Poor fuel economy

Hi all, my new Jimny (2000 1.3 petrol) seems to be doing quite badly on fuel economy. It barely scraped through the MoT on emissions, with the engine needing to be held at high revs to pass. We discovered that the downstream O2 sensor was not in the exhaust, just a bung. We have now refitted the downstream O2 sensor, but there hasn't really been any improvement. 

The car is mainly used for short trips on country roads, but we have also tested the MPG on longer motorway runs. I will list some of the recent UK MPG figures below, calculated brim-to-brim.

29.5
30.9
28.1 (first with refitted O2 sensor)
30.2
28.5​
​​​​​​
The last two were after motorway runs, so should be better than the others. 

What are some common causes of poor MPG? Are there any sensors that can be replaced to improve this? There aren't any warning lights showing, but my first thought would be to replace both O2 sensors (and the cat, as the thread is ruined). The brakes don't seem to be dragging, and there isn't a fuel leak. Thanks!

Main causes of poor MPG are:

short trips and going too fast. The engine isn't warmed up properly and uses more fuel, gearbox & axle oils are more viscous and drag a little more when cold, a Jimny has the aerodynamics of a brick so higher speed drinks fuel.

Blocked air cleaner, or someone messing with the intake and restricting the flow.

Blocked exhaust, the manifold catalyst breaks up and blocks the main catalyst, restricting gas flow, or it may be dented flat.

Pre-cat oxygen sensor, these go out of spec. as they age and the engine runs rich- shows up as poor MPG and low lambda reading at MOT

Sparkplugs, leads, coilpacks a missfire will lead to less power, so a heavier foot and lower MPG.

(You've covered fuel leaks and binding brakes already)


The post-cat sensor doesn't have anything to do with fueling, it checks the catalyst is working because the oxygen content should be less than that at the pre-cat sensor (which does control the mixture)

Dangling in fresh air it ought to have put a light on for emissions.

Tbh, low 30s is about right for an early Jimny doing short trips.

I was shocked when I got mine, it did less MPG than my Land Rover HiCap- but the heater worked, and I could fit it onto my drive. (Part time 4wd was a bit of a backwards step too...)

 

You didn’t mention what tyres you had on or your average speed?

When I had a 2011 Gen 3 I used to drive at about 50 - 55mph, but I did have 215/75 AT tyres which did reduce fuel economy. Overall my Gen 3 was in good condition, it was serviced regularly and I replaced, plugs, air filter, oil, etc regularly.

I drove my Gen 3 carefully, but at this time of the year I didn’t get much more than 32mpg, so 30mpg for your 2000 could be about correct.

My 2018 will happily return 32 to 36 but it's significantly newer is running standard tyre sizes and has been serviced meticulously it's entire life. Keeping it well serviced and standard is going to be the best way to get the best mpg.

Whoops! I completely forgot about tyres, having the correct size road tyres correctly inflated gives the best MPG.

Anything that makes a rumbling pattering noise as you go along tarmac wastes fuel (but will be very useful off road in the correct environment)

Anything oversize will work the engine harder and use more fuel, the speedometer error will also make it appear to be travelling less distance than it actually does so making the apparent loss of MPG even more pronounced.

Those are very normal figures for an early gen3. There's a strong possibility you don't have any issues.