As fuel prices are rising I have been taking a greater interest in my fuel consumption and have been looking at the instant fuel consumption readout.

When I am cruising downhill between 40 - 80 miles an hour when I lift my foot off the accelerator I notice the readout goes something like this :- 37.3 43.7, 54.6, 63.7, 70.2, 68.6, 73.7, 19.9,14.5,19.9, 70.2 etc.

Is it normal for the fuel consumption to suddenly drop like this? It can also seem to happen on the level but there is not such a large fall in consumption for apparently no reason.

Any ideas what could be causing this?

 

Under high vacuum no load pulse width to the injectors is just about cut off and depending on the speed youll see numbers up to 99mpg...normal
take a picture and tell everyone you installed a water to hydrogran brown gas convertor and look at the mileage you get. (i saw a guy last time $4/gal hit and he was trying to see those systems)

 

Haha, in NYC stop-and-go mine is usually fluctuates between 7-8mpg and 50-60, typically hovering in the 11-16 range!

 

I use the average fuel option as it gives me a better idea of what the car is doing. Instant is a bit too quick changing.

 

Hi Brutal

I understand why the mpg would appear high but what I don't understand is why with no accelerator it should suddenly drop to say 15 mpg and lower on occasions

 

Quite true. However, the instant readout is a good tool for new owners to train their right foot.

 

probobly cause the ecu will control rpm and pulse width for condition like power steering need, trans downshifting/upshifting, ac, etc and blip rpm up or increase pulse width momentarily. I really wouldnt put too much thought into it. Those just like ext temp are calculated algorithymes and not always exact.

 

At least the instant mpg on these cars aren't space consuming and pointless like those in BMWs. I never understood why they have that gauge. All it does is constantly move left and right while reading on a scale that is faaarrrr from linear. If you have driven a BMW, you will know exactly what I'm talking about.

I guess a gauge that can move more rapidly than a tachometer is kinda cool?

 

Was your A/C on (or defroster, or even in Auto mode)? The compressor cycles on and off, and the ECM will increase idle fuel supply slightly when it cycles on - at the initial turn on load it could drop the instant fuel consumption briefly.

 

I have an XJRV8 (2000). I only get 16mpg or so in mixed driving and I have noticed exactly the same fuel wastage when off the throttle going down hill.

I have no obvious engine issues. I had an o2 sensor replaced recently but poor economy persists.

Did you ever get to the bottom of what was the cause?

 

Probably the ECM is programmed to inject fuel occasionally to maintain temperature in the cats, as they can produce rotten egg smell in that situation.
Many years ago, when cats were first fitted to cars, residents who lived at the bottom of a very long downhill road in Sydney, Australia, complained of a constant smell of rotten eggs. Seems that car manufacturers switched off fuel injection on the overrun, to achieve better fuel economy. Cool cats= rotten egg smell. Legislation was passed to worsen the fuel economy of every car sold in Oz.