Fuel efficiency bad - need help solving this problem

[Poida]

I seem to be getting nowhere near the mileage that is generally expected by the miata, I'm averaging 15l/100km (250-300km a tank), and that's with shifts no greater than 3.5k and driving quite conservatively.

What are the main parts associated with causing my car to run so thirsty, so I can replace them if necessary.

Are you able to point me in the direction of how I can go about fixing this issue?

I know the miata isn't the most economical car, but it seems everybody else is getting marginally more out of their tank than me.

I've changed spark plugs, fuel filter, air filter so far, any ideas on what's next?

Side note: I have noticed that when given considerable throttle, I hear what sounds like a spluttering type noise that comes along with exhaust noise, like liquid/air escaping, could this be related?


Should I get it looked over by a mechanic who specialises in the MX-5? Can anyone recommend me a good one near Essendon area?

I've got an NA6.

Thanks
Peter

[Guran]

Yes that's extremely high consumption. My NA6 averages around 10L/100km and the car is basically only used to drive to/from the track where it then gets dríven extremely hard (flat throttle and every shift at >7000rpm). Part of your issue could be that by shifting below 3500rpm, you're always labouring the engine. Keep the revs between 3500 and 6000 and it'll be much happier. But there does sound like there's a bigger issue - impossible to diagnose with limited info provided. Definitely recommend you take your MX-5 (Miatas are only in the USA) to a mechanic ... any mechanic. But if were to hazard a guess it would be plug leads.

[elitee]

Plugs
Fuel filter

That is where I would start

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[Steampunk]

Check that the oxygen sensor is installed properly in the exhaust.
The electrical plug is a single wire plug (usually black) right next to the plug for the coil pack (usually grey).

If you are handy, replace the O2 sensor and go through a couple of tanks of fuel to ascertain a difference.

Have you checked the computer to see if there's anything aftermarket that's affecting fuel?

A faulty water/coolant temp sensor can also cause poor economy.

When you say you shift around 3,500RPM how much throttle are you giving it?

[project.r.racing]

O2 sensor
Coolent temp sensor
Low coolent level.

It will be one of these.

Whats the temp guage needle sit at when running normally? just left of middle?

[93_Clubman]

Peter, a few questions:
Have you base-line serviced it (ie full service), or do you have proof of a recent full service?
What is the service history like, if any?
How are you calculating fuel consumption?
The NA6 does run fairly rich, & oxygen & coolant temp sensors are pretty much a consumable, which when they go will increase fuel consumption.
Max Kirwin Mazda @ Preston - the service manager is/ was a MX5 Club Vic member. Or a useful service list from de bounce if you want to DIY:

To sum it up in a list and tick it off as they are done.

Replace timing belt, tensioners, cam and crank seals, water pump, hoses and possibly thermostat.
Check/replace the front drive belts
Replace spark plugs
Replace air cleaner
Replace fuel filter
Replace engine oil and oil filter
Replace power steering fluid
Check engine and gearbox mounts
Check radiator top tank and hoses
Replace the coolant
Replace gearbox oil (make sure you undo the correct filler plug on the passenger side)
Replace the diff oil
Check/replace gearbox upper and lower boots
Check the brake pads
Grease the caliper slide pins
Adjust hand brake
Replace brake fluid
Check the front wheel bearings/hub for play
Check clutch slave for leaks under the rubber boot
Replace clutch fluid
Get a full 4 wheel alignment by someone who knows what he is doing
Subaru upper cylinder cleaner
Tyres and tyre pressure
Clean the water/top drains

At the end you know all the servicing is done and as each task is completed you will also have a appreciation of the cars condition.

viewtopic.php?f=18&t=46863&p=588886#p588886

To which he adds:

My list above was provided for a car of average use and I believe still valid as an initial baseline.

To add to the list for a low Km older car I would be cautious of anything made of rubber as previous mentioned.
This includes the brake system (master and callipers) and the clutch system (master and slave) as the seals may have perished through lack of use.
I'm not saying to go out and replace them, regularly check them for any sign of leakage or unusual feel.

Also check:
Suspension bushes and steering/driveshaft boots.
Seals around the rear lights.
The rain rail and roof drains.

Ideally get it up on a hoist and spend some time looking for issues underneath then drive it for a couple of thousand Km and check it again.

[manga_blue]

Replace the O2 sensor first. It indicates the richness of the mixture by measuring the proportion of oxygen coming out of the exhaust. As it gets older it reads weaker and weaker so the engine computer compensates by delivering more fuel, destroying your fuel consumption. The NA6 uses a single wire O2 sensor. Bosch rates the economic life of a single wire sensor at 60,000kms.

All single wire sensors have the same output characteristics, so as long as one fits it will work. You should pay less than $100 for a replacement, as low as $40 if you're lucky. Stick with the reputable brands: Bosch, NipponDenso or NGK/NTK.

[Dweezle]

My na8 had awful fuel economy, similar to what your saying. It turned out to be a leaky injector.

So now that you have so many possible causes, Good Luck haha.

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[Mr Morlock]

You should be getting around 8-9 l/100KM. If much of the list means little to you and like most of us you either do not have the tools or training then take it to someone whose job it is to sort cars out ie a mechanic. Rest assured that the majority of car enthusiasts get their man to do it.

[16bit]

what about wideband o2 sensors?

[A.Chen89]

A normal oxygen sensor will spit out the correct output for the ECU, but will only read between 14-15 Afr. I.e slightly rich, stoics or slightly lean.

A wideband will be able to tell you the Afr over a wider range. Something like from 8 to 20. I.e pig rich to unbelievably lean

A wide band sensor and gauge will help you diagnose the issue, but you can't plug it into the stock ECU to replace the narrowband. You need to calibrate the analogue output to suit so the ECU sees what it's expecting.

[16bit]

so one would be better off getting an stock fitting item like this?

same part number as in the bosch catalogue.

http://www.ebay.com/itm/170767535099?ss ... 1438.l2649

[manga_blue]

Yeah, but that's a 4 wire for a NA8 or NB. An NA6 needs one like this
http://www.ebay.com.au/itm/NTK-1-SINGLE ... 890&_uhb=1

[NitroDann]

Unplugging the o2 sensor fixed my economy issues an there is sound science why if you need me to go into it.

Dann

[Poida]

But if were to hazard a guess it would be plug leads.

Leads were replaced in 2009 according to the last owners documentation (around 8000kms ago). Definitely going to take it to a mechanic to get everything checked out. Thanks for the shifting tip!

Plugs
Fuel filter

Haha, they were the first things I changed, thanks though.


Steampunk - I'll give the 02 sensor replacement a go. What do you mean by checking the computer?
I'll investigate into the water/coolant temp.
Low-mid throttle, a gradual build up to 3.5k, never usually full or close to full throttle.
Thanks for your help.

project.r.racing - I will be checking these shortly.
I've never really noticed, if I recall correctly, yes, just to the left of the middle, I'll check and report back though. Thanks.

93_Clubman - No I have not base-line serviced it as of yet, looks like that's one of the first things on my list of to-dos, doesn't look it's had a full service recently judging from the papers.

Has been regularly serviced (indicating from service papers) it's entire life, all main maintenance schedules and other servicing have been performed between 1990-2001 (0-80500km), after 2001 there have been minor services completed at least once a year, the timing belt and that had been replaced in 2011 (at 98000kms). If there's anything specific you want, let me know.

I first looked at the odometer before filling up the tank full from practically empty, and compared it to when I filled up again, making note of how many litres were actually pumped into the tank initially and how much kms I got from those litres, as well as using the tripometer as an indicator. Pretty much just measured how many km's I got from from the amount of fuel I put in and averaged it out.

Thanks for the help.


manga_reader - Very informative thanks, it seems like it's in need of replacement.

Dweezle - Haha, another possible cause indeed, thanks.

Mr Morlock - You are right, the easier to replace consumables are fine, but I lack overall experience/tools for something like this, I believe I will take it to a mechanic, and get a full service done to hopefully cover everything, and then just gain experience by performing maintenance afterwards, bit by bit.

NitroDann - I've read this before, I am quite interested in understanding why this occurs. Thanks.


Thank you all for your help and comments

-Peter