Fuel consumption... looooong post

fooesboy · Post by **fooesboy** » Fri Jul 12, 2013 10:34 pm

Hey guys,

I feel like my Miata is using a lot of fuel. I have been keeping an eye on how far I drive each time I fill up and the litres that go in.

Last lot was 387.4 km for 35.33 litres of fuel. That gives me 9.12 litres per 100km. What makes me wonder is that my other car (twin turbo Legacy) uses the same amount of fuel for the same driving.

I drive a 1989 1.6 with a K&N filter, extractors and a free-flow exhaust. No other modifications as far as I am aware. Almost all of my driving is open road and I rev the engine to about 4 most of the gear changes. Cruise just under the 110 km mark (about 3'400 rpm) I always use 95 octane.

The spark plugs have been changes, I've fitted new tyres and had 2 wheel alignments. and the car has been serviced twice since I purchased it in February.

Fuel usage has always been higher than I was expecting when I purchased the car 7.8 litres per 100km.

Any thoughts or suggestions before my mechanic gets a free reign and spends all my money chasing a problem that might not even exist.

Thanks for your help.

Chris

(sorry about the long post)

authentic · Post by **authentic** » Fri Jul 12, 2013 11:27 pm

Yeah I asked the same question when I first got my Roadster haha. I was getting like 400km out of a tank, so you're not alone.

I haven't done much to try and fix it (I just keep putting it off like the lazy student that I am), but its gotten slightly better with the 2.5" system that I have now installed.

Let me know how it goes!

Stevo · Post by **Stevo** » Sat Jul 13, 2013 6:33 am

I'm getting around 7.9 lt/100 km on open road driving and that is driving "enthusiastically".
Fast acceleration, cruise at 100/120kph, chop down to 3rd to overtake, rev out to >6000rpm etc and I still get 7.9lt/100km
Try cleaning air filter, gap or change plugs or just send it in to the garage and tell them about your fuel consumption issues.
Incidently, when I slow down a bit and drive like a "normal" person, the fuel consumption drops to about 7.3 lt/100km open road driving

Snapfrozen · Post by **Snapfrozen** » Sat Jul 13, 2013 7:42 am

You might find it's your driving style. How do you drive, where etc?

WideOpenThrottle · Post by **WideOpenThrottle** » Sat Jul 13, 2013 8:26 am

I think all early 1600 Na's are hungry...especially with high mileage ones as mine was the same....you got to factor in power steer,air-con too,plus the fact they run too rich from the factory!
Our 1800 Nb was always better on fuel.

And what about your wheels/tyres...too tall and heavy and it will use more fuel as well.

tomtikki · Post by **tomtikki** » Sat Jul 13, 2013 8:47 am

Yeah it sounds lame but driving style can make a HUGE difference to mileage, being in the right gear for a hill so the engine isn't struggling etc.
The high gearing means open road driving just isn't efficient, I've used a whole tank on the open road & only gained about 50km extra from the tank where as our mini ('02 Cooper) will do an extra 150km!

Since my Mx doesn't have a fuel light I just make sure I fill it between 400-450km.
Bloody annoying considering it's weight etc!
Ive given up trying to be efficient in it & fang it almost everywhere.

marcellarius · Post by **marcellarius** » Sat Jul 13, 2013 9:49 am

I've always felt my NA8 has been drinking too much fuel; I managed to get about 500km out of a tank last week, mostly motorway driving.

I've meant to investigate my O2 sensor -- they have a limited lifespan, and the OBD checks are pretty rudimentary, they won't pick up a bad sensor until it's completely dead.

SLYDIT · Post by **SLYDIT** » Sat Jul 13, 2013 7:45 pm

the thing that jumps out at me from your post is the extractors. "Most" extractors place the oxygen sensor too far away from the head down by the gearbox and it gets too cool and doesnt function correctly although it wont throw a check engine light. the car will run way rich even at criuse and stupid rich at idle.
I would have a look at your oxygen sensor wiring and see if it has only one wire going to it. if it only has one then theres your main problem....mx5s with extractors need a heated oxygen sensor fitted.
also you could go with the clocking crystal mod that will lean out your base mixture by 10%. the mx5 runs super rich at full throttle. (my turbo runs a leaner mixture than a stock car)

fooesboy · Post by **fooesboy** » Mon Jul 15, 2013 10:31 am

SLYDIT wrote:the thing that jumps out at me from your post is the extractors. "Most" extractors place the oxygen sensor too far away from the head down by the gearbox and it gets too cool and doesnt function correctly although it wont throw a check engine light. the car will run way rich even at criuse and stupid rich at idle.
I would have a look at your oxygen sensor wiring and see if it has only one wire going to it. if it only has one then theres your main problem....mx5s with extractors need a heated oxygen sensor fitted.
also you could go with the clocking crystal mod that will lean out your base mixture by 10%. the mx5 runs super rich at full throttle. (my turbo runs a leaner mixture than a stock car)

Thanks for the reply, just the sort of infomation I was after.

Do you know if it is possible to retro fit a heated oxygen sensor? The guy at the garrage suggested to have the airflow meter checked. Does this sound like a good idea?

I've had a lot of expences lately and don't have lots of money to throw around chasing this problem.

Chris

SLYDIT · Post by **SLYDIT** » Mon Jul 15, 2013 7:11 pm

easy to fit. just get a 3 or 4 wire O2 sensor and hook up a 12V feed from your switched ignition source to the heater circuit of the sensor,(mine is being fed from the 12V switched signal at the ECU). the signal wire connects just as the other one did. MY wiring is re routed through the grommet in the floorpan where the air con drain goes through.
i wouldnt bother with an airflow meter check. the two most common issues with fuelling are o2 sensor and the ECU water temp sensor failing.

fooesboy · Post by **fooesboy** » Sat Aug 31, 2013 8:46 pm

ok guys, time to report back.

I've filled up the car 5 times since I have changed the O2 sensor. Done a mixture of driving and can say that the new O2 sensor has not made a big difference to my fuel usage. It's now down to 8.8 litres per 100km of driving.

Don't know what to try next... I'm open to suggestions.

Chris

SLYDIT · Post by **SLYDIT** » Sat Aug 31, 2013 8:58 pm

did you fit a heated sensor?
the car runs off the 02 sensor most of the time when warmed up, the only open loop it does is when cold /accelerator pump/ at WOT above 4500rpm..
id be checking the resistance of the water and air temp sensors are within spec, is your K+N filter a pod filter or a panel filter in the stock airbox? if its a pod then it will be sucking in hot engine bay air which will rob you of power and economy.
Oh and check that it hasnt been fitted with bigger injectors from an 1800....you never know.

how is your car starting? is it a bit sluggish?

still reckon a clocking crystal mod is worth a shot

Snapfrozen · Post by **Snapfrozen** » Sat Aug 31, 2013 9:11 pm

I was just about to recommend a crystal mod. Have you considered throwing it on a dump with a wide band to check your afr?

fooesboy · Post by **fooesboy** » Sat Aug 31, 2013 9:21 pm

Did not fit a heated O2 sensor.... long story, but basically BNT ordered a standard 1 wire unit and when I said that I wanted a heated one the guy there was just being a bit of a dick....

The K&N was a panel, changed it back to a stock one when first chasing this problem as first thoughts where that it might have been over oiled.

The car starts fine, then does not idle very well until it has warmed up. Will die sometimes when reversing out of the driveway and going back to idle to change into 1st.

I tried to search the clocking crystal mod and could not come up with anything...

How do you tell what type of injectors are fitted?

fooesboy · Post by **fooesboy** » Sat Aug 31, 2013 9:23 pm

Snapfrozen wrote: Have you considered throwing it on a dump with a wide band to check your afr?

Sorry, but What????

I'm a bit thick sometimes, can you please explain?

SLYDIT · Post by **SLYDIT** » Sat Aug 31, 2013 9:25 pm

you NEED to fit a HEATED 02 sensor....end of story.
i'd do that before i did anything else.
exactly where is your 02 sensor located? (Picture?).
if the 02 sensor isnt hot enough, it will be lazy in its operation and its signal will die off at idle, but not enough to throw a check engine light.but enough to make the car run way rich

Then id be checking your coolant temp sensor for the ECU.its on the back of the head under the coilpacks (green plug) the wiring loom for this and the coils and the cam sensor has been known to fail at the point where its cable tied to the manifold.

i think from memory the 1600 injectors are blue and the bigger ones are tan or red.

your bad cold running could be a gunked up throttle butterfly, youll need to clean it and reset your idle rpm. heres how...
1- run the car till warm
2- turn on car
3- connect jumper to TEN and GRD on the diagnostic connector
4- (car in neutral)
5- adjust the idle speed adjuster screw
6- set to about 850 rpm
7- turn off car
8- reconnect jumper
9- disconnect and reconnect battery
10- turn on car, let it run, check idle speed. drive.

SLYDIT · Post by **SLYDIT** » Sun Sep 01, 2013 7:24 pm

fooesboy wrote:
Snapfrozen wrote: Have you considered throwing it on a dump with a wide band to check your afr?
Sorry, but What???? I'm a bit thick sometimes, can you please explain?

i think he meant dyno.

Snapfrozen · Post by **Snapfrozen** » Sun Sep 01, 2013 9:15 pm

Heh. Epic autocorrect. Yes, on a dyno

fooesboy · Post by **fooesboy** » Tue Sep 03, 2013 2:08 pm

OK, been back to BNT, now feel like I've been butt raped for price but now am the proud owner of the universal O2 sensor with 3 wires!

I will go and have it fitted tomorrow.

I've added a couple of photos showing the sensor locatio

fooesboy · Post by **fooesboy** » Tue Sep 03, 2013 2:12 pm

[quote="SLYDIT"]

i think from memory the 1600 injectors are blue and the bigger ones are tan or red.

your bad cold running could be a gunked up throttle butterfly, youll need to clean it and reset your idle rpm. heres how...
quote]

The injectors are blue on my car.

How do you clean the throttle butterfly?

fooesboy · Post by **fooesboy** » Thu Sep 05, 2013 5:44 pm

The new 3 wire O2 sensor is now fitted. Looks to be making a bit of difference as there is now no black smoke when the motor is run up. Will fill it up tonight and keep an eye on the fuel...

SLYDIT · Post by **SLYDIT** » Thu Sep 05, 2013 8:37 pm

good stuff. let us know how it all works out.

BTW if youve been runnig too rich for a while it may pay to do a plug change to new plugs just in case yours are carbon'd up.

fooesboy · Post by **fooesboy** » Fri Sep 13, 2013 9:52 am

Just gone through a tank... in record time

10.06 liters per 100km

Post by **jif** » Fri Sep 13, 2013 11:40 am

resident fuel efficiency nerd checking in !

Some thoughts:
- power mods tend to affect fuel efficiency (negatively!)
- cold weather affects FE also. As does cold starts. It takes the oil about twice as long as the water to get up to temp, so in winter it might be 10+ min before its all warmed up. If the heater can't blow scorching hot air in 5-7 min the thermostat might be sticking open (and running rich)
- brake drag is not helpful either
- 95 octane is overkill - if you "need" it then you prob also need to de-coke the engine a bit .. carefully spritzing (not squirting) water into the intake is a cheap way to do this

- tried higher tyre pressure ? I used to run 34-36 psi instead of 28.
- driving style ? Smooth and steady does it. Too bad someting like the ultragauge doesn't work on an early car like this.
- maybe adjust the Air Flow Meter flap tighter so it runs a bit leaner. Actually, getting actual air:fuel ratios first would be a useful first step.

I used to get 11-12 km/l around town , with a best of 15 km/l on a 300k round-trip (boring drive though) "just to see" how high I could get it. turbo NA8.

Something that gives instant feedback like an ultragauge/MPGuino would be useful for testing changes.
http://ecomodder.com/wiki/index.php/MPGuino

http://ecomodder.com/forum/showthread.p ... 14838.html

Our big Toyota v8 gets ~10km/l if I'm just pussying around in it , and much worse if the instantaneous-mileage display is off. Ha. It makes a difference :]

-j

On 13 September 2013 09:52, fooesboy <mx5talk@mx5forum.co.nz (mx5talk@mx5forum.co.nz)> wrote:

Just gone through a tank... in record time :?

10.06 liters per 100km

[Posted via external email]

marcellarius · Post by **marcellarius** » Fri Sep 13, 2013 11:44 am

What sort of driving? Urban? Rural? Motorway?
Do you really rev it out or drive it like my granny would?
Do you drive fast? -- higher speeds mean more drag. Roof up or down (affects the drag coefficient a bit).