Car is a TJ 3.5l Manual.

Last few weeks i have noticed the trip computer avg fuel consuption has jumped from normally sitting around 10.8/9 to as high as 12.9. Nothing has changed about my driving as I normally just drive this car to work and back.

One out of the ordinary thing that has happened is a week ago the water pump died and the car lost all its coolant. Got this fixed and also had the timing belt replaced as the water pump pulley had started to damage the timing belt.

Got the car back today and was hoping the fuel consumption had something to do with the water pump being on its way out and that it would return to normal. No such luck. Fuel consumption is still sitting on 12.9.

Two even strangers things I just noticed today were if I just let the car idle the fuel consuption will just keep climbing. Left it idling for about 10 to 15 min today and it went upto 27l/100km :wtf:
The second strange thing is that it won't show anything for the instant fuel consumption, just shows --.--
Did a soft reset on the computer, made no difference. Have not done a battery disconnect reset yet.

Any ideas on what this could be? my own searching comes up with either 02 sensor need replacement or a fuel leak. I cannot smell or see any obvious fuel leak though.

Timing belt may have been fitted a tooth or two out - my TJ had a similar excessive fuel consumption when I bought it - the belt was so far out the car eventually failed to start - on correction it came back to the 10-11L mark (auto)

hmmm that is interesting because when i mentioned the strange fuel consumption to the mechanic when i picked up the car he thought the same thing. Specifically that the bearing crapping out on the water pump pulley had caused the timing to go out.

But he then proceded to ask the other mechanic who actually did the work if the timing was out and he said no it was fine. But im am wondering if he just said that because i was standing there and he actually never checked the timing?

Im a bit of a noob when it starts getting into areas like timing so can someone tell me if the timing is something that has to be set/checked when a timing belt is changed or can it be done with out touching the timing?

You need to differentiate between ignition timing and cam belt timing.

Ignition timing is set by the ECU and is not adjustable. So there is nothing to check.
Cam belt timing is done by lining up the marks when installing the belt.

My TJ is sitting on 16.2L/100 KM at the moment, cold weather and short trips do that.

QUOTE:
Two even strangers things I just noticed today were if I just let the car idle the fuel consuption will just keep climbing. Left it idling for about 10 to 15 min today and it went upto 27l/100km

Yes, so? Stationary car means km travelled = 0, fuel still gets burnt, so your average will go up, especially if you have just reset the trip meter. Let it idle a bit longer, and it will go up to 64 L/100 KM, or possibly higher. (64 is the highest I've seen)

Time to put your thinking cap on, I think!

You are correct, the mathematics do indicate that avg fuel consumption would continue to rise when you are idle. But in reality after 3 years of ownership I have never seen it rise that high that quickly while at idle and never seen it get to 12.9 and stay there during normal everyday driving. Some is not right with this car even after replaceing the water pump.

And my thinking cap is burnt out atm, studying for my final electricians exam atm and all thats in my brain are voltage drop and fault loop impedance equations. Random car gremlins are not what i need atm.

The car has to be moving for the instantaneous fuel consumption to display. Idling is when the the trip computer is least accurate as it appears (from my observations) not to compensate for the effects of injector latency in the short injector pulse widths at idle. The effect will be much more obvious if little distance has been traveled since the last reset.

Without the ability to log fuel trims from the ECU, the only way to check the condition of the oxygen sensor without removing it is to:
1) disconnect battery (to reset ECU)
2) disconnect oxygen sensor
3) reconnect battery
4) test drive

Ideally the test drive should be at least 20 minutes long and at arterial road speeds without much in the way of stop-go. If the fuel economy with the sensor disconnected is better than it was with the sensor connected, physically checking the sensor would be warranted. Otherwise, reconnect the sensor (with the ignition off!) and look elsewhere.

Resetting the ECU is absolutely critical to this check, otherwise the ECU just uses the fuel trims learned with the possibly bad sensor and the result will likely be inconclusive. The oxygen sensor connector is near the bottom edge of the passenger footwell side of the centre console about in line with the dash.

in reality after 3 years of ownership I have never seen it rise that high that quickly while at idle and never seen it get to 12.9 and stay there during normal everyday driving.

And my thinking cap is burnt out atm, studying for my final electricians exam atm and all thats in my brain are voltage drop and fault loop impedance equations. Random car gremlins are not what i need atm.

I'd be quite happy getting 12.9 out of my TJ.
Short trips, wet and cold weather, stop/go traffic play havoc with the fuel consumption.

My lowest is 7.7 on a long, sedate country drive in warm weather, my worst is always short trips in winter at 16+.

To be honest, there is probably nothing wrong with the car and your mind is trying to distract itself from your studies! (Been there, done that).

To test your fuel trims/Oxygen sensor, just get the car warmed up and look at your instantaneous consumption at a steady speed of 60, 80, 100 KPH. Those numbers tell all.

Check the o2 sensor mate, check it is responsive and working properly, this can often cause a jump.

Also reset it, take it on a nice consistant drive at say 80 or 100 up port wakefield road or similar and see where it settle at cruise, this is a good initial indicator if something is wrong or not

Thanks for the advice guys. Will test the 02 sensor when I get a chance. Anyone know roughly how much a new one is?

Fuel consumption seems to have gone back to normal now, sitting on 11l/100km. Just took alot longer for it to settle back to normal than what i am used too.

On a side note the overheating issue I mentioned in this thread http://www.aussiemagna.com/forums/showthread.php?t=94025&p=1569350&highlight=#post1569350 seems to have reduced now. Its not completely gone as the temnp guage does still creep above half if the traffic is bad enough but I have not had to resort to turning the aircon on to force the secondary fan on. Seems the old waterpump was just not doing a very good job since i have had the car.