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[Brynjaminjones]

I've noticed lately that the XJ is using a bit more fuel than normal - I used to consistently get around 18mpg, but now I can't seem to get above 15.

I know it's not a big drop, but I want to see what you lot recommend checking to bring it back to normal.

The plugs, wires, cap and rotor are all about 18 months old, oil is fresh, air filter is new since Breakout. Idle is smooth, but it is running a little hot.

What should I check?

[wildwood]

Try and sort the temp.

Do a fill and run to empty and refill to get accurate consumption figs.

Maybe a plug in diagnostic thingy to check any weird codes.

Tis a puzzler.....

Have you gone up a size in tyres? Cos that'll do it.

[Brynjaminjones]

Tires are larger than stock, but they were on when I was getting better MPG, and they didn't actually reduce it at all as they're not exactly huge.
Could the O2 sensors cause a drop this small? The wire on the one on the cat doesn't look in the best state, but I don't know if that would affect how it works at all...

[wildwood]

Just had a look on Mallcrawlin.com and a geezer there was going on about doing away with the clutch on the fan and using an alloy spacer to lock it solid.......he did it because his GC and XJ were running hot in humid Texas summer traffic with the AC on.
It worked and he is running cool and got an additional 12 BHP.

Just a thought on you're running hot...... Doesn't help the mpg tho...... But maybe linked???

[donk498]

Check if your rear brakes are binding slightly after the rebuild.

[wildwood]

  you said brakes,now,work on two clicks so maybe tooooooo tight???

[Brynjaminjones]

I'd thought that, but now that the brakes are worn in a bit I do have to pull the handbrake higher than two clicks. How do I check if drums are binding? Is it a case of jacking up and turning the wheel?

[gazjeep]

When you have taken it for a run, have a feel of the wheels NOT THE BRAKES !!! ...... to see if one or more is warmer than the others, if one is warmer, brake is dragging 

[Brynjaminjones]

Cheers, shall do that this afternoon. I checked the front brakes yesterday and they were both equally hot after some hard braking. I didn't check the rears though, as I've never noticed any heat coming from them.


Another thought - would bad wheel alignment have much effect? I did clip a kerb quite hard a few weeks back (I obviously wasn't trying to get the back end out around a junction  ), and although the Jeep has always pulled slightly to the left, I'm not sure if it's worse now or not (this was shortly after driving a well aligned car for a while, so it could be that that's made me more aware of the Jeep pulling!).

[gazjeep]

Bryn, if the wheels are anything more than tepid to the touch then the brakes are binding. Always check all 4 for comparison

[Jonny Jeep]

18 to 15 is a near 17% drop so hardly a small drop. And yes, the O2 sensor could definitely cause it, but only the upstream. try running a tank of fuel with the sensor unplugged. If the mileage goes back up, replace the sensor. CEL will come on with sensor unplugged but it's nothing to worry about. The computer will be forced to run off programmed fuel tables.

I unplugged my upstream sensor when the heater went bad and I was waiting for a replacement NTK sensor from rockauto. I didn't suffer any degradation in MPG.

[JamesH]

Could be an O2 sensor, have you scanned it for fault codes?

[Dave69]

18 to 15 is a near 17% drop so hardly a small drop. And yes, the O2 sensor could definitely cause it, but only the upstream. try running a tank of fuel with the sensor unplugged. If the mileage goes back up, replace the sensor. CEL will come on with sensor unplugged but it's nothing to worry about. The computer will be forced to run off programmed fuel tables.

I unplugged my upstream sensor when the heater went bad and I was waiting for a replacement NTK sensor from rockauto. I didn't suffer any degradation in MPG.

if you unplug the O2 sensor the running will be rough initially while the system sorts itself out. no change in running is an indication of a bad sensor. normally this would be flagged as an error in the diagnostics

[Jonny Jeep]

With the O2 sensor unplugged there will be no rough running at all, unless something else is wrong. There's nothing to sort out. The O2 sensor isn't used at start up for at least 20 seconds anyway, temp dependent. After that, the engine will run faultlessly using it's programmed fuel tables if everything else is OK.

[Brynjaminjones]

Gaz, checked all four wheels again tonight after a 4 mile drive. Front were both equally slightly warm, whilst the rear were cool.

The sensor I'm concerned about is the downstream one, but that's just because of how it looks. I've not scanned it either - Dave, I don't suppose you have a scanner?

Not wanting to sound stupid, but how do I unplug the sensors?

[Dave69]

down stream sensor is normally only used to verify the correct operation of the cat converter. the pre-cat sensor has the effect on the fuelling. trace the lead back to the loom and there should be a plastic connector, either white or grey in colour

jeep must have the control from the lambda sensor controlled differently, disconnect one on a toyota and you see the effect straight away and check engine light displayed within seconds.

sorry one piece of kit i didn't buy. what you could do is ask your garage for a quick emissions check. this will then tell you what the fuelling is actually doing

[gazjeep]

Bryn, get one of these mate, works on Wrangler & my Panzer so should work on XJ

http://www.ebay.co.uk/itm/CAN-BUS-OBD2-EOBD-Engine-Diagnostic-Fault-Error-Code-Reader-Scanner-Tester-Tool-/351081572353?pt=UK_Diagnostic_Tools_Equipment&hash=item51be17a801

[JamesH]

The rear O2 sensor isn't normally the problem because it doesn't do as much as the front one. Either get a reader like that or a Bluetooth ELM327 interface and a diagnostic app on your phone or tablet.