle blade and housing only takes a minute. So it's silly to skip over it,'' one technician commented.
The condition may be as obvious as a brown or black ring of deposits inside the throttle bore. But experience shows a harmless-looking film of varnish on the throttle blade also can cause driveability problems. Clean the throttle assembly when the engine's cold. Working on a hot engine is counterproductive because the heat tends to evaporate cleaning chemicals too quickly. Using a can of spray cleaner fitted with a spray nozzle is helpful because the nozzle directs the spray exactly where you want it. Allow the cleaner to soak into the deposits for at least 10 minutes.
Often, you have to scrub off the coke and carbon with a brush. Several automotive tool companies offer soft-bristle brass brushes designed to remove throttle deposits quickly and safely. Some technicians make do with common bottle brushes, old toothbrushes and/or non-scratching pads such as Scotch Brite pads.
Carefully scrub off both sides and the edges of the throttle blade as well as the throttle bore area near the throttle blade. Resoak and rinse with spray cleaner as needed.
Patiently flush contaminants from the idle air bypass passage. Depending upon the throttle housing design and the severity of the contamination, you may be able to clean this passage effectively on the vehicle. However, experience confirms that removing the throttle housing assembly is the only way to clean some idle air passages.
Some techs routinely remove vacuum hoses from vacuum ports in the throttle housing, inspect the ports for carbon and flush them with spray cleaner as needed.
Be careful not to get spray cleaner in the throttle position sensor (TPS) or the idle air motor. When in doubt, removing these parts beforehand is the safest approach.
After decoking the throttle blade and bore, you may have to reset the TPS adjustment anyway. At the very least, always check the TPS and minimum idle air readings after cleaning the entire throttle assembly.
When you have to desludge an idle air motor pintle, always hold the unit with the pintle pointing downward. This way, the spray cleaner runs off the pintle-away from the motor.
Meanwhile, manufacturers such as Wynn's offer flexible wands that enable the technician to spray cleaning solvent deep into the crevices of a badly sludged intake plenum.
Several chemical companies make spray cleaners designed specifically for decoking throttle assemblies. But ordinary carb/choke spray cleaner is usually adequate for the job.
Some specialized cleaners carry traces of lubricant for the benefit of the throttle shaft and throttle shaft bushings.
Several cleaner precautions are in order. First, never dip a throttle assembly into a cold caustic cleaner such as traditional carburetor cleaner. Avoid dipping the assembly into mineral spirits-type cleaning solvents.
Before choosing a spray cleaner, ask other technicians what they recommend. Ask if the brand they use cleans effectively without harming plastic and rubber parts such as O-rings, grommets, bushings and TPS housings.
Some automakers discourage use of spray cleaners containing MEK (Methyethylketone) because the substance is abusive to plastic and rubber parts. But small concentrations of MEK improve the cleaner's performance and won't harm plastic and rubber, according to chemist Dennis Rosson, senior engineer and technical service manager at BG Products Inc., a Wichita, Kan.-based chemical maker.
Mr. Rosson said that when in doubt, a simple bench test will determine if the spray cleaner is abusive. Collect some common, spare rubber and plastic underhood parts. Put the parts into a suitable container and fill the container with enough spray cleaner to cover the parts. Quickly and securely cap the container and soak the parts for a while.
The cleaner is not harmful if the parts don't swell, soften up, or distort when you remove them from the container, Mr. Rosson said.
Later-model Ford throttle assemblies carry ``DO NOT CLEAN'' warning labels because powerful spray cleaners may dissolve the coke- and varnish-resistant plastic coating on these assemblies. However, experience shows that some high-quality spray cleaners will not harm this plastic coating. Ask the chemical supplier if his cleaner is ``Ford-safe.''
Mr. Rosson emphasized that when all is said and done, technicians usually must rely on the track record of the chemical maker.