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Watch Out for Bad Gasoline

by Larry Carley copyright
Adapted from an article written by Larry Carley that appeared in Underhood Service magazine

gasoline quality

Gasoline quality is an ongoing concern for motorists. The quality of the fuel that goes into your car or truck depends on how the fuel was refined, the additives in the fuel, and the handling and storage of the fuel.

Gasoline must meet certain minimum octane requirements to burn normally without causing detonation (spark knock). Gasoline must also be formulated to meet regional air pollution requirements so it will burn cleanly. The fuel must also contain additives that prevent rust and corrosion inside the fuel system, and additives to keep the fuel injectors, intake valves and combustion chambers free of varnish and carbon deposits. If a fuel does not meet all of these criteria, it can cause problems in your vehicle.

The same rules apply to diesel fuels (including Biodiesel). Diesel fuel must meet cetane requirements for good combustion, contain the proper lubricants to protect the high pressure injection pump and fuel injectors, and additives to prevent rust and corrosion in the fuel system.

We continue to hear complaints about driveability issues that are typically caused by a build-up 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 may cause your Check Engine light to come on).


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 buildup.

* 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.

dirty intake valves
Intake valve deposits from gasoline that does not contain enough detergent

* Deposits that form on the intake valves can restrict airflow through the intake ports, causing a loss of high speed power. The deposits can also 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 can also cause valve sticking and valve burning. Intake valve deposits are formed by normal combustion byproducts, but may build up more rapidly if the valve guides or seals are worn and the engine is sucking oil down the guides.

Dirty intake valves are a common problem on late model engines with Direct Gasoline Injection (GDI) because the fuel is sprayed directly into the combustion chambers rather than the intake ports. Consequently, there is no fuel spraying on the backs of the intake valves to wash away deposits. Oil vapors that are siphoned back into the intake system via the PCV system can stick to the intake valves and create carbon deposits.

* 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 fuel's octane rating isn't 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 build up 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!

A condition known as Combustion Chamber Deposit Interference (CCDI) can also 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).


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.

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 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. Gasoline retailers must meet the high Top Tier standards with all their grades of gasoline (not just premium) to be designed as a Top Tier supplier. In addition, all the gasoline outlets carrying the brand of approved gasoline must also 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, Esso, ExxonMobil, Kwik Trip/Kwik Star, MFA Oil Company, Phillips, QuikTrip, Road Ranger, Shell, The Somerset Refinery and 76. For a complete listing of gasoline retailers who are selling Top Tier gasoline, Click Here.

top tier gasoline suppliers
Top Tier gasoline brands.


Unfortunately, fuel quality isn't 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 state's Department of Weights and Measures. Even so, the focus of most of these programs is to make sure consumers aren't 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!

Most gasoline refiners don't 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 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 they'll start to experience drivability problems.

Worse yet, if a bad batch of fuel leaves a refinery and ends up in people's 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 drivability problems may also 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. E85 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.


When a vehicle is experiencing deposit-related drivability, 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 drivability 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 can't 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 afterwards 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 3000 miles).


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 does 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.

Survey Finds People Often Buy The Cheapest Gas

According to a consumer survey by Phillips 66, 80 percent of young drivers claim to be knowledgeable about proper car care, but their behavior at the fuel pump says otherwise. The survey, taken by 1,018 American men and women age 21-30 who drive and are primary purchasers of gasoline, reveals that nearly half (49 percent) regularly fuel up with unbranded gasoline, despite the fact that it is known to cause engine deposit build-up over time that can inhibit gas mileage and engine performance.

While 84 percent of respondents say they are more likely to care about their car's engine performance than its appearance, one in four (25 percent) millennial drivers admits they are NOT putting the best gas into their cars for long-term auto health. Their reason? Price! They often buy the cheapest gas they can find regardless of its quality.

Survey findings prove that for more than half (54 percent) of those polled, cost is the most important factor when selecting which gas to use. In fact, 55 percent of respondents think the type of gas they use does matter for performance, but only 42 percent are likely to take the brand at the pump into consideration before purchasing. And while nearly half of respondents (41 percent) think additives signify quality gas, only three percent think this feature is key when selecting their gas.

The survey says that although most drivers care about their cars, they often buy the lowest priced gasoline to save money in the short-term, without considering the performance impacts the resulting engine build-up may be causing long-term.

TOP TIER Detergent Gasoline, like Phillips66 and other brands, include MORE detergent additives than is legally required to improve car performance almost immediately and minimize deposit formations in fuel injectors and the engine's intake valves.

According to the survey, 98 percent of millennial drivers who use inferior gas would upgrade, with 45 percent willing to switch if superior or branded gas increased gas mileage and 35 percent willing to make the change if they believed there was a difference in the quality of the gas. The problem is, many think all gasoline brands are the same -- which is NOT true!

Consumers may think they are saving money by seeking out cheaper gas. But what they may not realize is that the discount gasoline they are buying actually can cost them money in the long run by compromising their fuel economy and causing deposit build-up in their engine. By using a TOP TIER gasoline, motorists can keep their engines clean, maximize fuel economy and performance, and avoid the need for expensive fuel injector service down the road.

bad gas More Fuel Related Articles:

Fuel Contamination: What To Do If You Put The WRONG Fuel In Your Car

Gasoline Fuel Treatments

Gasoline Performance Problems

Fuel Octane Ratings & Recommendations

Troubleshooting & Cleaning Fuel Injectors

Alternative Fuels

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Related Links:

Top Tier Gasoline

Gasoline Quality Guide for Technicians (PDF file)

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