Re: 2000 TJ 3.5L Auto Sedan

Hey all, my fuel efficiency is declining from 1L:8km to 1L:5.5km. I've done everything I thought of, new spark plugs, professional fuel injector clean, new oil & air filters & max. tyre pressure but it doesn't seem to improve much.

Could there be a leak in the fuel delivery system? Is the fuel delivery hose very close to the bottom of the car where it could get in contact with the ground? Would it be expensive and difficult to check?

Firstly wrong section.

To those reading - economy has gone from 12.5lt/100km to 18.18lt/100km.

Have you had your oxygen sensor checked/replaced?

Is your cat convertor blocked?

Have you had your MAF checked?

Could there be a leak in the fuel delivery system? Is the fuel delivery hose very close to the bottom of the car where it could get in contact with the ground? Would it be expensive and difficult to check?
Is there fuel on the ground when you let it idle? That'd pretty much rule out leaking fuel.

How many k's has it done, might just need the o2.

Is there fuel on the ground when you let it idle? That'd pretty much rule out leaking fuel.

How many k's has it done, might just need the o2.

I didn't take much attention to the ground. I would of thought any fuel leakage would evaporate pretty quickly. I thought of possible leak as I suspect the bottom might get in touch with the ground once on a dirt road going over an embankment.

It is at 101k right now. How do I check if faulty O2 sensor, blocked cat convertor or MAF (what's MAF?) is to blame? I am not technically competent.

I am not technically competent.

that statement just reinforced my thoughts from your signature :)

i bought a new O2 sensor off E-bay a while ago, there are about a dozen threads exactly detailing the O2 sensor to get it's just one plug, and a spanner away from installing..

the MAF is also known as the AFM (air flow meter) it's that oval plastic thing with a box, with wires connected to it that is bolted to the back of your airbox,

i'm assuming you'd have a can of contact cleaner? pull the MAF out, and give it a good once over with the contact cleaner.. this helped mine a fair bit, i assume it was due to having a quite heavilly oiled K&N filter in front of it.. i know nissans suffer really bad from this with thier 'hot wire' setup..

with the Cat.. prolly easiest you just take it to a mechanic to have it checked.. not the easiest thing to do without a foot or two (depends how big/small you are) of clearance..

as it is a steady decline... id say it would be one of the first two reasons.. if it all of a sudden started to get hidious economy.. then i'd say the cat gave out..

If you've never had your o2 changed its fair to say its due at 100k. I'd give that a go before anything else now, its probably the least expensive thing to change and can't hurt.

a fairly rapid onset change in fuel consumption can also indicate a slipping transmission. Do you notice any slipping when putting the engine under a bit of load.

a fairly rapid onset change in fuel consumption can also indicate a slipping transmission. Do you notice any slipping when putting the engine under a bit of load.

Nothing seems to slip except the fuel consumption. At first, I thought the fuel gauge was faulty. But after a few tanks, I ruled that out.

Can you guys give me a rough idea how much a new O2 sensor cost? Is this (http://cgi.ebay.com.au/MITSUBISHI-OXYGEN-OXY-O2-SENSOR-MAGNA-VERADA-PAJERO_W0QQitemZ300285595794QQcmdZViewItemQQptZAU_ Car_Parts_Accessories?hash=item300285595794&_trksid=p3286.m20.l1116) suitable?

I've seen some other thread mentioning about some kind of chip. I can't remember the name, was it MCU? Is it the same as this O2 sensor?

the MAF is also known as the AFM (air flow meter) it's that oval plastic thing with a box, with wires connected to it that is bolted to the back of your airbox,

i'm assuming you'd have a can of contact cleaner? pull the MAF out, and give it a good once over with the contact cleaner.. this helped mine a fair bit, i assume it was due to having a quite heavilly oiled K&N filter in front of it.. i know nissans suffer really bad from this with thier 'hot wire' setup..


Would contact cleaner mean home use detergent or distilled water?

No, contact cleaner is a special spray you buy from just about any auto store. Its made for cleaning electrical contacts.

also used for cleaning computers etc.. auto barn is expensive, but jaycar it's cheap :)

my o2 sensor was about $56 plus shipping.. but i had to solder the old plug / wires onto the new ones, as that one didn't come with the plug.. the one with the plug was about $80... i thought meh... $24..... i can swing a soldering iron.. it's just colour for colour..

also used for cleaning computers etc.. auto barn is expensive, but jaycar it's cheap :)

my o2 sensor was about $56 plus shipping.. but i had to solder the old plug / wires onto the new ones, as that one didn't come with the plug.. the one with the plug was about $80... i thought meh... $24..... i can swing a soldering iron.. it's just colour for colour..

Do you notice an immediate improvement after the change?

If the O2 sensor was faulty, would it give a bad fuel efficiency on the highway too coz the difference in fuel efficiency on the highway was barely noticeable.

If the O2 sensor was faulty, would it give a bad fuel efficiency on the highway too coz the difference in fuel efficiency on the highway was barely noticeable.
Not necessarily. I used to get 18+L/100km around town, but highway economy would still be normal (as low as mid 7's) which usually game back to an average around the 16L/100km mark, changed the o2 sensor and that average dropped to 12-13L/100km. I've still got a leak in my exhaust I'm fixing up soon, so that should be causing the couple of L above average it is, but yeah you get my drift :)

Not necessarily. I used to get 18+L/100km around town, but highway economy would still be normal (as low as mid 7's) which usually game back to an average around the 16L/100km mark, changed the o2 sensor and that average dropped to 12-13L/100km. I've still got a leak in my exhaust I'm fixing up soon, so that should be causing the couple of L above average it is, but yeah you get my drift :)

You never told me you changed it and got such a difference out of it!

You never told me you changed it and got such a difference out of it!
Didn't I? The first couple of weeks I got low 11's, then it went up to mid 12's. I still get a high week every now and then, but for the most part its decently lower :happy:

Any leaks in your manifold gaskets? This can cause increased fuel consumption - particularly if they occur upstream of the O2 sensor.

Is there a drop in power output too?

Could be a Vacuum hose leak or a gasket leak. (Inlet plenum, rocker cover etc...)

Is the Maf plugged in?

Is there a way to test the o2 sensor is faulty before buying a new replacement?

I hand my car in with the mechanic today. He seems doubtful the o2 sensor is faulty. He said usually there is a warning light on the dash if it is to blamed. Can anyone tell me if he is right or wrong?

Is there a way to test the o2 sensor is faulty before buying a new replacement?

My mechanic tested mine. I can't recall what he did though - I think it might have been something to do with measuring the voltage on it while operating?. Mine was done at a bit over 100,00kms and was on its way out - it wasn't functioning properly for the first few minutes when turned on, although it was fine when it was up and running. Car seemed to run a little smoother afterwards but didn't do much for fuel consumption (particularly on long drives).

A sick oxygen sensor does not necessarily throw a fault code. They die slowly and misinform the ECu causing incorrect fuelling. Just replaced mine at 116k.

Well, got my car back. Got told to use premium unlead coz it's cleaner. Guess he doesn't know much about the o2 sensor...

according to bosch
4 wire O2 sensors generally have a life expectancy of 200,000 km
3 wire 150,000km
2 wire 100,000km
1 wire 50,000km

magna's have a 4 wire sensor if i recall so any failure below 150000 km i'd be looking for an EMS fault or poor maintenance history

02 sensor on a warm engine will fluctuate fairly rapidly from .1 to .8V @2000 rpm if it is then this says that the O2 sensor is ok and the system is calibrating the fuel mixture

but 02 sensors can become slow with age, and they can become inaccurate

missfire, oil consumption, bad fuel, over fuel, under fuel, excessive and extended hard driving can all damage a sensor.

and yes the mechanic i right to steer you onto premium.... cars on premium run better and have better life due to its smoother burn

What exactly is a "EMS fault"?

I am already driving like a grandma... and only goes up to 400km before the low fuel light lit up. It used to lit up around 480km.

Isn't the car designed for RON 91? Why would RON 95/98 make any difference? Does magna ECU detect which type of RON fuel in the tank and adjust burn rate accordingly?

no, 3rd gen magnas do not have knock sensors. 95/98 RON fuel cannot be utilised in this engine, but many report smoother operation and better fuel consumption with higher RON fuels. I concur.

EMS = Engine Management System
the slower more even burn of higher octane fuel allows for leaner operation when cruising and hence better fuel economy
ideally in the cylinder we want combustion not an explosion or detonation higher octane combusts more so than low octane
it also contains more detergents which means a cleaner engine..and a cleaner engine is a happy engine

which gets me thinking... how clean is your throttle housing? a dirty throttle housing can mean you have to use more accel for the same power
also have you changed your trans fluid with OEM fluid? that can be a huge help

EMS = Engine Management System
the slower more even burn of higher octane fuel allows for leaner operation when cruising and hence better fuel economy
ideally in the cylinder we want combustion not an explosion or detonation higher octane combusts more so than low octane
it also contains more detergents which means a cleaner engine..and a cleaner engine is a happy engine

which gets me thinking... how clean is your throttle housing? a dirty throttle housing can mean you have to use more accel for the same power
also have you changed your trans fluid with OEM fluid? that can be a huge help

I am not sure... how do I check? All I know is, the fluids are changed around 83k with the timing belt & some pulleys. I assumed they use OEM as I was using mitsubishi dealer at Rosebery back then. Then not long after the next oil change at around 91k, I noticed a degrade of fuel efficiency. I took the car to an independent mechanic and change all sparkies at 99k with the air filter and get them to clean all fuel injectors. I couldn't think of any other things to look for.

So if you guys say the 3rd gen o2 sensor last upto 200k, it's most likely not the sensor fault, is it?

EMS = Engine Management System
the slower more even burn of higher octane fuel allows for leaner operation when cruising and hence better fuel economy
ideally in the cylinder we want combustion not an explosion or detonation higher octane combusts more so than low octane
it also contains more detergents which means a cleaner engine..and a cleaner engine is a happy engine

which gets me thinking... how clean is your throttle housing? a dirty throttle housing can mean you have to use more accel for the same power
also have you changed your trans fluid with OEM fluid? that can be a huge help

Premium, either 95 or 98, contains more than the minimum mandated amount of detergent. Car makers require that unleaded fuel contains detergent to clean the injectors. In ordinary 91-93 unleaded they use the absolute minimum quantity and quality.

The Premiums are premium not only due to increased octane, but also because of more/better detergent. This cleans the injectors better, and if used from when car/engine is new will keep the back of the inlet valves fairly clean too. This helps air flow into the engine, so more HP. Some premium fuels are able to move deposits from the back of medium carboned up valves too but as fuel companies often operate shared/common pipelines and distribution the brand on the bowser and the brand of fuel being pumped into your car can vary with each delivery. Caltex Vortex contains a very effective detergent, but you can't be sure thats what you are getting at a Caltex servo, and now with Woolies involved who knows what comes out of the pump?

The other thing in favor of Premium is that is a heavier fuel; contains more energy per ltr. Ordinary unleaded is about 0.73 specific gravity(ie. a ltr weighs 0.73kg). Premium is generally at least 0.75 and can be up to 0.78 if the octane has been brought up by blending in a lot of heavy aromatics such as toluol and xylene. That means there is around 3% plus more energy in every ltr(up to about 6% if there is a lot of toluol/xylene), so the engine burns less fuel to make the same HP. In closed loop mode the ECU cuts back the injector duty cycle as the exhaust Lambda changes so you save fuel. In open loop mode you may not see any economy improvement. It all depends on how well you drive.

Actual fuel burn rates have got more to do with the components blended into the fuel rater than the actual octane level.

Cheers,
Alan

hmmm i was always told that higher octane caused the slower burn rate.... learn something new every day

hmmm i was always told that higher octane caused the slower burn rate.... learn something new every day

A common misconception. Burn rate is more a function of mixture preparation quality, combustion chamber shape, engine speed and throttle %, plus what actual fuels have been blended together on that day to form the compound we refer to as petrol/gasoline. Octane just raises the "explosion" threshold, so combustion pressure can peak sooner and higher without going into uncontrolled burn, detonation.

If lab tested, a dozen 98 RON fuels would all have different burn characteristics, and want slightly different ECU mapping. Big race teams routinely buy a full years fuel from the same batch and then go about developing/mapping/dyno testing based on the characteristics of that batch.

Good race drivers/riders can pick varying throttle responses too, so in the top grades of motorsport the fuel companies will be asked to batch fuel to suit the fancy of particular fussy drivers/riders, or to correct a fuel with problematic throttle response. The end product will still qualify as say 98RON petrol but will be quite different, and perhaps cost up to US$8.00 - US$10.00 per ltr.

Cheers,
Alan

yet again Alan another awesome piece of information. It puts theory to a point where I find that, for example, BP Ultimate seems to feel better than Shell V-Power in my car. It is much much smoother to drive with BP, less shaking at idle and so on.

I once put that Shell V-Power Racing into my old car, and it felt like absolute crap while I emptied out the whole tank as best i could

A sick oxygen sensor does not necessarily throw a fault code. They die slowly and misinform the ECu causing incorrect fuelling. Just replaced mine at 116k.

Same goes for most sensors, its a pain as a light would be great, but instead you have to go over the car with a multi meter.