Over the past 12 months the highway fuel economy in my car has been slowly getting worse, It used to be around 6.5-7lt at 110 and around town was 10.5-11lt but I drove a 600 kms trip yesterday and the economy is now up around 10.5lt at 110 and exactly the same if I drive around town.. This isn't combined driving just purely highway or town driving and I have the same economy? Side note it needs a new cat and I will be replacing the oxygen sensor as well which I know affects economy but doesn't that affect town economy also?

o2 sensor effects all economy yes, it adjusts your fuel trims based on readings it provides to the ECU.

Do you have the or service the car often? Spark plugs, air filter etc effect the economy as well.

"needs a new cat" . . . perhaps check your fuel economy with the new one on?

Could be a lot of things. Is this an auto? How many revs is it doing at 110? Should be 2200rpm in 4sp with 3.5L. I assume you have the tyres at the correct pressures. Have you added anything such as roof racks that would add extra drag?

It's a manual and revs around 2800@110.. Tyres are at correct pressure and no roof racks.. And yes I service my car every 7,500kms, I'm changing the cat cause it smells and the oxygen sensor as it is the original one so probably about time

It's a manual and revs around 2800@110.. Tyres are at correct pressure and no roof racks.. And yes I service my car every 7,500kms, I'm changing the cat cause it smells and the oxygen sensor as it is the original one so probably about time

If mine has got the original oxy sensor (think it probably does), should I change it anyway considering it will be 13 years old? My economy isn't too bad and hasn't changed since I bought the car but it may change with a new oxy sensor. How difficult are they to change??

If mine has got the original oxy sensor (think it probably does), should I change it anyway considering it will be 13 years old? My economy isn't too bad and hasn't changed since I bought the car but it may change with a new oxy sensor. How difficult are they to change??

Give us your steady speed instantaneous fuel consumption at 60, 80, 100 KPH on a warmed up engine and we can tell you straight away if you need a new oxygen sensor.

Why pay for and swap something that isn't broke?

I'm changing the cat cause it smells
Check the discussion here first: http://www.aussiemagna.com/forums/showthread.php?t=104931. The smell quite possible has nothing to do with the condition of the 'cat' - unless it's one with fur and your dog got it ...

Give us your steady speed instantaneous fuel consumption at 60, 80, 100 KPH on a warmed up engine and we can tell you straight away if you need a new oxygen sensor.

Why pay for and swap something that isn't broke?

I'll check it out tonight and get back to you on that one Max. Yeah very true mate.

Just finished testing my instantaneous fuel consumption they are 6.5lt@60 8.0@80 and 9.5@100.. Finding a flat road isn't as easy as it sounds ha ha.. What are the figures roughly meant to be?

sounds high; my TW wagon on a 40km steady 100kph run, flat road, used 7.9/100; the inst. reading at 100 is varying around that mark, if I recall correctly.

Yeah that sounds high for sure.
I think @ 100 mine is normally around the 7 mark (manual).


Were you having all the weird electrical problems?
Could be related.

Yes still having the electrical problems, I originally thought they could be related however the electrics have only been about 3-4 weeks and I have seen my fuel economy getting worse and worse over the last 12+ months.. I only drive long distances in it about once every month or two and the fuel economy has been getting worse every time I take it

sounds high; my TW wagon on a 40km steady 100kph run, flat road, used 7.9/100; the inst. reading at 100 is varying around that mark, if I recall correctly.

+1
TJ sedan gets 7.7 average over 300 km country drive - didn't check inst at 100 kph though.
The drive included hills and slowing down in small towns, 4 spd auto + A/C on, so I'd think 7 or so for inst?

My TJ2 over the last two years has gone from average round town of around 11 litres per 100km to up to and over 14 ltrs..and on the freeway was high 8's ltrs per 100 now cannot get it under 10.5 ltrs..
last November changed timing belt and all bearings,new water pump,new fan and alternator belts,new bearings as well..
new starter motor,gearbox flush and added tranny cooler..
new plugs,leads,brake rotors and pads..Also new o2 sensor..
serviced the CV joints and greased..
buggered if I can work out why the increase in fuel as it got no better with all bearing done..I am stumped..

I've done 230,000kms on standard o2 sensor without issues. What makes you think it's the sensor?

If there's a wrecker nearby you could pick up a spare set on connectors to piggy back the signals in to your cabin (read makeshift test harness).

Testing between pins 3 and 4 at cold (20c) for an impedance of 7-40 Ω
Then checking between pins 1 and 2 for a voltage between 0.6-1.0 once the car has warmed up.

If anything outside these parameters you probably have a faulty unit.

I have about 330,000kms on the original sensor and I don't belive the highway economy is related I figure the sensor is quite used and could do with replacing as the whole car is getting a once over when I find the time.. But if they are meant to be a lifetime thing I will test it and if it's fine I'll save my money

My TJ2 over the last two years has gone from average round town of around 11 litres per 100km to up to and over 14 ltrs..and on the freeway was high 8's ltrs per 100 now cannot get it under 10.5 ltrs..
last November changed timing belt and all bearings,new water pump,new fan and alternator belts,new bearings as well..
new starter motor,gearbox flush and added tranny cooler..
new plugs,leads,brake rotors and pads..Also new o2 sensor..
serviced the CV joints and greased..
buggered if I can work out why the increase in fuel as it got no better with all bearing done..I am stumped..

Have you replaced the fuel pump recently? If not, (it could be original one) maybe think about replacing it and see if that fixes it.

Have you replaced the fuel pump recently? If not, (it could be original one) maybe think about replacing it and see if that fixes it.

Fuel pump? Is that a joke? :|

I have about 330,000kms on the original sensor and I don't belive the highway economy is related I figure the sensor is quite used and could do with replacing as the whole car is getting a once over when I find the time.. But if they are meant to be a lifetime thing I will test it and if it's fine I'll save my money

Yeah if a new sensor is going to give the same reading you may as well pocket the $100-150

No..But how would that help.??

Just finished testing my instantaneous fuel consumption they are 6.5lt@60 8.0@80 and 9.5@100.. Finding a flat road isn't as easy as it sounds ha ha.. What are the figures roughly meant to be?

The instantaneous figures look normal. Are you sure you don't have a petrol leak?

Pretty sure I don't have a leak, as the fuel consumption is normal for around town driving, and if I had a leak it would be more noticeable around town as a tank of fuel takes longer to empty.. Fuel consumption at 100 is definitely high, that is a dead straight and flat road and it's in the 9's, any slight incline and it goes up to 15

Have you cleaned out the throttle body lately? Carbon depositing, gumming and varnishing in the intake system can, over time, affect fuel economy.

That and the maf sensor was my next thought because I'm assuming it would affect highway fuel economy more then around town as my average speed is 110kph which would theoretically need more throttle then an average speed of 30kph

If a throttle butterfly doesn't have smooth, unrestricted operation, I can see how that can return bad figures; as with a dirty MAF.
Both are cheap to rule out, get some Subaru Upper Cylinder Cleaner from a Subaru spare parts department; and some MAF Cleaner from Repco, shouldn't cost much more than $30-35 for the two items.

To check whether the oxygen sensor is working reasonably, disconnect the sensor, reset the ECU (disconnect battery for at least 10 seconds and then reconnect; this step is critical to the check) and then take the car for a drive to check fuel economy. If engine behaviour (particularly at idle) and fuel economy appear better with the sensor disconnected then the sensor could well have failed or be in the process of failing and further investigation is warranted (such as pulling it out to check for caking of carbon on the sensor body); if not, reconnect the sensor and look somewhere else.

The results of this check are most reliable if the engine is already warmed up at the start of the check, otherwise the warmup enrichment may make it harder to evaluate the economy differences. The test drive also needs to be a reasonable distance (not just around the block) to ensure reasonably accurate consumption measurements.

The design life of this type of sensor was 160000km; while many sensors last longer than this, they all fail eventually. Their response times (to mixture changes) also get slower as they age which results in the ECU not being able to control the air/fuel mixture as precisely as it would with a new sensor, though this has far less effect on engine performance and fuel economy than sensor failure.

My TJ was really bad for a while much like yours, couldn't work it out, garage told me to change the cat, Oxygen sensor but I found out the garage had "adjusted" the throttle body settings. I had taken it to a Mitsu dealer, different to the one I usually use, they found the problem with a simple test. Maybe someone is "fixing" your car to make more business for themselves. My car is 12 years 270thou K on the clock owned since new, original cat and sensor, was told they never need replacing. Is Auto and average 7.5 highway 10 per 100 k's City/town driving

If the fuel pump was faulty you wouldn't have the car running. Mine stopped two years ago, new unit fitted into tank. all good

Besides mal fitting the 6g75 engine I've done all the maintenance and fiddling myself.. I'm planning on looking at it tomorrow.. Where about do you unplug the oxygen sensor from? Is it underneath or is the plug inside the cabin?

So to test the O2 sensor I drove 15kms up did a U turn and 15kms back, instant fuel around 9.5-10.5 and average 9.9.. Unplugged the O2 sensor, reset the Ecu and did exactly the same drive and instant fuel around 7.5-8.5 and average 8.1.. Idle is way better too.. After resetting the Ecu with the O2 sensor unplugged it wants to stall and actually did afterwards when I had the air con on.. Air con was off for the testing

So to test the O2 sensor I drove 15kms up did a U turn and 15kms back, instant fuel around 9.5-10.5 and average 9.9.. Unplugged the O2 sensor, reset the Ecu and did exactly the same drive and instant fuel around 7.5-8.5 and average 8.1.. Idle is way better too.. After resetting the Ecu with the O2 sensor unplugged it wants to stall and actually did afterwards when I had the air con on.. Air con was off for the testing

Boom. Let me know your address and I can send you a spare one.

The ECU goes to a default map when it can't get a signal from the o2 sensor, or MAF, or other sensors for that matter. If your case it sounds like the o2 sensor may be off and sending signals to richen the mixture over time.

The ECU goes to a default map when it can't get a signal from the o2 sensor, or MAF, or other sensors for that matter.
Not correct in the case of the oxygen sensor - the ECU continues to use the normal maps but doesn't operate in closed loop mode.