What’s In The Tank?

Jan. 1, 2020
This past decade has seen automotive technology changing at an exponential rate. One change that often misses the attention of most of us involved in drivability diagnostics is the fuel itself.

Changes in gas-powered engines have been the norm each year since Nikolaus Otto patented his internal combustion engines in the late 1800s. This past decade has seen automotive technology changing at an exponential rate. One change that often misses the attention of most of us involved in drivability diagnostics is the fuel itself.

Crude Oil — Where It All Begins
Crude oil starts out as a substance made up almost entirely of hydrocarbons. Furnaces sitting next to fractionating towers heat the cru to more than 700 degrees. The hydrocarbons begin to boil as the lighter components vaporize. As these lighter vapors rise, they condense on tray like platforms at various heights. Some of the lightest (fewer carbon molecules) products such as propane come off the top of the distillation tower while the heavier substances like motor oil are condensed and captured at lower levels in the tower.

Conventional carbon cleaning may not be effective on GDI engines, and require the application of a cleaning agent directly to the back of the valve.

Over the years, as engines have become more refined, so has the process of refining crude oil. A process called cracking was developed to break longer carbon string molecules into the shorter six to 10 carbon molecules found in gasoline. Cracking utilizes high temperatures and pressures to accomplish this method of getting more gasoline out of a barrel of oil. The process raised the octane level as well.

Octane — Then and Now
Gasoline actually is a pretty complicated liquid fuel. Since 1995, the EPA has required all gasoline sold to have detergents blended in. Other additives include anti-icing agents, anti-oxidants, anti-wear agents, metal deactivators, corrosion inhibitors and oil soluble dyes to identify grades. The bulk of gasoline can be brought down to two basic concepts: octane and heptane.

Octane is an 8-carbon molecule and is defined as the fuel’s ability to resist knock. Knock is the irregular ignition of the air/fuel mixture in the cylinder. Normal combustion begins with the spark plug’s spark event and spreads out in a uniform manner. During knock/detonation, hot spots of carbon in the cylinder can become other sources of ignition which then in turn causes irregular flame fronts. The end result is a cylinder pressure (and temperature) that spikes up to the levels that result in damage to combustion chamber components such as pistons, valves and spark plugs. In a compression ignited (CI) diesel engine, the valves and pistons are designed for a more explosive combustion process but that’s not the case in the average spark ignited (SI) gas engine.

Another negative effect of detonation comes with the expanding gasses wanting to send a rising piston back down instead of allowing it to achieve Top Dead Center (TDC) and have a power stroke that actually produces power. On top of that, one of the major contributors to smog, oxides of nitrogen (NOx), is produced when detonation occurs. When you hear your engine knock(s), you are making NOx.

Replacing a fuel pump? Never a bad idea to perform a post-repair autopsy to be sure you know why it died. Here, debris is clearly visible.

The other major concept of gasoline, heptane, is a seven-carbon molecule that ignites very easily – just the opposite of octane. In the 1920s, chemists developed a scale that rated Octane at 100 and Heptane at 0. We still use that scale. For example, if you select 87 Octane at the pump you are getting 87 percent Octane and 13 percent Heptane along with a few other chemicals with varying carbon lengths.

Octane enhancers have been around since the early 1920s with the most time honored being tetraethyl lead from where we got the slogan “fill it up with Ethyl” and the term “leaded” fuel. The lead would attach to the molecules of gasoline and help resist knock. It also was a good lubricatant for valve seats.

By the early 1970s, lead was discovered to be a health hazard. Lead poisoning of catalytic converters by the mid-70s and oxygen sensors by the early 80s brought leaded fuel to a near novelty with an ultimate EPA total ban by 1995. Although more advanced forms of cracking and reformulation in the refinement process helped to raise gasoline’s octane ratings, something had to replace lead as an octane booster. Methyl tertiary butyl ether (MTBE) did the trick but became an environmental problem. Not only is it a carcinogenic, it mixes so well with water. Leaks in underground fuel tanks can contaminate drinking water wells miles away. This has led to a planned phase-out of MTBE along with billions of dollars in lawsuits and cleanup costs. 25 states have banned the use of MTBE. Ethanol has proven to be a suitable alternative. Many industry insiders are saying E-15 might become the new MTBE.

Though the differences in fuel have become minimal in recent years, octane behavior can be varied. Do you recall seeing those R + M / 2 stickers on the fuel pump? Its the formula chemists use to determine actual octane of gasoline and is an average of two methods of testing octane. The Research Octane Number (RON) method (R for short) measures the octane of the fuel in a test engine with a variable compression ratio under controlled conditions. The Motor Octane Number (MON) method (M for short) is a more precise method that is tested in an engine under a load with variable ignition timing. MON will run eight to 10 points lower than RON.

Unless the car is certified as a flex fuel vehicle, adding E85 can be just as bad as diesel to a gasoline-powered engine.

Which Octane to Recommend?
When considering which octane to use, always advise your customers to purchase the octane required by the owner’s manual. Compression ratios, variable valve timing, engine temperatures and other factors dictate what octane an engine requires. If 92 octane is more expensive than 87, would it make sense that it would burn better, contain more energy or get better mileage? The answer is a qualified no. On the contrary, all things equal, using a higher octane fuel than required simply wastes money.

I’ll qualify the statement “all things equal.” Fuel sold at higher altitudes, Denver for example, runs a point or two below fuel sold at lower altitudes for each grade. The reason is at higher altitudes, the air/fuel mixture will not burn as rapidly (less dense air), therefore knock is less of an issue. If the detergents are at the same level between regular and premium grade fuel, (usually premium grades have more detergents) the engine’s carbon deposits are minimal, the engine’s cooling system is working to specs and the spark curves and knock sensor hardware/software was properly developed by the OEM, an engine will not get any more power or economy with premium fuel compared to regular.

However, if one of those variables is making the engine move towards the detonation threshold causing the PCM to reduce power/economy with ignition timing retardation, you might see increased performance and economy with premium fuel. On the other hand, if the vehicle’s spark timing strategy is designed for accommodating multiple octanes, you certainly will see an improvement with premium fuel.

Ever had a customer add the wrong fuel to the tank?

How does that work? Ford is an example of one OEM that doesn’t set a spark curve in stone and backs it off when knock is detected. Some Ford engines will increase spark advance up to the level of a knock threshold and then back it back a tad. Put in 87 octane and you will get a spark curve that complements that. Put in 92 octane and you get a more enhanced spark curve.

Not all fuel is created equal. The engine in your customer’s car may run slightly better on one of those combinations than another. It’s rare but it happens. Most of the time the difference is a placebo effect (only in their mind), but who are we to argue with a customer who likes XYZ fuel? If premium won’t do it, there are plenty of aftermarket additives that enhance octane.

My recommendation is to emphasize the “easy does it” approach to all fuel additives. While there are some good additives we swear by not every single one of them is beneficial. Some are just plain harmful, many are warranty killers and most are horribly damaging if overused. Octane booster may contain Toluene, alcohol or any number of chemicals that can swell elastomers, emulsify fuel tank plating (creating rust and lead poisoning of cats and oxygen sensors from the tern plating) and cause corrosion throughout the fuel system. The same goes for any other additives such as fuel line anti-freeze and fuel system cleaners. My guess is we will never beat the “if one bottle is good, two will be twice as good” lifelong belief system our customers sometimes have.

Top Tier Explained
Audi, BWM, GM, Honda, Mercedes-Benz, Toyota and Volkswagen recommend top-tier detergent gasoline. Those OEMs have stated a common belief that the EPA minimum detergent requirements don’t go far enough to keep engines clean. The past 18 years of EPA-mandated detergent requirements have seen a slow but sure decrease in many fuel companies’ detergent concentrations by up to 50 percent in some cases.

E85 might be cheaper, but odds are you’ll use more of it. E85 has less energy per gallon than gasoline and requires a richer air/fuel ratio.

Here are some of the top-tier fuel retailers: 76 Stations, Aloha Petroleum, BP, Chevron, Conoco, Country Mark, Entec Stations, Exxon, Hawaii Fueling Network (HFN), Holiday Station Stores Inc., Kwik Trip/Kwik Star, MFA Oil Co., Mileage Stations, Mobil, Ohana Fuels, Phillips 66, Quik Trip, Road Ranger, Scheirl Oil, Shell, Texaco, Tri-Par Oil Co. and U.S. Oil.

Detergents are changing as well. Consider the early 90s prior to the chemical detergent Techron coming into the market. Fuel injectors were becoming clogged, so fuel retailers put in detergents to clean up the injectors. The trouble with that was some of the detergents being used in part caused deposits to form on the backs of the intake valves on PFI and SFI engines. There were other sources of valve deposits – namely condensed (and carbonized) crankcase vapors and EGR. These deposits were hard when the engine was warm but soft enough on a cold engine to absorb some of the fuel mixture (like a sponge) on cold starts. This resulted in hard cold starts, rough/hesitating engines when cold and stalling issues.

The solution was a method of cleaning dating back to pre-WWII aircraft engines that experienced a similar problem. Several automakers devised a cleaning system using a finely ground up media of walnut shells that was blasted at the backs of closed intake valves via compressed air. The cleaning unit then used a vacuum to pull out any residual blast media out of the head area prior to reinstalling the fuel injectors.

Get additional additives (usually detergents) right at the pump.

Today most fuel retailers make great mention of how their fuel cleans dirty intake valves. Kroger even has an extra piece of hardware on some of their gas pumps to “boost mileage.” Simply press a button for how much better you want your fuel economy to be and anywhere from $2 to $9 will be added to your credit card as you pump their gas and the extra additive “Super Mileage Booster,” which simply is a carbon fighting detergent.

Now fast forward 20 years to Gas Direct Injection. (GDI) GDI doesn’t use an injector pointed at an intake valve. Rather it situates the injector directly in the cylinder. The cleanest fuel in the world might not keep valves clean in an engine with GDI. The valves still will get some residual fuel spray inside the engine along with the EGR and PCV deposits. Early adaptations of GDI have encountered numerous complaints of fuel economy and performance degradation. This condition can occur as early as a few thousand miles after leaving the dealer’s showroom floor.

Due to the softer and stickier nature of these valve deposits, the old walnut shell blaster is not recommended – the soft media will stick in the deposits rather than blow them off. BG Products has reported some success with their repair shop customers using BG 44K and BG MOA as a preventive maintenance product. They also report limited success at reducing deposits that have already formed on the backs of the intake valves with one of their chemicals that is poured into the head against a closed valve (upper intake plenum removed).

Who has this tool in their toolbox?

Diagnosing Valve Deposits
If you suspect severe valve deposits you have two diagnostic options. The first option is a peek with a bore scope. Prices on these handy little jewels have come down into affordable ranges in the past few years. Your second option would be a little less intrusive but requires a little research if you haven’t done it before – Volumetric Efficiency (VE) testing. Google the procedure; all you need is a scan tool and a VE calculator which can be found on-line or included in ATS’s E-Scan. The E-Scan is a PC based scan tool that performs several unique drivability tests including VE testing. If your VE is confirmed to be lower than specified due to mechanical conditions, it might be time to turn some wrenches to have a look at the intake valves. They might have enough carbon on them to impede the engine’s ability to breath. More recent engine adaptations of GDI have seen redesigned PCV systems, internal EGR via variable valve timing and more careful injection timing to prevent sticky intake valve deposits.

Oxygenates
The Clean Air Act of 1990, which brought us OBDII, further changed the composition of gasoline requiring up to 65 different blends for various regional considerations such as air pollution levels and seasonal conditions. Five separate EPA regulations outline specific requirements for motor gasoline: (1) limitations on lead-based antiknock agents, (2) mandated detergent additives, (3) limitations on the Reid Vapor Pressure, (4) mandated oxygen content, and (5) the content of reformulated gasoline (RFG).

The two most common oxygenates are methyl tertiary butyl ether (MTBE) and ethanol. In order to achieve a 2.7 percent required baseline, refiners or distributors must either blend in 15 percent MTBE, derived from natural gas, or 8 percent Ethanol, derived from renewable feedstocks such as corn stover or cellulose. With MTBE almost put out to pasture (as mentioned previously) that leaves ethanol in either ETBE form (Ethyl Tertiary Butyl Ether) or E-10 (gasohol) as the popular oxygenate for regions requiring oxygenated fuel. Expect minor fuel economy reductions with oxygenated fuel.

Alcohol
Alcohol in the tank of a stock vehicle not designed as a flex fuel vehicle is a disaster. That’s why OEMs for years have limited the alcohol in non-flex fuel vehicles to 10 percent ethanol (alcohol produced from grain) and 5 percent methanol. (Alcohol produced from wood byproducts) Gasoline is powerful; one gallon of gasoline provides the equivalent energy of 32.000 watt hours of electricity. That’s enough to run a color TV for a month.

Ethanol is powerful too. Just not as powerful. Gallon for gallon, gasoline provides, on average (numbers vary with seasonal RVP changes), about 125,000 BTUs of energy. E-85 (85 percent ethanol/15 percent gasoline) only provides 80,000 BTUs of energy. That means it could take you 1.5 gallons of fuel to go the same distance 1 gallon of gasoline will take you. On the plus side, every gallon of ethanol produced reduces American’s dependence on foreign oil, helps farmers and reduces air pollution.

OK, I know! We can’t get them to read the owner’s manual, let alone the print on the gas cap.

Even more important than economy, if the fuel system hasn’t been designed for alcohol various elastomers can fail along with internal tern (lead solder) plating in tanks that are of the old steel design. Fuel pump failures are common. Current pump gas is limited to E-10 but in the works are plans to plunge the nation into the world of E-15. That extra 5 percent ethanol content doesn’t sound like much but it does make a difference. Vehicles built prior to 2001 are likely to have problems of alcohol induced damage to their fuel systems and are exempting from using E-15. Owners of historic cars and motorcycles are naturally up in arms over the move. The president and CEO of the American Automobile Association (AAA) Robert Darbelnet made news speaking out about the problems of E-15 last fall.

We’ve all had this scenario in our shops, asking the customer owning the no start/really bad running vehicle, “Why did you pump E-85 in your non-flex fuel vehicle?” The truthful answers? “It’s cheaper.” “I thought it wouldn’t hurt.” “It was an accident.” Sometimes we don’t get truthful answers. Sometimes the tanker refueling the retailer screws up. Diesel goes in the gasoline in-ground tank or E-85 goes in the 87 octane tank. How can you tell if your customer’s non-flex fuel vehicle has been dosed with E-85? The answer might be simpler than you think.

Contaminated Fuel Diagnosis 101
The wrong fuel won’t burn well or at all in an engine. Period. Your sense of smell is your quickest test. Next is your scan tool. Fuel trim numbers will be positive (if the engine even runs), indicating there needs to be more fuel due to the fact (in this scenario) the fuel in the rail isn’t allowing for good combustion. Lean codes might be present. The engine might have enough fuel trim adaptation to actually run decent or could lean out so much at wide open throttle (WOT) that something bad happens inside the cylinder. Some flex fuel vehicles use a sensor in the fuel line that monitors the fuel’s alcohol content. The PCM reads this percentage and adjusts the fuel injector on time to compensate for the less powerful fuel.

Other flex fuel vehicles monitor fuel trim changes when the fuel tank’s sending unit indicates there has been fuel added. If the trim goes up and the only thing that happened (in the PCM’s mind) is fuel being added, there must be alcohol in the fuel. On that note, if a flex fuel vehicle in your shop gets a PCM re-flash, the old learned alcohol percentage resets to 0. You either need to drain the tank and put in straight gasoline or test the alcohol content yourself and reset the PCM to that percentage with a scan tool. Finally, there are tests for fuel problems. You could send a sample to a chemist but that takes time and lots of money.

Here’s how you test for water contamination and/or alcohol content.

1. Retrieve a measure of fuel from the effected vehicle and allow it to settle for 15 minutes.

2. If a separation line is present there is water present in the fuel.

3. Pour 8 units (cc, ounces, ml whatever unit you like) of sampled gas in a container accurately marked.

4. Add 2 units of water to the sample. Apply lid and shake. Allow to settle for 15 minutes.

5. If the fuel sampled is straight gasoline a separation line will form at the 2 unit mark.

6. If the fuel separation line is above the 2 units mark, there is alcohol in the fuel. The difference of where the separation line should be and where it is gets divided by the total volume of what you are testing and that becomes your alcohol content percentage. Easy enough?

Suspect alcohol in the fuel? Put your ohmmeter leads in it. Any reading indicates alcohol, since it is conductive while pure gasoline is dielectric.

Alcohol absorbs water and alcohol will mix with gasoline so it almost appears the alcohol gets rid of the water. It doesn’t; it just suspends it. This essentially is how gas line anti-freeze works. Comprised of isopropyl alcohol, the fuel line anti-freeze prevents water from accumulating at the bottom of the tank by suspending it and allowing it to flow with the gasoline / alcohol mix to the engine to be burned.

Fuel Volatility
The fuel’s ability to stay in a liquid state when being pumped to the engine and vaporize into a fine mist suitable for reliable and complete combustion is measured by its Reid Vapor Pressure. Winter fuels need a more volatile fuel to atomize at lower temperatures (higher RVP closer to 15 PSI) while summer fuels need more stability at higher temps to prevent vapor lock (lower RVP of 7.4-9 PSI).

The EPA mandates the numbers with adjustments made for not only the season, but warmer and cooler regions. And yes, today’s engines can vapor lock just like an old carbureted engine although it is rare. I had the privilege to relive this old problem with a 2010 Chevy Equinox with a 2.4 GDI engine. After a hot soak during most conditions but the coldest of winter days the engine would stall or hesitate in a manner that emulated vapor lock – the old condition where the fuel boils somewhere between the tank and the carburetor. Bosch made the system, so I asked one of their drivability instructors about it.

He confirmed my suspicion: fuel was boiling in the rail on a hot soak on some of these models. I’ve also encountered the opposite RVP issue – crank / won’t start cold conditions where the previous shop threw everything but the kitchen sink at the engine to no avail. Good spark, good fuel pressure, good spray pattern and volume, good compression and valve timing. Ought to start! The customer was a farmer who kept both gas and diesel tanks on the farm and still was using summer gas into the fall. When the car was cold, the low RVP fuel wouldn’t atomize, thereby causing a no start. While the recipe for gasoline has changed over the years, the occasional repair shop ritual of “change the fuel – fix the car” is a recipe that will never change. 

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