Pemex Gasoline: Issues? – The Article

Use only approved containers for gasoline . . .
Does anyone else ever wonder about the quality of Mexico’s single brand of Pemex gasoline versus the USA’s plethora of brands?

As a combination car-guy / chemist, I have seen both the car problems and environmental problems that gasoline and gasoline stations can cause, but I never imagined that our local Pemex station would have such issues with sediment, rust, water, too much sulfur, “varnish” that causes deposits**, and …

Since Mexico has only one supplier, how can an ordinary driver make the most out of a limited field – and how might they find out if their local gas station is pumping out good stuff?

First things first: without a GC or GCMS, there’s no good way to find out exactly how much detergent your gasoline has (think: PEA or the older PBA), but insufficient amounts of detergents can cause so many problems** that the US EPA has set minimum levels of detergents required in gasoline: 40 C.F.R. § 80.161 Detergent additive certification program.

Since data on these sorts of things are tough to come by without your own laboratory, what can an ordinary person do to get an idea of the quality of your Mexican gasoline – and are there any simple rules of thumb that one can use to take the best care of their car’s fuel system?

Our first bit of advice: Buy your gasoline from stations that have relatively new storage tanks. Since most underground storage tanks (USTs) sit at or below the water table, (especially in the Yucatan), even with cathodic protection, steel tanks rust and leak over time ~ putting gunk like in the foto above right into your car’s tank. The USA had roughly 7 decades of using (rusting/leaking) buried steel tanks, so, a culture of home-spun wisdom (along with some old-wives tales) sprung up around where to buy gas, when to buy gas, what to avoid, fuel system driers, and fuel system cleaners.

Fuel Quality:
Even if you buy the most expensive gasoline – typically premium that has higher octane (burns more slowly) and lower sulfur – still, the spraying, evaporation, & combustion processes in a gasoline motor are dirty.
They all leave deposits** over time.
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These deposits gum up carburetors, gum up fuel injectors, and accumulate in combustion chambers. These problems combine to reduce performance, reduce acceleration, and reduce gas mileage. This means that peak performance is maintainedby regularly using a good fuel system cleaner. In the USA, even when buying “top tier” high quality gasoline, best performance is maintained by using a good fuel system cleaner every 7-10 (12 gal) fill-ups.

We have found that the first US fuel system cleaner approved by Mercedes and BMW is still one of the best: Chevron brand “Techron”. “Techron” is available at a number of Mexican parts stores – e.g. Autorrepuestos in Merida, but as with most quality items: Quality and high performance is not cheap. Techron typically costs $80 – $100 pesos per bottle that treats 12 gal of fuel. Cheaper cleaners simply do not work in our 40 years of experience. Techron cleans the tank, the fuel pump, the injectors or carburetor, the valves, the combustion chamber, and combustion sensors.

Quirks of Pemex Gasoline:
Unfortunately, after 21 years of driving part time in Mexico, and 6 years of full time driving with Mexican gasoline, it is clear that it takes a bottle of Techron about every 3-4 tankfuls to stop hesitations, maintain acceleration, and good gas mileage. Another quality of Mexican gasoline is that it “goes bad” faster than US gasoline. Unfortunately, gasoline does have a shelf life. “Old” gasoline has lost many of its lighter volatile components, so it does not ignite as easily (making it hard to start the lawn mower or trimmer, as you yank the rope, but have less “pop”). You can reduce this problem by storing gasoline in a vapor-tight container – (which is not the typical red can). Red plastic gas bottles can be sealed by placing a piece of flexible gasoline-resistant plastic over the neck of the bottle, and carefully screwing-on the cap/nozzle. Similarly, we should maintain the vapor-tight nature of our car’s fuel systems – replacing any leaking vapor return lines, gas cap’s seals, or failed vapor recovery canisters. If you hear a nice whoosh of escaping vapor when you open your gas cap, the system is likely still vapor-tight. If no there is no whoosh, then there might be a vapor loss problem.

There is another gasoline aging problem that even a vapor-tight containers and well maintained cars face: Fuel oxidizes due to contact with oxygen in air. Oxidized fuel has a distinctive “bad gas” odor that is funky and difficult to describe – but if you smell it once, it is unforgettable. There are 2 ways to avoid the oxidized fuel problem: either use the gasoline quickly enough that it does not have time to oxidize, or for stored gasoline like in mowers or trimmers, use a fuel stabilizer, like Stabil. A few ounces of Stabil is enough to keep gasoline from oxidizing for roughly a year.

Mexico’s version: For some reason unknown to us, Pemex gasoline oxidizes (goes bad) faster than US gasoline. US gasoline can typically sit in your car’s tank for up to 2 months without having problems. Here, we have had Pemex gasoline go bad in as little as 3 weeks to a month. Since I only drive my pickup truck about once a week or less, my solution is to buy only $200 pesos of fuel at each “fill-up”/gas stop – unless it is used for a long trip.

What’s up with that picture earlier in the article?
This is what I found in just one (1) liter of Pemex gasoline.
This liter of gasoline was pumped directly from a relatively new looking Pemex station in Merida. By putting the gasoline TEMPORARILY into a clear, clean, soda-pop bottle, you can see how much water – and how much sediment was in this 1 liter of Pemex Magna gas. For perspective, there was roughly 2 teaspoons of water in this liter – no Pemex truck was there filling the tanks – I bought the gas early during the cool of the morning, yada, yada, yada. Putting a some of your station’s gasoline into a clear flexible container can let you see what settles out, but:
Again, only use approved containers for gasoline . . .

I would guess that this station has been there at least 10 years, but was recently remodeled to look sparkly and new – so, the problem may be leaking USTs at the station, or the water and rust may be coming from some larger storage tanks used to fill the gasoline transport trucks?

All I know is that I am now a bit more eager to only buy Pemex gasoline from a station that really is only a year or 3 old.

Detail oriented readers may have noted that all the solid crud in the bottle (rust and dirt?) were isolated in the water layer. Since water is heavier than gasoline, water pools at the bottom of our car’s gas tanks – and it acts as a sediment filter – trapping the nasty solid stuff in the water layer, even when shaken. This provides a visual warning for 2 other items: (1) If you run your tank completely empty – you may be sucking in 2-3 years of accumulated sediment/rust laden water into your fuel filter and system. (2) If you dump a shot of alcohol (ethanol is preferable to not harm rubber or plastic fuel system parts) – like the gas dryer stuff people use in the USA, then you may release several years worth of accumulated rust/sediment into your fuel system in one shot.

In either case, this can explain why some savvy travelers keep a spare fuel filter (or 2) in their car when taking long trips. Cheap insurance?
~ for when buying gasoline at older roadside Pemex stations when traveling? ~

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**Sidelights on fuel system deposits:
Over the past few years shops have noticed that they are having more complaints about drive-ability issues including: poor fuel economy, hesitation, stalling, rough idle, hard starting, engine knock, and even misfiring.

These problems are typically caused by the buildup of “varnish” deposits in fuel injectors or throttle body, and by carbon deposits on the intake valves, head, and combustion chamber. These buildups are caused by reduced amounts of detergent & dispersants – which happens at the gasoline refiners.

This seems to be an issue with Mexican gasoline, but have been less frequent in the USA since the mid ’90’s, when the EPA forced refiners to add detergents to meet Additive Concentration requirements (see EPA reference above). When refiners and distributors add only minimal amounts of detergents, they save money, but it causes engine problems, increases emissions, and lowers gas mileage.

Modern cars have knock sensors and other computer controls, so they adjust fuel to air & engine timing to accommodate lower quality fuels. As the computer adjusts air-to-fuel rations and retards ignition timing to inhibit knocking, it simultaneously reduces horsepower, increases emissions, and reduces fuel mileage. Engines with higher compression ratios (like Mercedes, VW Passats, etc) do suffer physical damage if low quality gasoline is used over a longer time periods

Unfortunately, we have found ZERO recent government or private studies to determine fuel quality either in the USA or Mexico, so, we have no current data on the levels of detergent & additives that control deposits. Experience with Pemex gas says that its quality is not good – giving lower gas mileage even with clean well-maintained fuel systems.

Why Fuel Related Deposits Are a Problem:
Fuel varnish deposits accumulate inside fuel injectors, giving poor spray pattern, large fuel droplets that do not vaporize quickly, causing engines to run less much less efficiently. In addition to rough idle, you might experience hesitation, poor fuel economy and increased HC emissions. A lean fuel mixture also increases the risk of detonation and pre-ignition. Fuel system deposits in the motor tend to form during the “heat soak” period that occurs after a hot motor has been shut off. Shorter trips and more frequent trips cause these deposits to build up faster.

Deposits on intake valves restrict airflow through the intake ports, causing high speed power losses. The gunky deposits also act like sponges, temporarily absorbing fuel sprayed by the injectors. This “sponging” interferes with the mixing of air and fuel, causing a hesitation and reduced acceleration. Excessive deposits eventually cause valve sticking & valve burning. Typical intake valve deposits are formed by normal combustion, but they build up more rapidly if the valve guides or seals are worn and the engine is sucking oil down the guides. A single treatment with a good fuel system cleaner like Techron or BG can remove all these deposits – as noted by a blackening of engine oil after use of a good cleaner on a dirty motor.

Continuing on engine deposit problems: Deposits in the combustion chamber and on top of pistons can increase the engine compression and cause the motor to need higher octane fuel to maintain performance.
Carbon deposit build-up inside the combustion chamber also increases the risk of hot spots forming that can cause pre-ignitions. The hot spots ignite fuel before the spark plugs fire, causing increased combustion pressures. Modern cars have knock sensors to detect pre-detonation caused by deposits. The sensors detect the problems, which the ECM/ICM (computer) interprets and then retards spark timing – stopping any knocking without the driver noticing, but retarded timing also decreases acceleration, and increases fuel consumption & emissions – which attentive drivers may notice.
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Diesel Issues:
There are 3 types of diesel vehicle fuel sold in Mexico:   “Pemex Diesel (Anexo 9)”, “Diesel Marino Especial (Anexo 10)”, and “Pemex Diesel Bajo Azufre (Anexo 11)” (Low Sulfur Diesel), but many PEMEX stations only carry one type of Diesel.    e.g. In 2011, a Trailer Towing forum reported:
“All diesel fuel sold in Baja California is refined in the USA 15 ppm ULSD.”

But that only covers Baja California.    A further 2011 report on ULSD in Mexico describes: ‘Ultra Low Sulfur Diesel (ULSD) is the type of diesel fuel which has very low sulfur content. ULSD is more environmentally friendly as compared with Low Sulfur Diesel fuel (LSD). Starting with 2006, ULSD has been widely available across North America and Europe. This occurred because the countries from North American and Europe adopted new emission standards which involved extensive use of ULSD. Some countries from the rest of the world took similar measures while others lagged behind.

In 2004, a team of experts set up a plan which would substantially improve the quality of air in Mexico. The main recommendation of this plan was to have Mexico comply with the new emission standards already in use in many countries. Even though the Mexican environmental agency (SEMARNAT) stipulated that PEMEX should make ULSD widely available by 2009, it did not implement any new emission standards. As a result, PEMEX failed to meet the deadline, and ULSD can be purchased only in the northern border area, Mexico City, and Guadalajara.

Starting with 2007, all Diesel vehicles manufactured in US have been designed for ULSD. There has been a lot of debate over the impact of LSD on newer diesels that run on ULSD. Many experts and owners agree that LSD does not affect the performance of the vehicle. The real issue lies in the fact that a person might lose their warranty for some components in case they use LSD on a ULSD vehicle.

The efforts of the Mexican government to make ULSD available have met with limited success so far. Owners of vehicles which run on ULSD can purchase the fuel in Northern Baja. Unfortunately, PEMEX gas stations located in Southern Baja sell only LSD fuel brought from the mainland. To find the stations with ULSD, look for the words “bajo azufre”, which translated means “low-sulfur“, below the word “Diesel” on the gas station signage.”

Finally:
Since ULSD is not offered everywhere across Mexico, VW, Mercedes, and other cars with the  Adblue DPF system should only use the ULSD to avoid damaging the DPF system.  Back when the USA did not offer the ULSD, Volkswagon, Mercedes et al could not sell/operate their clean diesel DPF based systems in the USA.   These drivers need to keep an eye out for Low Sulfur Pemex signs:

Note that the RED sign posted here is for Premium GASOLINE that is ultra low in sulfur: “UBA”

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Final Thoughts:
I don’t think the fuel situation is overall that bad here.    There was this one problem described above for the one station’s gasoline – based on the foto – but I’ve been buying gasoline for the trimmer there for 6 years, storing it in the same transparent container, and this is the first time I’ve seen a problem. Will I buy gas from that station again? Lo no se…

I’ve pulled enough gas tanks and measured gas mileage enough times to know that our cars run significantly better with a good fuel system cleaner both here and NOB, but we do have to add the cleaner about twice as often here to keep fuel economy high and stop hesitations and stumbles.

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Feel free to copy while giving proper attribution: YucaLandia/Surviving Yucatan.
Steven M. Fry