More XJ Mods

[Taylor95]

Wow your idle fuel consumption is really low! Is that US gallons? I wonder if getting the new intake manifold style and new headers would help me in that area.

I think the spoiler is definitely worth it. I'm not sure how much it would help in your situation though, because you keep a few things on your roof. I imagine it would yield at least 1-2 highway mpg on a jeep like mine. That's one thing I'm still hoping to get.

[alexbrown64]

Yep, thats US gallons. Works out to 1.8lt/hour @ hot idle (88c. 190f).
There is a big difference if it is in gear at the lights. It goes to about 2.3lt/hour, and then if your in gear at the lights and have the aircon on, well, it burns 2.7lt/hour. That 50% increase in fuel consumption idling at the lights in gear with the aircon running.

[Taylor95]

That is impressive. As I stated earlier, my Jeep consumes 0.6 gph at hot idle. This was after removing my mechanical fan. Can you attribute any of your mods to your low idle fuel usage? I have done a lot of the same stuff as you but my Jeep uses like 30% more fuel than yours at idle.

[alexbrown64]

When I put my obd1 mt2500 scanner on, my fuel trim is at -5%. My injectors are at 4ms. I do have 777 injectors. My rpms are just over 700. The engine had to be up near 180-190f to get the good hot idle readings. My mpguino is calibrated spot on. Resetting computer makes readings worse. Adaptive learning makes it get better. ANY slight vacuum leak makes it worse. I have a tranny cooler that kept kicking in early, as I have the trigger for that hooked up to my old triple fan setup. The load on the alternator was making fuel use at idle worse, so I disconnected it. Also, my aux fan was coming on early. I changed the sensor, and now it doesn’t come on til 210f I think. Anyway, removing All load from the alternator helped a lot. The aircon is a massive one that sucks juice at hot idle. All my pulley bearings are new, and my alternator is new. My engine is tuned to perfection, all new sensors, plugs, coil, cap, leads. No cracks in header, as it’s new. I run an oil additive called Xado. Very expensive, but I had a noisy lifter and it fixed it and made the engine run quieter.

Cheers,

[alexbrown64]

Well, the unofficial results are in. I did over 100k's of mixed driving today and the Jeep is running so smooth and the economy has definitely improved.

Just to recap, i fitted a touch screen MPG gauge to my OBD1 XJ 4.0. Wired through the injectors and speedo. After calibration it is very accurate and can give me instant and avg lt/100, distance to empty etc..

After getting base readings of avg combined 18.2 lt/100 and an 80kph (50mph) reading of 14.5lt/100 a few weeks ago, it was time to do some work on the Jeep to improve things.

This included swapping out the ZJ clutch fan to a Hayden XJ clutch fan. New O2 and MAP sensors. Pump tyres up to 40 PSI. New fuel filter. G70 friction modifier in diffs and Nulon E10 in engine. Reset computer.

After driving today, the computer is adapting well and bringing the economy back into check. At 80kph i now average around 12.5lt/100. This is a US 18.82MPG. Combined at the moment is averaging around 15.9lt/100 which is a US 14.79MPG.

Considering my XJ has a lot of mods and weighs a lot more, i think the figures are starting to become reasonable. A brand new unmodified 1998 XJ 4.0 Auto had an official government EPA rating of a combined 16MPG and 19MPG highway rating. https://www.fueleconomy.gov/feg/noframes/14742.shtml

I'm not far off those figures now. A steady foot helps alot. Also, my transmission has the power/economy button. Economy locks the torque converter up at 45/50 kph (power setting is about 70kph), so that driving around town, i notice it uses a lot less fuel.

Cheers,

[Taylor95]

Those are pretty big gains! What are those things you are putting in your engine and diffs? Currently I use 10w-30 full synthetic and 70w-90 synthetic gear oil.

My Jeep is a lot more efficient now that I replaced the battery and alternator. I drive mostly highway (~65 mpg or about 105 kmh) and I average 22 mpg. I'm very happy with this but I think I can do better. Most of my sensors are original, so I think replacing them might slightly improve my numbers.

[alexbrown64]

Taylor95, 22mpg is very good. Your getting 3mpg better economy than what was reported for a new one.

The additives i put in the XJ diffs and transfer case contain PTFE friction modifiers. I use it to reduce friction and make things run smoother.

My engine has done 201,000 miles (325,000 kilometers). It was starting to sound a little mechanical with very mild lifter noise. The additive has just quietened it down and made it run smoother. Sounds more like a sewing machine now.

[Taylor95]

I'll definitely need to look into those.

Have you looked into any engine mods? I definitely want to get a set of roller rockers (greatly reduced friction), a mild cam and maybe rebuild the whole thing with increased compression.


On forums I've heard people say that the roller rockers would give you an easy 1 mpg. They are expensive though.

[serialk11r]

Originally Posted by Taylor95

On forums I've heard people say that the roller rockers would give you an easy 1 mpg. They are expensive though.

That claim sounds highly suspicious. Motorcycles and a lot of high performance cars use bucket lifters. A roller rocker's pad still slides on the valve stem, and the roller is a plain bearing, so the friction is on the same order of magnitude (as opposed to e.g. roller bearing vs hydrodynamic journal bearing, where you have a 10x decrease).

Higher compression ratio definitely helps a lot since you're only at 8.8 if I'm reading specs on the internet correctly. If you want to mess with cams, going higher duration while keeping overlap the same will reduce torque but increase part load efficiency.