Lately, it seems technicians have been hearing more complaints about driveability issues that are typically caused by a buildup of varnish deposits in the fuel injectors and throttle body, and carbon deposits on the intake valves and in combustion chambers. These problems include engine knock after cold start or when the engine is working hard under load, hesitation problems when accelerating, rough idle, poor fuel economy and even misfiring (which often turns on the Malfunction Indicator Lamp).

Detrimental Deposits
When deposit control additives are not adequate in gasoline, harmful deposits can build up inside the engine:

Fuel varnish deposits that form inside the injectors restrict fuel delivery and cause the engine to run lean. This may cause lean misfire, rough idle, hesitation, poor fuel economy and increased HC emissions. A lean fuel mixture also increases the risk of detonation and preignition. These deposits tend to form during the heat soak period that occurs after the engine is shut off. The shorter the trips and the more frequent the drive cycles, the faster these deposits build up.



Deposits that form in the throttle body can reduce airflow through the idle bypass circuit affecting idle quality and smoothness. These deposits are formed by fuel vapors that rise up through the intake manifold.

Deposits that form on the intake valves can restrict airflow through the intake ports, causing a loss of high speed power. The deposits also can act like a sponge and momentarily soak up fuel spray from the injectors. This disrupts the mixing of air and fuel causing a lean fuel condition, hesitation and reduced performance. Deposits also can cause valve sticking and valve burning. Intake valve deposits are formed by normal combustion by-products, but may build up more rapidly if the valve guides or seals are worn and the engine is sucking oil down the guides.

Deposits that form inside the combustion chamber and on top of the pistons increases the compression ratio of the engine and the octane requirements of the fuel. Too much compression can cause spark knock (detonation) if the fuels octane rating isnt high enough. Over time, detonation can damage the head gasket, piston rings and rod bearings if it is not controlled. The knock sensor will detect detonation and tell the PCM to retard spark timing. This will take care of the knock, but retarded timing also increases fuel consumption and emissions.

A buildup of carbon deposits inside the combustion chamber also increases the risk of hot spots forming that may cause engine-damaging preignition. The hot spot ignites the fuel before the spark plug fires, causing a sharp rise in combustion pressure. Under extreme conditions (high rpm and load), preignition can burn a hole right through the top of a piston!

Bad Case of CCDI
A condition known as Combustion Chamber Deposit Interference (CCDI) also can occur when the carbon deposits are so thick the deposits on the piston and head make physical contact. This area, known as the Squish Area (piston to top of chamber), has a clearance that is about as thick as a paper clip. This can cause a loud, metallic banging sound when a cold engine is first started. The deposits are soft and will gradually flake off. But the flakes may lodge between the valves and seats causing a loss of compression, misfiring and rough running when the engine is cold (a condition called Combustion Chamber Deposit Flaking or CCDF).

Deposit Control
The formation of harmful deposits can be controlled by adding detergent-dispersants to gasoline, the most common of which is polybutene succinimide. Used with a petroleum carrier oil, detergent-dispersants help keep the intake manifold and ports clean. These chemicals are more effective than the carburetor detergents that were once used in gasoline, but they must be used at concentrations that are three to five times higher than the older carburetor detergents.

Deposit control additives, such as polybutene amine (PBA), were introduced in 1970 to help keep injectors and intake valves clean. The only drawback with PBA is that too much of it can increase combustion chamber deposits. Polyether amine (PEA), by comparison, cleans fuel injectors and valves, and does not increase combustion chamber deposits. In fact, it helps remove accumulated deposits inside the combustion chamber to reduce the risk of spark knock.

Top Tier Gas Makers
Gasoline retailers must meet the high Top Tier standards with all their grades of gasoline (not just premium) to be designated a Top Tier supplier. In addition, all the gasoline outlets carrying the brand of approved gasoline also must meet the same standards. Gasoline retailers who are currently on the Top Tier list include Chevron, Chevron-Canada (B.C. only), Texaco (Chevron-supplied only), Conoco, Entec Stations, Kwik Trip/Kwik Star, MFA Oil Company, Phillips, QuikTrip, Shell, The Somerset Refinery and 76.

Gas test
It should be noted that we have not found any recent studies by the government or anybody else that addresses the issue of fuel quality or whether the level of detergent and deposit control additives in gasoline is sufficient to keep todays engines clean.

Unfortunately, fuel quality isnt something that is easily policed. Many states have programs in place to monitor fuel quality on either an ongoing basis or incident specific basis. Most are run by the states Department of Weights and Measures. Even so, the focus of most of these programs is to make sure consumers arent being cheated at the pump and get the full gallon they pay for. Some programs also check fuels to make sure they do not contain too much alcohol. The specific density of gasoline can be field tested to determine its volatility and alcohol content. But testing octane and the amount and type of additives in the fuel requires expensive laboratory testing. So this type of quality testing is rarely done.

According to one leading gasoline retailer (who sells a Tier One fuel, by the way), many gasoline marketers have reduced the concentration of fuel additives in their fuel up to 50% in recent years!

Foul Fuel
Most gasoline refiners dont want to sell the public bad gas because they obviously want repeat customers. Even so, they also know that deposit formation is a gradual thing that occurs over time. So if they cut back on the additive package to save a few cents per gallon, nobody is the wiser and least not right away.

The problem occurs when people buy the cheapest LAC (see below) gas they can find every time they fill their tank. The low level of additives (or low quality additives) in the fuel will not be adequate to keep their engine clean, and sooner or later theyll start to experience driveability problems.

Worse yet, if a bad batch of fuel leaves a refinery and ends up in peoples vehicles, it can cause even more serious problems. There have been instances where too much residual sulfur in a bad batch of gasoline has caused a rash of fuel pump failures.

Immediate driveability problems also may occur if the fuel is contaminated with water, contains too much alcohol or the wrong type of alcohol (methanol instead of ethanol). Alcohol is a great octane booster, but for ordinary gasoline the amount of ethanol should not exceed 10% (or 5% for methanol). The only exception here is E85 fuel for flex-fuel vehicles that is 85% ethanol and 15% gasoline. G85 contains less energy per gallon than gasoline and runs leaner, so the vehicle must have a special fuel sensor so the PCM can compensate for the alcohol to maintain the proper air/fuel ratio.

Flushing Out Deposits
When a vehicle is experiencing deposit-related driveability, performance or emissions problems, the deposits obviously have to go. The troublesome deposits can be removed a variety of ways. One cost-effective solution to deposit-related driveability issues is to simply add a can of fuel system cleaner to the fuel tank. The cleaner will slowly remove the deposits while the vehicle is driven. The only drawback with this approach is that it takes time maybe one or two tankfulls with the additive to make a noticeable difference. That may be too long for some people.

For those who want a more immediate fix, the cure usually consists of cleaning the throttle body with an aerosol cleaner, flushing the injectors with a concentrated solvent or cleaning product, and/or feeding an intake system cleaner of some type into the engine while it is running to clean the intake ports, valves and combustion chamber.

If injectors fail to respond to on-car cleaning, they can be removed for more thorough off-car cleaning on special equipment or replaced if they are clogged and cant be cleaned. For heavy carbon deposits inside the combustion chamber, a top cleaner product may be added to the engine and allowed to soak for 15 to 20 minutes to loosen the deposits. An oil change afterward is recommended because some of the cleaner will end up in the crankcase.

For extremely stubborn intake valve and combustion chamber deposits, blasting the valves through the spark plug holes or intake ports with a soft media such as walnut shells is an easier fix than pulling and disassembling the cylinder head for hand cleaning.

To prevent the formation of new deposits once the engine has been cleaned, you should recommend using a top tier gasoline or adding a bottle of fuel system cleaner periodically to the fuel tank (say every four or five fill-ups or when changing the oil every 3,000 miles).

Fuel Cleaning Additives
One very important point to keep in mind about aftermarket fuel system cleaners that are added to the gas tank is that they use different chemistries to achieve different results. As we said earlier, some chemicals, such as PBA, can clean injectors and valves, but may actually increase combustion chamber deposits. Other chemicals, such as PEA, can clean the entire fuel system as well as the combustion chambers.

For dirty fuel systems, you should use a product that is concentrated enough to provide some real cleaning action. But for maintenance, you only need a product that keeps deposits from forming.

One new fuel system cleaning product that has been recently introduced that claims to do something no other product does: it actually cleans and protects the contacts on fuel gauge sending units. The contacts on the sending unit are typically plated with silver-palladium to resist corrosion. But over time, residual sulfur in gasoline can corrode the contacts causing the gauge to rear erratically or not at all. Replacing the sending unit is an expensive job because you have to drop the fuel tank, so a more affordable alternative is to simply add a bottle of this product to the tank and let it take care of the corrosion.

DID YOU KNOW… In 1995, the U.S. Environmental Protection Agency set minimum standards for additives in gasoline to prevent the formation of deposits in fuel injectors. Gasoline refiners had to certify that their additive packages met these standards, but some experts now say the original standards were set too low and do not provide adequate protection with some fuels and engines. The minimum EPA-required level is referred to as the Lowest Additive Concentration (LAC), and is typically found in the cheapest priced gasoline. At the other end of the fuel quality spectrum are Top Tier gasolines. These fuels are recognized by the vehicle manufacturers as having the most effective additives and in the highest concentrations.