Avgas fuel colour

[john ball]

Why is it that sometimes when I drain the fuel from one tank it is a very different colour blue than the other tank. Also it has a slightly sticky feel when it is a much darker colour around the fuel drain ?

On a slightly different note --- would AVGAS do any harm to the fuel system on a vintage/classic car that used to run on 4 star ?

[Pilot DAR]

The Avgas blue should be fairly consistant. It's colour intensity is regulated as an element of the ATSM or other standard to which is must comply. It might appear more intense where there is a concentration, as some has evaporated, but sticky should not be happening at all, unless some other liquid has been dissolved in the Avgas and is now included.

The high areomatic in Avgas. which were not in "four star" would probably be quite harmful to the fuel system of an older car. You'd have "rubbers" in the fule system getting really punky. I would not do that!

[Ultranomad]

The dark and sticky thing could be a sign of fuel seepage, when minor quantities of fuel keep evaporating, gradually leading to an accumulation of the dye.

Regarding avgas in a vintage car, keep in mind that 100LL typically contains a lot more lead than leaded automotive fuels of the olden days (roughly 2x if I remember correctly), so watch out for excessive valve deposits.

[A and C]

Most of the Vintage car & bike people add Avgas to about 15-25% of the total fuel load, most do this to provide the lead to lubricate the valves without risking the unwanted deposits refered to above.

A few of the higher performance bikes & cars like Avgas at up to 50%, these are usualy the later vintage types, the E-Type Jag and Triumph T150/T160 spring to mind.

[Mariner9]

The dye used in Avgas is 1,4 dialkylamino-anthraquinone. While the colour is regulated in ASTM 910 and Defstan 91-90 specs, there is quite a large range (lovibond colour 1.7 to 3.5) allowed, therefore colour variation is not unusual.

The sticky residue around your fuel drain is not concentrated dye, it will be "gum" which is the all encompassing term for oxidation by-products. The presence of gum may signify a "weeping" drain so it would be worth getting them looked at given the price of avgas!

M9 :8

[john ball]

Thanks to you all for the usual informed answers.

Yes, there is a very slight weep from the drain -- not actually dripping just a bit wet to touch, so I will investigate. Firstly, by trying pushing up and turning to let it seat in a different possition.

[Pilot DAR]

I vote that M9's is the best answer.

I used to attend the ASTM D910 meetins, along with the D439. I recall that discussions about dyes. In particular, there was a proposal in one of the other groups to dye off road diesel blue. I explained that would be a remarkably bad idea, as there was probably off roard diesel around airports, and it would wind up being pumped into an airplane. I think it was stopped.

If turning your sump drain is stopping the weaping, it's time for a new O ring in that drain. It's not a big job. Some (particularly older) O rings swell in 100LL over time.

[Jodelman]

would AVGAS do any harm to the fuel system on a vintage/classic car that used to run on 4 star ?

Dunno!

But it would be illegal in the UK. Whilst Avgas costs more than Mogas, there is less tax on it and the nice people in the tax office don't like it if you use it in your motor car.

[ShyTorque]

I use a tuned up old engine with a very high CR in a car for off-road stuff. It was designed to run on 4 star, so some long time ago I looked up the amounts of TEL (tetra-ethyl lead) in each of the two fuels.

100LL AVGAS still contains about five times as much TEL as 4 star did. I have used a gallon or so to boost the octane rating of a tankful, one gallon in five of unleaded. The engine liked it, it seemed smoother at high rpm.

What was noticeable afterwards was a white-grey film in the exhaust, presumably the lead compounds produced by combustion.