Bad Batch of Fuel - Symptoms & How to Remedy


Last weekend, i came across a situation that has rarely happened to me over the last 5 years of karting, which, upon further reflection, prompted me to speak about it in the hopes of helping others on here which could run into it at some point or another, and i figured sharing some wisdom could help in avoiding potentially catastrophic failure on your engine.

I ran into a bad batch of fuel.

Preface: This happened in Lonato, Italy, over the course of Sunday the 14th of Jan.

As most of you know, the vast majority of “amateur” karting (ergo, outside of International Events) in the EU is done on pump fuel. More often than not, an organizer will designate a fuel station nearby the track within its competition regulations, take a sample of 98 octane fuel on the Friday of the race and use it as a baseline for fuel testing procedures. It is as such common practice for us to run 98 pump fuel during practice days as well, which is of course more economical but exposes you to the possibility of running into a product that is contaminated in one way or another, as the quality of the fuel depends on the amount of usage that the pumps receive (if there is a lot of traffic, the tanks will be filled more often and will as such provide fresh fuel more often than not), along with the reputability of the distributor itself.

Most of the time, when fuel is “contaminated”, it is by a too high amount of water / ethanol, outside of the range specified by the supplier (for reference, most 98 octane pumps provide fuel at 5% ethanol, at least in France / Italy).

The problems / symptoms of an over - ethanolized fuel are:

  • The lack of capacity for the fuel to complete proper combustion and maintain a flame front which is fast enough to combust the mixture at/close TDC.
  • Due to the above, unusually high EGT readings, especially the low EGT readings.
  • EGT readings which do not react to carburation shifts : The logical approach is to richen the mixture when encountering abnormally high temps. With bad fuel, the richening of the mixture will more often than not have no impact on EGT values.

All of this leading to detonation, as the low combustion speed leads to sporadic combustion after TDC (which can be more or less severe). In my case, my piston looked like swiss cheese after two sessions.

This is very serious and can happen very fast. as the next step, if not remedied, is the piston beginning to disintegrate and throw aluminum pieces in your bottom end, potentially grenading your beloved engine.

The main takeaway of all of this to avoid issues is:

  • Always keep an eye on your EGT temps, after every session.
  • Check your piston surface after every two sessions. Not only does it help you with carburation, but it will also warn you of such a scenario.
    -If you notice EGT values not moving even after big shifts in carburation, do not use the fuel in your jug anymore.
    -If you notice heavy detonation, stop running immediately, and proceed with a piston change.

Hope this helps :slight_smile:


Thanks for sharing…just wondering if you noticed driveability issues? Your checklist makes sense but wondering if there was an obvious, seat of the pants feeling too?

If you have the means, you can pour water into the fuel & it will bind the ethanol & sink to the bottom of the container, water being heavier than gas. Then just extract the water-ethanol part by whatever means (eg, pump, invert container & drain out the water-bound portion). Keep in mind, this is only practical with a translucent container.

Thanks for sharing, and glad you were able to catch it before big boom.

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You Can make a very basic test to the fuel to find out how much Ethanol there is In the fuel.
Some 98oktane Can have 10% ethanol In it, and i would not be surprised if the specification says min 5%.

Ethanol itself is very high-octane (109 RON) and blends to substantially increase fuel octane rating. It’s a key component of why eastern premium pump gas is 99-101 RON these days.

However, it leans out the mixture significantly, as it only needs a 10:1 air-fuel ratio rather than nearly 15:1. Going from 0% ethanol leaded race gas to 10% ethanol requires 8-10 “numbers” of jet size increase as you go to 3% oxygen in the fuel.

If you find yourself going lean with a higher-ethanol blend, just increase your main jet size and enjoy the additional power. If you find excess spark plug erosion, go to a colder heat range. Passenger cars now use heat range 7 or 8 plugs rather than the 9 or 10 they used to.

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It would seem to me the OP presents two issues. The ethanol concentration and the ability of ethanol to absorb water. As Charles points out ethanol requires more fuel to not lean out. The EGT would show this in increased temps. If it was water that was the issue wouldn’t that lower EGT temps? I would think if it was water the driver would feel that in the performance too.

Hey rob,

Performance wise, it is difficult to tell as the day started off with the bad fuel, so that became my baseline reference. It is possible that i would have felt some difference if i had transitioned from good to bad fuel during the day.

From my understanding, the main issue with contaminated fuel is the lack of capacity for it to burn at the expected sufficiently fast speed when compressed and closed to TDC. The slowing of the flame front creates the possibility for the creation of pockets of isolated, unburnt and atomized fuel which tend to combust sporadically towards the decent of the power stroke, AKA detonation which leads to piston failure. (in my case, it happened within the span of 2 sessions bearing in mind i increased the main jet size from 180 to 185 upon being alarmed by the EGT after session 1)

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@Charles_Kaneb is spot on. As alcohol % increases, you must deliver more mass of fuel to maintain balanced combustion. If the same mass flow of fuel is present, the mixture will become more lean as alcohol content is increased. The higher temps observed were the result of a lean condition.

Running pure alcohol requires larger jets and maybe even fuel pumps to deliver MORE fuel. Doing so will bring temperatures down. Engines that run pure methanol (alcohol) can run higher compression ratios, burn cooler, and can therefore produce more power in the same displacement. Alcohol does present some challenges with lubricants and elastomers, so you have limitations for oil and seal/o-ring materials.

Obviously changing jets is not a solution for a single bad batch of fuel. :woozy_face:

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