Fuel efficiency bad - need help solving this problem

[NitroDann]

As magna blue said a faulty o2 will cause the ecu to dump extra fuel in. If you disconnect it the ecu can no longer use its incorrect signal. As result it ignores air fuel ratio trimming via o2 reading and reverts to a base map.

Its not 100 % as good as a working o2 but its really really close.

Dann

[manga_blue]

I'd like to agree with you, Dann, but then we'd both be wrong.

Anyway, all the other suggestions here - coolant temp sensor, injector cleaning, temp/thermostat checking - are all worth checking too. Possible but less likely are stuffed AFM sensor, dragging brakes, tyre under-inflation and blocked exhaust.

[NitroDann]

My wideband dissagrees with you.

Perhaps there was something deeper with my car that has eluded me.

Dann

[93_Clubman]

Leads were replaced in 2009 according to the last owners documentation (around 8000kms ago).

Steampunk - What do you mean by checking the computer?

There was a time when MX5 OEM replacement spark plug leads were only lasting a couple of years, but they seem to be better these days. Still consumable item for many though.

Think SP was suggesting that high fuel consumption might have been due to an aftermarket ECU, or alternatively a factory ECU being run with a signal interceptor & modifier like a piggyback ECU.

Only asked about fuel consumption calculations as some only use odo & fuel gauge.

[16bit]

just purchased a bosch o2 sensor.

plan on installing it on the weekend.

should hopefully net me more than 300km a tank.

[Steampunk]

Steampunk - What do you mean by checking the computer?

Think SP was suggesting that high fuel consumption might have been due to an aftermarket ECU, or alternatively a factory ECU being run with a signal interceptor & modifier like a piggyback ECU.

^ what clubbie said.
the computer resides under the passenger footwell, check the wiring (don't open the casing) for anything that seem out of place, or if it appears to have been removed and reinstalled.

[16bit]

anyone looking to order an oxygen sensor I ordered mine from bosch online store. it was the same price as the ebay ones i could find and I got it in 2 days.

cost me 129 aud delivered for the lso7 unit.

going to install it this weekend and see if i can get better fuel economy from it.

[mx-593]

I think I'm having the same problem as you :/ Does the car tend to shake and jerk accelerating after corners ? I can't even go at 10km/hr in gear 2
Could I be having the same issue ?

[16bit]

As magna blue said a faulty o2 will cause the ecu to dump extra fuel in. If you disconnect it the ecu can no longer use its incorrect signal. As result it ignores air fuel ratio trimming via o2 reading and reverts to a base map.

Its not 100 % as good as a working o2 but its really really close.

Dann

i did this and the car feels much better.

it does seem sluggish when cold and seems to have a bit more torque just getting about.

going to leave it un plugged until i get my o2 sensor removal tool in the mail.

be interesting to see how much this helps fuel economy.

[16bit]

sorry it DOESN'T seem to be sluggish when cold anymore.

[sailaholic]

My o2 I just need a spanner? Is yours in a weird spot or something?

Sent from my GT-I9100 using Tapatalk 2

[16bit]

yeah couldn't get to it with a spanner.

not enough space between the runners and air con pipes.

got my tool in the mail on friday and changed it over last night.

worked a treat.

the bosch wiring connectors are pretty easy to work with and seem to be largely idiot proof.

car is def better cold, no more hesitation.

also seems to have a little more torque throughout the rev range so the but dyno would say its now not running so rich.