Low on fuel? Don’t wait for the warning light – Part 2

Posted on February 3rd, 2021 by Rob Marshall

Low on fuel? Don’t wait for the warning light - Part 2

Following GEM’s earlier blog on the road safety issues associated with low fuel levels (https://blog.motoringassist.com/news/low-on-fuel-dont-wait-for-the-warning-light/), Rob Marshall examines some of the technical implications:


While running a low tank of fuel can make your car more economical, if anything, due to a reduction in weight, it could cost you in the longer term.


One common issue is that the bottom of fuel tanks can harbour detritus, which risk being be sucked-up by the fuel pump especially when the level drops to only a few gallons. While this is less of a problem now that most cars’ fuel tanks are made from plastic rather than steel, and do not corrode, small contaminants can still enter the tank during refuelling.


Condensation can build within the tank during winter months, the resultant water sinks to the bottom of the tank. Additionally, the ethanol element within petrol absorbs moisture and, once it reaches a critical point, the ethanol/water gloop separates from the fuel and drops to bottom of the tank. In short, whether dirt or incombustible gunk, you do not want them entering your engine.


The fuel pump(s) can also suffer. Petrol possesses a lubrication quality and so, should the tank be run excessively low, the fuel pump could draw-in air, which starves it of lubrication and damages it internally.


While they are affected by similar issues, modern diesel cars have an extra complication. Their DPFs (diesel particulate filters) tend to be emptied of soot, by the engine dosing fuel into the exhaust system, thus raising the temperatures and burning off the soot accumulation. However, this regeneration cleaning procedure will not occur unless certain parameters are met, one of which is an adequate fuel level. Therefore, should you run your fuel level down on purpose, the filter may not be able to clean itself and you increase the risk of particulate filter blockage.