Few questions about an idea I have... 99 XJ

Hey all.

I have a 99 XJ 4.0 sitting on 32" ATs with a 4ish" lift. I have also a donor 4.0 from an 06 Rubicon that will be mixed with my CJ7's 258 to make a 4.6 for the CJ. The cam, lifters, springs, injectors, and fuel rail are practically new... only 7k miles. Would these throw the 99's computer off-kilter if I put them in?

Also, is there a way to fix a front transfer case yoke onto the back shaft to fix a modified front driveshaft to, or am I better off getting a SYE kit??? I'm poor and have the skills to do it if it can be done, but I don't want to go into this and wind up throwing good money after bad, either.

Also, is it normal to get 12.8 mpg in an XJ of this stature??? I had an 03 Rubicon, stock 4.0, lifted 6" on 35s, with the added weight and drag of onboard air, a 9000-lb winch, a full-size spare, heavy-duty tire carrier, and one helluva front bumper to match... I still got 14-15 mpg out of it! The XJ already has a K/N intake... could a muffler help? 4.10 gears(whoever lifted this Jeep did it more for looks)???

If you've read this far, then thanks for bearing with my late-night ramblings, and I appreciate any help you all have to offer.

The '99 computer should cope with the '06 parts just fine. However, is it worth the effort if the '99 is running fine?

Your XJ mileage is pretty low. If your mileage quoted is mostly highway, IMO the gears are not your problem. Something else is wrong, possibly with as sensor?? Have you tried scanning for codes or replacing the O2 sensor?

I am not sure about the yokes. I have never heard of this being done, but if you have the spare yoke, it might be worth a closer look.

SYE eliminator I haven't heard done on an XJ, but check the Wrangler area for discussions.

Low mileage...if the previous owner didn't change the gearing, your speedo could be off, showing that you're not going as far as you actually are. You may want to have it calibrated, or see about having the speedo gear changed. Also, the axle ratios won't hurt, you'll get better performance and probably a boost in mpg if you change them to the 4.10's.

SYE's certainly do happen for XJ's as well. There are really only two ways to do it, and both involve a kit. The full SYE kit, or a Rubicon Express hack-n-tap kit. Both give you a solid yolk in the rear, but the SYE will give you a shorter transfer case, resulting in a longer drive shaft, and eliminates vibrations.

With either choice you will need to replace the rear drive shaft with one with a slip joint in the middle. Some have had success in having a front XJ DS cut down to length and rewelded, but you really need to do it properly to avoid warping when welding it back together. There are manufacturers who will make you a custom DS, like Tom Woods.

If your motor was shot, I'd say go for it. However, there is alot involved with doing this swap, if you don't even really need it, why bother? If it ain't broke, don't fix it.

HTH

Rob D

Thanks guys. I'll be in the market for O2s for sure, seeing how this thing has almost 120K on it... aren't you spota replace them at 50-75K? As for the re-gear, if we didn't live in the mountains I wouldn't even consider it. And she runs fine right now so we won't swap engine parts out... yet.