Can't get better than 15mpg

[unlimitedrubicon]

OK, that makes sense that the pistons would be proud of the block if you were using a 4.2 crank and 4.0 pistons / rods.

[kf_chris]

It's not the engine I wanted to build but it's got a lot more power than a 4.0.
I'm just less than happy about the constant 15mpg.

[unlimitedrubicon]

Given that it had an 8 1/4 rear end I am assuming this is an XJ, is that correct? What year is it?

How long has it been since all of the fluids have been checked / replaced? I have found that on older vehicles with a lot of miles on the fluids, gear lube especially, replacing them with fresh synthetic makes a huge difference.

Have you checked that your clutch / torque converter (depending on trans) is not slipping thus dropping your efficiency?

How much does it weigh? Is it fully stock – with no additional weight from armor / racks / cage / etc.? Do you carry a lot of stuff in it? Even just 100lbs can make a measurable difference in fuel economy.

[kf_chris]

Was the 8.25 with 3.55 gears. Now it's a 8.8 with 4.10's. No change in mpgs

Trans is new, like 2500 miles new. xfer case just got rebuilt. Diff fluids are both new.

I don't carry a spare, or have big bumpers or a rack. it's an empty jeep most of the time. I'm baffled

[lone_eagle78]

hope you can figure something out as far as better mileage, I am just hoping to get that high. I have a worn out (250k miles) 4.0 in mine at this very moment, and I only get 7-8. I just got a jeep with a 4.6 stroker and am putting that motor and trans into mine, so I am hoping to get to 15.

[banipal19]

Have you checked your AFR with a wideband O2 sensor? Maybe your injectors are dumping too much fuel into the cylinders

[kf_chris]

I haven't used a wideband. I did a good bit of research on getting one before the build and decided I didn't need to. I may have it on a dyno (when I can get the bitch running again) and check the AFR then

[kf_chris]

It ended up being the throttle return spring as mentioned above. Hope that the TPS isn't a problem it's throwing a code, so I'll have to have it checked later. Be looking forward to a dyno trip eventually!

[ibanezjeepguy]

static 9.92
dynamic 8.65

think about it... your motor wants to compress all that air and fuel mix thats being sucked into the cylinder BUT your cam wont allow it.
if you dynamic ratio is correct as you list it... compared to the actual unadulterated CR, then your engine would probably be most efficient closer to the 3,000 RPM range, maybe even approaching 4...
think about it... if a "stock" 4.0 has a static CR of around 8.7:1 and a dynamic of around 8.45:1, its general sweet spot for power and economy lies right around the 2,000-2,200 rpm mark. In creating such a long stroke and high compression engine without actually utilizing ANY of that compression at all is essentially a waste.

If you haven't already, I would hook up a fuel air ratio gauge and tune up your MAP sensor so you're not dumping in excess fuel but if your plugs look ok then you're probably not anyway.

For the HHO gas generator... in closed loop mode, your o2 sensor will see the excess oxygen in your exhaust and enrich the fuel supply to compensate.
Did you compensate for that factor?

Good luck man!