I've been monitoring my fuel usage in my Automotive Wolf software for almost two years.

In the last few days I've been parking outside rather than in my garage. Today I found out that my usage went from 16.5l/100km to 19.5l/100km.

I was dragging around a towbar which I've since removed but other than that nothing has changed.

Could it be that parking outside in summer is making me lose as much as 1/2 litre a day in evaporation? Or could it be fuel leaks from the angle that the car is parked on (it is up a small but steep concreted incline)? Perhaps a combination?

Curioso

a guess

that is possible
does the petrol cap suck or blow air like a hissing sound
when you undo it to fill up

I thought the fuel system was sealed

maybe try a new petrol cap

I have read an artical "as the motor gets hotter it less efficient by a much as 15%"

THere is a lot of sucking and hissing like releasing an airlock when you remove the cap, but how does that translate to excess fuel evaporation? Does that mean the cap isn't sealing properly and letting out more fuel?

if you get that reaction the cap is ok

so dont know what is causing the fuel loss

I'll keep the outside parking to less than 30 minutes a day and see if that changes anything. I doubt 30 minutes would do much as the petrol tank would only just be starting to warm up by then. First time in almost two years of recording every fuel purchase and its Litres/100km rate that I've got anywhere near 19l/100km.

a faulty purge control solenoid

Not sure which purge control valve solenoid you mean but I've got a carby TP and there is one of those attached to the charcoal cannister. But in my case, the charcoal cannister was disconnected years ago from the carby because it was diagnosed as being used up or full and therefore unable to "breathe" or whatever it's meant to do.

Had the carby analysed for running rich today by a fuel system guy - it turns out it was. Probably a float thing that he's going to fix next week as he has to pull the whole thing apart and I just sprung this on him yesterday so he didn't have time.

The carbie guy fixed it - he had to unblock the jets and reset the choke. The float was ok.

I think it might have been me spraying lanolin lube around the area to stop corrosion that caused it but he thought it was due to using cheap (91 octane) fuel. He recommended using premium high octane stuff.

Does anyone think putting the high temperature burning fuel in a car this old (1990) is a bad idea? I'm thinking it would be better to use 95 octane in a 50/50 mix every month or so, as I don't want to burn the brass jets with Premium or Vortex or whatever they call the high-octane fuel that burns at a much higher temperature.

Curioso

On my TP auto carby I only ever used (91 octane) fuel

I could not use ethonol 10%

I think your idea of going 50 /50 on 95 octane
every month or so is worth a try

see if the fuel consumption figure improves

My fuel consumption rate has already dropped to 15.02 litres per 100km from 18.25-19.69 litres per 100km since late November. My true rate will no doubt plummet further than that as this 15l/100km rate only had three days of driving with a fixed carburettor. A full tank on a fixed carbie will probably be around 13 litres. I'll post it when I get the data.

I've been hovering between 14.72 and 15.97 litres per 100km since the last reading of 15.02. While that's a bit disappointing, it has been peak summer since then, necessitating running the aircon almost constantly, plus extra fuel evaporation. So I expect that to drop as it gets less hot.

On the plus side, with 280km of highway driving, the rate plunges to 12.18 litres per 100km. I got over 460km on one tank with around 280km of that on the highway. I haven't had that kind of mileage per tank since the car was about 10 years old, so that's not pad for a 20 year old car.

Thats not bad , i get around 664 Km to a tank on straight driving , i think my tanks bigger tho. Not bad in city either . I think its doin pretty well yur car for its age ,

Mine's 64 litres, how much is yours?

68.5 liters , the non AWD models have 72 and 75 liter tanks here , mostly in part because of the non silencers on the exhaust system that is near the tanks