Poor fuel economy? *Please read this!* (the *definitive* poor fuel economy topic)

[mjf65]

I do not think tyres or fuel type can cause this bad fuel consumption. You can check if you have a brake depress offset by feeling the temperature of all your rims when stopping after a drive - it should all be at the same temperature and relatively cold. When the brake pedal is not properly adjusted and a slight break command is sensed, one or more wheels may continuously have a slight brake effect while driving (hardly noticeable), causing heat and the rim to heat up. This is the easy try-yourself stuff.
This sounds like the problem with the wifes Jazz. I've noticed the mpg has been poor for a while and the pads are well worn down but still legal at 30k. I drove the car back from work tonight without any heavy breaking and the front discs were too hot to touch. My wife arrived in my Golf and they were much cooler? If the brake pedal applying unwanted breaking force my problem how do I adjust the pedal to sort it out myself rather than the dealer doing it for me?

Thanks for the info.

[79stef]

Not sure whether or not you still have the car but I was just reading through old posts and wanted to add what someone else had already pointed out. Have you checked the EGR valve is working properly. Having fixed mine (very easy fix), and since spoken to Honda, high fuel consumption is one of the results of a faulty valve. Worth a try

[DV]

Not sure whether or not you still have the car but I was just reading through old posts and wanted to add what someone else had already pointed out. Have you checked the EGR valve is working properly. Having fixed mine (very easy fix), and since spoken to Honda, low fuel consumption is one of the results of a faulty valve. Worth a try

I`m experiencing the same after the EGR valve replaced, better fuel consumption.

[guest1844]

Is it possible the engine is not warming up fully, ie a faulty thermostat?
I notice that when the blue light goes out the car still runs on a fast idle of about 1000rpm. It's another 5 minutes or so before you get the silent 750rpm. I'm guessing the blue light goes off at about 60C. That's the trouble with having a light - you don't know if its working correctly, and you can't see any trends or changes of behaviour.
I suppose you could add a temperature gauge, but these days you're probably better to get an OBDII reader instead, and see what the computer thinks it is.

[SuperCNJ]

Hi All,

I need some advice.

About 4 months ago the MPG on the Jazz suddenly dropped from an average of about 45-50mpg down to barely 33mpg. I understand that the cold weather has had an impact on it as the ECU compensates with more fuel and the use of the heater etc. But I would have thought that at most this would reduce it by 5-10mpg. We've seen the dash MPG reading up to about 65mpg before, though I do take it with a pinch of salt.

I just don't understand it. There are no obvious faults with the car, all seems to be in order, we use Shell normal unleaded and it has new tyres. The only thing I can think of is that I have a carbon T1-R cold air induction filter on it. It hasn't caused any problems before and having had a look around it, nothing obvious seems to be blocking it.

Any ideas?

[guest1521]

Check brakes aren't binding.

After running a mile or two without stop-start road conditions, one wheel warmer to touch - on the steel or alloy face of the wheel - than the others can be a clear and EASY clue. Binding=friction=heat. Just be aware that normally both fronts will be warmer than rears since three-quarters of braking force is on the front.

Or jack and check each wheel, see if each turns freely and easily by hand.

[SuperCNJ]

Thanks, I did suspect that before. As I had a seized caliper on my Civic which would roll to a stop quite abruptly but the Jazz doesn't seem to have this problem. I also changed the rear pads a month ago and there didn't appear to be a problem.

[chrisc]

Tyre pressure?  Wheel alignment?  Faulty mixture sensor?  Diagnosis via an OBD machine should point you in the right direction, but it sounds like faulty combustion to me

[olduser1]

I would try and identify what changed around the time of the increase in fuel.

[chrisc]

It might even be a stiff ankle..

[SuperCNJ]

Definitely not a heavy right foot since the missus does most of the driving!

Nothing was changed when the drop happened.

The only thing I can think of might be that the T1-R Cold Air Filter I installed a long time ago may need cleaning?

I checked the oil level last night which was fine.

Would having the heater on knock 10+mpg off your average?!?

[guest1521]

"Would having the heater on knock 10+mpg off your average?!?" Nope.

Clogged/very dirty air filter?

Thermostat not fully closed while engine is warming up? And so running richer for longer after cold starts.

A faulty coolant temperature sensor? It'd make it run richer, p'raps without a dash light appearing. 

Bad oxygen sensor? (But a dash light would most likely appear.)

Partly clogged exhaust from a crumbling catalytic converter? (Dash light'd likely appear)

If auto gearbox... not changing all the way up to top gear?

[chrisc]

When you do diagnose and fix the fault, please be sure to let us know, since it would be a pointer to preventative maintenance and an easier fix.


btw:  My wife is a bit heavier than me and tends to take off quite quickly in her car, so doesn't get nearly so good petrol consumption as miserly me.   (She never reads the forum so I am quite safe)

[Kenneve]

It seems to me that it is simply down to the weather and type of driving. My normal consumption is usually anything from 58 down to 45ish depending on type of driving.
Currently it is -2 degrees outside and this afternoon the average consumption reading was down to 36.3, mostly short local journeys.

[guest1844]

It might be the thermostat is stuck - the engine warms up enough to put the blue light out but not right up to temp. You can partly tell by the idle speed. With all electrical load off, it should idle about 750rpm. If it's not quite warmed up it will be a bit faster.
Best way is to get an OBD II code reader that also reads live engine parameters. One of them will be coolant temp. I think £50 should get you a decent one.