Temporary loss of oomph...

[bxcfilm]

I have had my 2019 Jazz EX I-VTEC 1.3 manual for a year and this has happened 2 or 3 times so far, so I thought it might be worth asking.

This morning, I was on the M25 doing a steady 60-70 and the engine sound changes a bit, sounding more throaty. And the engine loses some power, with the change down light coming on at 60 in 6th. After a couple of minutes of this, the Range suddenly drops by maybe 20 miles. A while later the engine sound changes back to normal, with normal power restored, and a while after that the Range goes up again by about 20 miles. Bear in mind I'm cruising at a steady pace before, during and after these events, so there's no obvious reason for the Range to change significantly. No error / warning lights come on at any point.

Any ideas as to what this might be?

[ColinB]

You don't say which bit of the M25 you were on, but I suspect a hill might have been involved. 6th gear in the Jazz is really only OK on the level (or nearly so). Trying to maintain 70mph on any decent incline - and there are some on the M25 - and you need to drop down into 5th or even 4th to keep the speed up.

The variation in the range figure is probably a coincidence. I've seen it wobble up and down by more than 20 miles for no apparent reason, especially when the fuel gets low-ish so the gauge struggles to measure a level accurately as the fuel sloshes around in the wide flat tank. Or, if you’ve maintained speed by opening the throttle and/or changing down, then you’re using more fuel so you should expect the range to drop temporarily. Don't think that's anything to worry about (unless you're one of those who run the range right down to zero before filling up).

[Marco1979]

You could try the same stretch of road at the same gear and speed again and set your display to the current mpg. That is a good indication for engine load. You may see the load going up because of head wind or an incline. The noise indicades that the engine has to work hard.

In a certain gear, minimum mpg (maximum load) is fairly constant and you can see that minimum by flooring it 

[Lord Voltermore]

I think that some cars simply calculate   range based on approximately how much fuel is in the tank  using the fuel gauge reading. This is converted with a simple mathmatical calculation based on  a typical average  fuel consumption. .

Fuel gauges are not very accurate, and vary  (eg going up or down hill etc) and  fuel consumption varies a lot depending on driving conditions . (eg 80 mph  on the motorway guzzling fuel, or cruising gently at 40 mph. )

Many cars (inc the mk4) now constantly and accurately measure  how much petrol the engine is actually using, and will periodically revise the estimated range figure  if  driving conditions change  for better or worse.   

I dont know if the mk3 can also do this, but  if the car is struggling to climb a hill in 6th it will  be using a lot more fuel per mile than it was before.  Which could explain why it revised the range estimate .

[monkeydave]

have you had your fuel pump recall done ?

[bxcfilm]

have you had your fuel pump recall done ?

Yes.

[bxcfilm]

You don't say which bit of the M25 you were on, but I suspect a hill might have been involved. 6th gear in the Jazz is really only OK on the level (or nearly so).

J6 - J5, but I don't think that's relevant. I have found the car to be pleasantly eager for a 1.3. Most of the time..

I should have mentioned also the other glitch which happened yesterday. I was accelerating away from a roundabout on a stretch where I can normally get up to 5th quite easily. It's uphill initially, but not steep. Suddenly the car was sluggish, such that I had trouble maintaining 30 mph in 3rd. After maybe half a mile the problem cleared, and by then the road was slightly downhill. This sort of thing has happened maybe three times in the year that I've had the car.

[TnTkr]

Have you been able to check the fault code memory?

[Marco1979]

Maybe some dirt in the fuel tank or fuel filter that temporarily blocks the flow? Just guessing…