Fuel economy troubleshooting 2004 3.8ltr NP

[TroyHodges]

I bought a PLXdevice to gather stats and errors via the OBD connector and so far I'm not getting great stats and no errors. Round town i'm averaging 20-22l/100km. Standard tyres, 2" lift, bullbar (no winch) and 2x roof bars, so nothing too excessive.

So far I've:
- Replaced the air filter
- Cleaned MAF using MAF Cleaner
- Cleaned the throttle body using throttle body cleaner
- I have just replaced to Oxygen Sensor (took some finding as this model/year only has one and vendors kept telling me it had 3... Eventually of all places SuperCheap found the one I needed) anyway I can see it switching voltage via the PLXdevice so I can only assume all is good. But no better economy yet. Should I drain the ECU? or let it adjust itself over time?

If this is not my problem does anyone out there have any ideas where to now?
I was thinking of the following but didn't want to change them all at once so I can tell what the fuel problem was caused by:

Plan of attack!!!
- Replace plugs to dual points?
- Replace fuel filter?
- Replace the ignition leads?
- Figure out if there is a vacuum leak? how?
- Figure out if the is a fuel pressure regulator issue? how?
- Figure out if I have faulty injectors? how?


I have seen reports that the 3.8ltr should be averaging around 16l/100kms at it's worst (eg towing) so I feel I have a problem and a lot of improvement to make. Other than the economy the car runs well, not fuel related hicups/coughs or splatters...
I have read many threads which is what I've gathered for my plan of attack above. Would be great if anyone can provide advice along the way, especially the hows? and the order of attack?

Thanks Troy

[hobbo]

I think for around town you Are getting correct mileage. NP 3.8L are Known for being heavy on fuel, I would take it for a good 500km cruise at 100km/h to see what it gets at it's best?

[TroyHodges]

I've seen plenty of threads with 16l/100 on average for the 3.8. Yes I expect it to be thirsty but I think 22-23 is going a bit far to say its normal.

[dolphin]

you need to take into account; how old is the engine as in K,s on the clock.
how do i drive the vehicle steady from the lights or hard on the foot.
how is my tyre pressures for road driving, not to soft .
how old is my exhaust system plus the catalic convertor .
my 3.5 litre is quite good on fuel with 165k,s on the clock plus a good free flow exhaust system from headers to tail muffler, i think i am getting around 13 litres per 100 around town and better on the open road. a good 650 k,s per tank.

[Bundyk]

I agree, 22-23 is heading beyond acceptable. With what you have listed I would say 16-18 average. Start with the fuel filter and see how you go. Do you have an exhaust leak at all ie cracked manifolds? My economy improved after fixing this by fitting extractors.

Might also be worth seeing if there is a way to see if the EGR system is working properly. On the topic of exhaust, is you cat converter(s) blocked in any way? This can also cause high fuel consumption

[wrexy]

Do not think you will get 16's towing with a 3.8. With a diesel that will be more realistic.

[bazzle]

Unfortunately for us the Mitsubishi 3.8L engines (non-Mivec and Mivec)

are not renowned for their fuel efficiency/economy. When towing

maximum loads in difficult terrain some members have reported economy

figures of around 25-30L/100km from both 3.5L and 3.8L Pajeros.

I have kept figures for my fairly standard 05 Exceed over a one year

period of mixed driving and at best managed 13.9L/km, and at worst

15.7L/km, driving with a very light foot and NO TOWING. From what I've

read on this and other forums these figures are quite acceptable for this

vehicle and so I haven't investigated any further improvement.

Even with a mechanically perfect vehicle there are many factors that

can impact on fuel efficiency, these include, but are not limited to:


*Ambient air temperature
*Atmospheric pressure and hence, altitude
*Choice of fuel (RON to suit engine compression parameters)
*Quality of fuel
*Aerodynamics of vehicle and load
*Ratio of vehicle mass to towed mass
*Tyre pressure
*Suitability of wheel/tyre combination in relation to final drive ratio
*Suitability of driver-selected gear ratios to the load and terrain
*DRIVER INPUT/DRIVING STYLE

If anybody can think of others please chime, it's getting a bit late for me...

[Goodsy]

My 2004 3.8 averaged 20L/100k's. Good was 18 and often was over 20l.
Same as you.
My had 265/75R16 AT's, steel bullbar, winch and a 2 inch lift.
I would hate to think what it would be like with a rack on top.

You are maybe a little over in fuel eco but sadly not that much.
The 3.8 is a thirsty beast even when driven like a granny.

What fuel are you using?

[hobbo]

Thats why I put my NM 3.5L on Gas, it wasn't bad on the open road, the best I got was 12L/100km but Stop/Start around town it was a Monster! 20L/100km average. These are Heavy Vehicles at over 2 tonne. You might be able to Improve it by changing the Plugs and Cleaning the Injectors but not by much.

[TroyHodges]

Great feedback guys,

I'm very light on the pedal, with this PLX KIWI showing me realtime fuel usage is scares crap out of you even when your gentle. On gentle take off the usage climbs up over 50-60/ltrs per 100, granted it is normal for fuel usage to climb on acceleration but after seeing this you soon curbs your accelerator pedal habits.

Tyre pressures usually run at 35? I'm just going of stamp on car so I might research a better pressure here.

Exhaust system age I'm not sure but that's another item I was considering. Is there a way to check the catalytic converter for blockages? I have seen people mutter "clearing the blockage" which I assume is removing the internals. I assume illegal but is this a way I can test if it is causing my fuel problem? Or is there other ways to test/check? My NP has 165000 on the clock, headers look fairly newish (recently acquired car so not sure of it's history) but the rest of the exhaust looks a bit aged so maybe this is something I should investigate. Whats an improved exhaust system cost these days? How to I identify a "good free flow exhaust" for another? I did have the manifold looked at when I got the vehicle as I could hear excessive exhaust noise from the front end. The mechanic did a quick check and found it was leaking from the flanges joining the two pipes into one. They simply tightened them up and noise went away for a month... I returned to same mechanic and this time they replaced the gaskets, and it's been fine since. Maybe I need to re-think my exhaust altogether... I'll start looking into the costs...urrgggg.

As for fuel type, I been just using the standard Unleaded 91 from the local servo. I've been not trying to change the fuel yet as I think my issue is more than fuel type.


Thanks for your feedback guys, been very helpful. I'll keep the post alive as to my progress, hopefully I'll find some improvement to share.

[Easykill1978]

many members have stated improved power and economy when using non plat plugs.

also check the basic things that affect it the most.

• air filter - brown means its blocking a good percentage of the air.. so i would change that for sure.. i change mine often..
• fuel filter - when i had my petrol i allways changed it every 3rd oil change

.

and lastly i went from 91 to 98 octane and never went back..sure it costs more.. but i got more power and easy first tanke extra 50km and it slowly climbed in range till it reached or got close to the same fuel/cost ratio as 91.

and lastly lastly

i would check the leads and coil packs.. and your earth..

you can check the leads at night, simply start her up with no lights on and open the bonnet.. if you see the blue lines in the leads, than leads are faulty and need replacing..

clean the earth contact and reattach

have your injectors cleaned or rebuilt.

[bazzle]

Hey Goodsy, your "driving like granny" made me smirk. Whose granny? SWMBO is a granny (only just, the grand kids are just babies) ..... she accelerates and brakes like a V8 supercar driver, hence I don't let her anywhere near the Pajero, couldn't afford the fuel. When our kids were young they all groaned when I was the assigned driver to take them to sports.....they reckoned I should let Mum drive so they would arrive before kick off. When I'm her passenger I feel like I need a neck brace....she tells me my reflexes are shot... I should just sit back and relax.She must be as good a driver as she tells me she is, her safety record is A+++++ and she's never earned a ticket in over 40 years of driving. I wish I could say the same. Go figure....

[TroyHodges]

I'm losing this battle...

So after replacing the O2 sensor I started to see the economy stats a bit worse than the previous sensor. I had a thought yesterday that maybe to ECU might need a reset to clear the bad habits it learnt from the old (possibly faulty) O2 sensor. Anyway I unplugged the battery overnight and this morning turned on everything to clear any residual charge before reconnection the battery. After that I took some stats today on the short trip to and from work, 10kms and it now appears to be averaging 28lts/100km. I am being so very very annoyingly gentle on the pedal. Urrrrggg. So I went from 23L/100 and after a new O2 sensor I'm now getting 28L/100.

At a loss.... Did I not reset the ECU properly? Anyone? I figure that the new sensor has made difference so perhaps the old one was faulty? I can see the new sensor changing voltage between 0 and 1v as expected as did the old one.... Does this sound this a relearning ECU problem? Or did the new sensor now exaggerate my 'other' issue I'm yet to find.....

Help please.......
Regards Troy

[Easykill1978]

maybe it wasnt faulty.


check your air filter and repalce if brown, not the top underneath where the air goes thru....

check and clean your maf sensor too...

[TroyHodges]

maybe it wasnt faulty.


check your air filter and repalce if brown, not the top underneath where the air goes thru....

check and clean your maf sensor too...

Yah that still a mystery, however given that the two sensors give me different results I assume one of them isn't working as expected.

As for the air filter, I'd replaced it only 2 months ago and cleaned the MAF then too.