Rather Poor mileage

Mr. Shine · Post by **Mr. Shine** » Sun Jun 10, 2012 5:30 pm

I run 95 and seem to average about 9.7km/L, which leads me to suspect fuel octane is not the issue unless I've been getting bad batches for the past year and a half...

KSCRIM · Post by **KSCRIM** » Sun Jun 10, 2012 9:17 pm

I have a 98 1600 NB, when I first got it I used 91 but noticed a slight knock after idling at the lights. The knock sensor backed the timing off to eliminate the knock after a second or so. With 95, there is no knock at all, ever. I use shell 95 or as it is called now "Z"
Going to 98 is a waste of money as the engines are built and timed to run at optimum on 95. The ECU can only retard the timing from the static setting, controlled by the knock sensor. (Nothing to do with the spark advance that is the normal function of the ECU)
The fuel consumption is at its lowest on 95, both in L/100k and $ /100k.
(I have had 2 2.0litre V6 mazda engines, a MS6 and a MX6, both had the same post idle knock using 91 and were completely happy on 95 with better L/100k and $/100k figures.)

I get about 7.4L/100k on long trips and 8.5 L/100k around town.
I have records going back to 2005 for each fill.

Mad Kiwi · Post by **Mad Kiwi** » Sun Jun 10, 2012 10:23 pm

I totally agree with the 95-98 suggestions but don't forget to try changing your driving style...It might be all the difference.

Mr. Shine · Post by **Mr. Shine** » Sun Jun 10, 2012 10:37 pm

KSCRIM wrote:I have a 98 1600 NB, when I first got it I used 91 but noticed a slight knock after idling at the lights. The knock sensor backed the timing off to eliminate the knock after a second or so. With 95, there is no knock at all, ever. I use shell 95 or as it is called now "Z"
Going to 98 is a waste of money as the engines are built and timed to run at optimum on 95. The ECU can only retard the timing from the static setting, controlled by the knock sensor. (Nothing to do with the spark advance that is the normal function of the ECU)

The NA doesn't have a knock sensor, although this means any knock should be noticeable with an NA...

authentic · Post by **authentic** » Sun Jun 10, 2012 10:39 pm

Well unless you have to drive MX-5 a very specific way, I don't think my driving style will change much. I don't drive like a nana all the time, nor do I try to set land speed records. Unless I'm supposed to do that?

I will definitely fill it with 95 next time and see how that goes. But that wont fix my current problems, I think its a lot more than that.

I've got some NGK leads on the way to replace my faded pink ones. Plugs looked ok when I did a compression test a couple of months ago, they didn't seem overly sooty. Ill have a look at them again when I replace the leads though.

That leaves the o2 sensor and the air flow meter to have a look at.

If I still have the factory fuel filter, could that cause high fuel consumption? Mine is a black filter.

aidwano · Post by **aidwano** » Sun Jun 10, 2012 11:44 pm

Fuel filter couldn't make the consumption worse, all it can do is get blocked and make you lean out.

There is no way 91 is causing your economy to be that bad. I have run 91 a couple of times with no major change in economy.

Throttle position sensor could be worth checking along with the AFM and oxy.

I have a lightly modded 1.6 (mild cams etc) and get 8L/100km open road and 10L/100km around town with plenty of full throttle.

WideOpenThrottle · Post by **WideOpenThrottle** » Mon Jun 11, 2012 8:37 am

Aside from the fuel what about your brakes? - they might be tightish on the rotors....put the car on a slight hill,engine off,in neutral, handbrake off & see if it rolls freely with no obvious noise.

Growler · Post by **Growler** » Mon Jun 11, 2012 2:52 pm

I'm not too worried about fuel economy.As long as mine goes fast when I put my foot down!

authentic · Post by **authentic** » Mon Jun 11, 2012 3:28 pm

WideOpenThrottle wrote:Aside from the fuel what about your brakes? - they might be tightish on the rotors....put the car on a slight hill,engine off,in neutral, handbrake off & see if it rolls freely with no obvious noise.

I'll give that a go too, never thought of that. Cheers

Denz · Post by **Denz** » Mon Jun 11, 2012 11:10 pm

I took my Car from Hamilton to Coromandel township and back, thrashed the balls off it from thames to the township and back (hit the rev limiter a few times, constantly in the lowest gear I could be etc) and got about 8l/100km

Keith Jones · Post by **Keith Jones** » Tue Jun 12, 2012 8:56 am

I can confirm what Mr Shine said about the government testing fuel to ensure compliance with the specification. They do this on a regular basis. We use this data at the University for our research.
They test for both RON (Research Octane Number), which is the number fuel in NZ is sold by, and MON (Motor Octane Number) which is a better indicator of on-road knock resistance. Between 1 July 2008 and 30 June 2010, which are the latest figures available, 91 octane varied between 90.8 and 94.6 and 95 octane varied between 94.9 and 96.4.

Post by **jif** » Tue Jun 12, 2012 10:27 am

I for one would be interested to know where the 91 rates at 94.6 !

On 12 June 2012 08:56, Keith Jones <mx5talk@mx5forum.co.nz (mx5talk@mx5forum.co.nz)> wrote:

I can confirm what Mr Shine said about the government testing fuel to ensure compliance with the specification. They do this on a regular basis. We use this data at the University for our research.
They test for both RON (Research Octane Number), which is the number fuel in NZ is sold by, and MON (Motor Octane Number) which is a better indicator of on-road knock resistance. Between 1 July 2008 and 30 June 2010, which are the latest figures available, 91 octane varied between 90.8 and 94.6 and 95 octane varied between 94.9 and 96.4.

[Posted via external email]

Keith Jones · Post by **Keith Jones** » Tue Jun 12, 2012 12:21 pm

Unfortunately that 94.6 was a one-off blip.
Does show the variability though.
The good thing is that it's is pretty much always on the high side around 92.

punkoutnz · Post by **punkoutnz** » Tue Jun 12, 2012 12:46 pm

Any idea how often they do these tests? The problems we've been having at work with fuel have been the past two summers, the guy reckons he did a few stations around auckland around this time last year. Be curious to know how often and if it's a sample group of stations, etc.

Keith Jones · Post by **Keith Jones** » Tue Jun 12, 2012 1:32 pm

Samples are taken in 11 areas from Whangarei to Bluff, with the most samples taken from the highest fuel consuming areas. The sampling process follows statistical methodologies to maximise the accuracy of the process. Samples are taken in the Auckland area about 3 to 4 times a month, ie. about once a week, from different fuel companies.

What problems have you been having with your fuel?
I don't have any data from 2011. So it's not impossible that there was a clitch.

punkoutnz · Post by **punkoutnz** » Tue Jun 12, 2012 1:46 pm

So many engines that have been running fine for years on 91 are suddenly having issues with poor performance, hard starting, lack of power, etc. And a switch to 95 octane has instantly remedied the issue. It has been quite bizzare.

Angreal · Post by **Angreal** » Tue Jun 12, 2012 2:28 pm

Mine still runs as it always did and it's still on 91

Mr. Shine · Post by **Mr. Shine** » Tue Jun 12, 2012 2:43 pm

punkoutnz wrote:So many engines that have been running fine for years on 91 are suddenly having issues with poor performance, hard starting, lack of power, etc. And a switch to 95 octane has instantly remedied the issue. It has been quite bizzare.

Sounds to me if they've been fine on 91 for years that it'd be an engine condition issue rather than fuel octane/quality, and the higher octane is fixing the symptoms but not necessarily fixing the actual problem?

punkoutnz · Post by **punkoutnz** » Tue Jun 12, 2012 2:51 pm

If it were like that for a single motor, yeah maybe. But when you're getting runs of them in their tens all with the same symptoms spanning a period of a few months... Then no, definitely a fuel issue.

Keith Jones · Post by **Keith Jones** » Tue Jun 12, 2012 3:05 pm

Sounds like some of the issues we get commissioned to investigate.
Don't suppose anyone is willing to pay to get an answer?

punkoutnz · Post by **punkoutnz** » Tue Jun 12, 2012 4:06 pm

Having to argue with people just to pay another 6c at the pump is often pushing the boundaries as it is let alone "give me money and I will find out for you" haha.
Gotta remember that outboards will show up issues a lot sooner than cars will, cos they're loaded up all the time during their use (they do only have one gear after all), so while you can "get away with running lower octane if you drive carefully" doesn't so much work in a boat where it's under load all the time. So if there's ever any issues with fuel and fuel quality, it's going to show up in a marine environment pretty darn quickly! Would definitely be curious to know if there has been a drop in the fuel quality here or if it's just a particular addtive that doesn't seem to be working as well as it should be.

Born_disturbed · Post by **Born_disturbed** » Tue Jun 19, 2012 8:36 pm

395kms on 36.54L of Z 95

Thats with 30ish mins of rush hour traffic each way Monday to Friday

And a highly spirited driving style most of the time

91 NA with a 97 SR limited 1.8 (133HP version)

Ian · Post by **Ian** » Wed Jun 20, 2012 9:34 am

So, have you checked all the obvious things that have been suggested, or are you hoping that 95 octane is the answer??? Because I can tell you now, it isn't...the answer.

WideOpenThrottle · Post by **WideOpenThrottle** » Wed Jun 20, 2012 12:56 pm

Another thing to consider is your wheels...if they are heavy 17" with tyres that are too tall it could be adding to your problem.

authentic · Post by **authentic** » Wed Jun 20, 2012 6:42 pm

I haven't fixed the problem yet, and I realise that simply changing octane wont fix my issues.

I've got a large list of things that could be causing the problem, and I am slowly checking them all.

Rather Poor mileage

Rather Poor mileage

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