fuel economy increase NEEDED desperately

[bugadifino]

Bear in mind I drive my XJ like I'm a 87 year old with a bowling hat on the parcel shelf....... I am doing a huge trip from Adelaide to Monkey Mia & back in 6.5 weeks with wife, 4 kids, & caravan in tow.

The results are in after fitting a new O2 sensor. I get a little more mileage than before, maybe 30-50 km more per tank. However, still way under what I would have expected.
(These are rough figures) With an 80L (60L usable) gas tank, & a modified 35L petrol tank we have a rough 400-450km range (approx $85 at $1.30/L).
Fuel costs alone are going to cause me to remortgage the house, I really need some REALISTIC ideas on what may be causing such shitty economy, & how to drastically improve it.

I'll try to attach a photo of it so you can see what crap is on it, tyre sizes etc...

[Laz]

Looks like your running 31's maybe and an inch or two of lift (punt) you didn't mention your base economy figure by fuel type - but i'd expect with about 95 litres (combined) of feul you should get 500-600 Klms without too much hassle

You'll get better econmy running the tyres at the correct pressure (you can do the chaulk line test across the tread, roll forward a couple of revolutions then check the wear on the chalk - centre missing too much pressure - outside missing too little - do this with the tyres at running temp)

Replace the air filter before anything else (repco)

Get the catalytic convertor checked out - mine was good for about 5L/100 when it was stuffed.
Most exhaust shops have one of those point and shoot laser temp gauges - they take the car for a hard 10 minutes run then check the temp on the front and back of the cat - if the back isn't 30% or so hotter than the front then it's not doing it's job (Burning up all those nasty unburnt hydrocarbons)

Next step is to get into something like a dynotune who have seen more than one jeep in thier life and get them to run a diag - this will tell you if the TPS (Throttle position sensor) or mixtures or 02 sensors are screwed.

I get around 19L/100 towing and about 16-17 normal driving - many of the newer models with coilpacks and different inlet manifold get a fair bit better.

Most ppl with xj's seem to get around 14L/100 round town and about 17L/100 towing - so I don;t know how much more of an improvement your going to see..

what kind of economy were you expecting out of gas? about 18L/100 round town and 22L/100 towing is probably on the ball.

More XJ ppls will be able to help mate hope this helps

[dsmit376]

thats pretty damn poor economy,
I own a 1998 xj sport and it uses heaps less fuel then that.

On fraser island with 5 people a boot full, & towing a trailer I used 100L for 550km of sand driving, half of which was in soft sand racing the tide on the way up(as one of my friends turned up late), thats an average of 18L/100km. I run a 3" lift with 31" tyres and maintain my car very regularly.
My xj on the highway empty will average 12.5 - 13L/100 and 15 around town.

First thing i'd do is check all your sensors to make sure they are in the correct range, things like TPS is easy you just unclip the leads and put a multimeter on the contacts, as you slowly open the throttle you should see the resistance smoothly move (up or down ive forgotten).

Change your air filter just like laz said, if the engine can't breathe it has to work harder to ingest fuel and air.
Alot of people who have no clue about cars just think if they have a new 4wd or near new 4wd that because its new if they take it offroad the scheduled servicing should stand, ie changing air cleaner every 10-15,000kms, totally wrong, i take my air cleaner out after every day of offroad and hit it against the tyre, you almost can't see yourself because of the dust. Replace it or atleast check and clean it regularly if you do any offroad work.


Chuck in a bottle of injector cleaner and go for a few hot laps giving it a bit of stick to flow some fuel to clean them.

Change your plugs and leads and set the correct gap before you put the plugs in.

Also after you have done all this reset your ecu/pcm so it can relearn, its not hard to do(different for different year models)
just undo the negative lead to your battery, touch the lead to the positive lead to short out the remaining energy stored in the capacitors, while its disconnected put your key in the ignition and turn it on to the run position, turn on your headlight switch turn off your head light switch, key off and reconnect battery.
The car will then relearn the next 50 dead cold starts.

Hope this helps

[99JEEP]

my old mans 97 xj has same set up as you and gets a max of about 500k out both tanks 300 gas 200 petrol. You will chew through the gas when towing as gas is not as economical as petrol thats why if a gas my xj i'll just put the tank in the back for that extra range. But to sort of reply to your question your not going to get much better range with that set up.

all the best Mark

[Neil]

Your economy is about standard with mine, when towing van I tend to use petrol when under load (ie hills etc) and cruise on flat on gas.

[Rodgebone]

you should be getting between around 600km with that setup....soooo,

air filter
plugs....035" gap (you may want to run one range colder plug with lpg)
ignition leads
cap/rotor
tire pressure at 30-35psi
correct alignment (especially toe setting)
with new O2s you should reset the computer as already suggested
tps has NOTHING to do with fuel economy UNLESS you have a flat spot during highway cruise (u would feel a hesitation shifting speeds at cruise)
make sure yer t-stat is a 195 degree unit - 180 will kill yer mileage
if yer like me and have a heavy foot you can expect nothing better than what you got...keep it under 2000 RPM and your mileage will increase
keep the underhood temps down...make sure yer hydraulic clutch fan is working properly...insulate your airbox (themotec is good)...insulate your exhaust manifold (thermotec again)
flush your injectors ($130 thru Poly 4x4)

hope that helps and enjoy Monkey Mia!

[hillsjeep]

Great post Rodge, I see another entry in the Index to cost repairs sticky coming.

EDIT: Item 25 added...