Yesterday I reinstalled my front axle shafts and got the front end spinning again. Left the driveshaft out for now just to take it one step at a time. Everything sounds good rolling down the road. No drivetrain noises! That's great, and makes me feel much better about my axle internals. I will get the front driveshaft back in and drive it for a couple days before giving it the final seal of approval.

I gave everything in the front end a good eye-ball and shake while under there. So far, so good on everything. Bolts are staying tight, nothing cracking off, nothing rubbing. All seems good.

Here's a detail I wanted to share. The JK unit bearings have wheel speed sensors built into them. Because I'm not using the speed sensors, I clipped the wires and used this as a grease fitting. JK people who have dissected these bearings after failure have noted there is very little grease from the factory inside these units and have started greasing here. There's some talk about whether this is okay for the sensors, but without needing the wheel speed info, I was able to pack these bearings full. Hopefully that helps just a little bit. One of the biggest downsides of the JK swap is it still uses Dana 30 unit bearings. While the JK bearings are technically bigger/better than the XJ bearings, they're still pretty wimpy. These are Spicer bearings, and they're not cheap. Fingers crossed they last longer than the one year I was getting out of XJ bearings.






While doing this, I investigated why my I haven't been able to steer all the way to my steering stops. I was surprised by what I found. Out of all things, I am binding the tie rod ends on my Fusion4x4 tie rod. This is actually by intention on the TRE's. They have a limited amount of misalignment built in to prevent the tie rod from flopping up and down too much. The amount of misalignment would probably be just right for the OEM steering angle, but now that I am asking more of it, I'm finding the limit. This tie rod uses Rare Parts (brand) tie rod ends. These TRE's are rebuildable. Part of me is curious to see if I can modify the joint to allow more misalignment, the other part of me wants to call it good enough and not worry about it. Add that project to the backburner list.


Last thing to note is I reinstalled a factory airbox in place of the cowl intake to do some testing with. Man, what a difference. I haven't noticed just how loud the cowl intake is until I've been really paying attention to it recently. The OEM airbox is so much quieter and nicer. Using the sound meter app, I was able to see a 4 dB reduction in average sound level (77 to 73) over the same drive, which was mostly driven from the noise level reduction under throttle. With the cowl intake, I recorded 94 dB inside the cab at WOT. With the OEM intake, 84 dB. A 10 dB reduction is huge. That represents about one half of the perceived loudness to the human ear. I still saw peak noise around 88 dB when hitting potholes, but those impulse spikes do not have nearly the effect on the average level as sustained throttle. Needless to say, I think I'm going to find a way to go back to the OEM intake, a snorkel, or something totally different. Regardless, I want to keep the intake out of the cowl. I had to move my coolant overflow reservoir to put the factory airbox back on. Its currently zip tied off to the side. Need to do something more permanent. I was able to see my intake temps are 40-50°F higher at 50 mph with the factory airbox compared to the cowl intake. Not the end of the world.

 

Just recorded this 0-60 pull. Came in at a pretty lame 15.5 seconds. These were rated for 8.2 seconds stock. Ouch. Note that is with 5.13 gears and 255/75r17 (32”) tires. Watch the speedo on the GPS on the mounted phone, not the dash.




So, uh, what gives?

 

I find it a bit hard to believe these things do 0-60 in 8.2 s stock, but 15.5 s seems slow. With your tire size and diff ratio, 60 mph happens at 5000 rpm in 2nd gear. Did you do the run in 2nd so it wouldn't shift into 3rd?

Cowl intakes don't make any sense to me. While the cowl receives outside air, the cowl area gets toasty warm, and the metal intake pipe will as well. And while the cowl is positioned pretty high, its subject to flooding from rain or washing, especially if the drains get clogged. On a significantly lifted XJ, taking cold air from the headlight area seems fine unless doing some serious water crossings.

 

I ran the test in D. Didn't do anything, just floored the gas and let it do its thing. Watching the video, it shifts into 3rd at 4750 rpm on the tach, which is 52 mph on the GPS. Using the Grimm Jeep calculator, a 29" tire would produce 52 mph at 4750 rpm in 2nd gear. The tires I have are pretty bald. I just measured them to be 30.5" rolling diameter. Considering the margin of error everywhere, it seems accurate enough.

Looking at it another way, to convert 60 mph to what the speedo is actually reading out, we can do 60 * (5.13/3.55) * (28/30.5) = 80. So, looking at the video I can see 80 mph on the tach happens at 3700 rpm in 3rd gear, and actually is at the same time as 60 mph on the GPS. Good evidence the GPS is reading accurately. Not sure where the discrepancy is coming from with the Grimm Jeep calculator.

The biggest benefit to the cowl intake is how much space it frees up in the engine bay. That part is hard to argue with. Other than the noise, and the fact that I've always been a bit skeptical of how well it seals around the filter, I've never had any issue with water ingress or anything else. It definitely pulls cooler air than the factory airbox. While I don't enjoying deep water crossings, I have dipped a headlight a couple times. The factory airbox probably would've taken in water in those situations.

In other news, I put my front driveshaft back in yesterday and drove it to work today. Thing was shaking like a dog. Lot of noise and vibration above 40 mph. Definitely something going on that needs investigation.

 

Have you weighed it recently? It does seem like it's working harder than it should...

My very first XJ was a completely stock 98 automatic and it was surprisingly quick - they have great power-to-weight when they're stock and healthy

 

My jeep was lifted 5" on 33s and I dunked the headlights more than once, causing the engine to sputter. But the water was no deeper than the rockers. The problem is when dropping into a water hole, the front dips into the water before the tires hit the bottom of the hill.

 

Weight crossed my mind as well. I've certainly added a fair amount of weight to this thing, but not an astronomical amount. I've never weighed it, so can't comment there. Always been curious. I should really do that...

Figure between my stiffeners, bumpers, sliders, suspension and drivetrain mods, winch, spare tire etc, etc, I've added 1000 lbs. That's probably a generous over estimation, but let's use that number for the sake of argument.

Google says curb weight was 3360 lbs with 190 horsepower, so 0.0565 hp/lb (17.7 lb/hp). Factor in my added 1000 lbs, that number becomes 0.0436 hp/lb (22.9 hp/lb). So I've decreased my power / weight ratio by about 23%. Not great, not terrible.

Factoring in my current tire size and gearing difference, I expect the 0-60 time to be 75% of the factory time ( (3.55 / 5.13) * (30.5 / 28) = 0.75 ), or 33% faster than stock.

So, if anything, I'd expect it to be about 10% faster in current state than stock. Take into account power loss from 200k miles, and its a wash. It shouldn't be almost double the factory 0-60 time though.



Regarding the vibrations/noise from the driveshaft: I put the rig on jack stands and ran it in 4 high this evening. I saw the front driveshaft wobbling around quite a bit. Figured the front shaft was bent and that was the source of my problems. I installed my spare shaft in its place and took it for a drive expecting everything to be great, but there was no change. There was still significant noise coming from somewhere.

I'm a bit suspect of my front output on the transfer case. When I was changing the driveshaft, I noticed the front output sounded awfully crunchy.



I pulled it into 4 high while driving and noted the noise gets significantly worse when under power. Only took it for a quick rip up the street, so didn't have too much time to listen, but I have more investigating and diagnosis to do. I am thinking the transfer case makes sense because it has been silent while driving with the front axle shafts installed and without the front driveshaft for the last few days. Even a few weeks ago when I was suspect of the front end, I only found noises to come when going around turns at speed that would make the diff turn the spider gears. The noise that I'm hearing now happens when driving straight, and installing the driveshaft brought it on. The front output was the only part not spinning while driving for the last couple days. Fingers crossed I don't need to completely tear my transfer case down. Feel like I can't catch a break with this thing right now.


edit: oh, also, by the way, when I let the jack down it was in neutral and rolled forward into my bench vise…

 

You should be able to take the jeep to the landfill to get an accurate weight. I'm out in the county, so we dont have trash pickup. The front section of the dump has 40 yard containers for household trash that is open until 7PM, while the scales for the hill close at 4PM but they leave the scales turned on. If your city/county doesn't leave the scales on, I'm sure the booth attendant won't get too upset if you just roll across the scales - I'd go at an odd time when they shouldn't be too busy. Mine is a modest build - 3/16 bumpers (not too chunky), plated frame rails, spare in the back, and boatsides - I think it was right at 5k last time I took it across the scales.

 

Won't be super accurate but should get you within a couple hundred pounds in either direction.
Start watching at about 13:10 in.

 

I appreciate the recommendation, but think I'll just hit the scale at the transfer station. Interesting choice of video to send to show that technique lol

 

Wow. I'm sitting right at 4k lbs with me in the vehicle. Front bumper, winch, some bolt-ons, 31" tires, and a bit of recovery gear. I guess a rear bumper, full size spare, loaded roof rack, larger tires, and more bolt-ons could raise the number, but dang, 5k lbs is impressive.

 

Removed my front driveshaft again and went for a drive in 4-high. Everything was smooth and quite. Feeling confident there's no issue with the transfer case. Still have no idea why things are so rough with the front shaft installed. Will get more investigation.

Changing focus to engine performance investigation, I put a vacuum gauge on my intake manifold. It was pulling a steady 14-15 in-Hg at idle and 17.5 inches at ~2500 rpm. Those numbers are a hair low compared to what I'm reading I should expect.




Also pulled my front O2 sensor out and plumbed a gauge into the bung to measure exhaust back pressure. When testing sitting in park revving the engine, I was reading basically nothing. The gauge would oscillate back and forth rapidly at idle, but settle down if I increased engine speed at all. Even at 3500 rpm, the gauge was reading ~0.5 psi. That doesn't seem right. There should be more back pressure. I assume the gauge works because I could produce about 2 psi from blowing into the tube.







I took it for a drive with the gauge hooked up. Climbing a hill at 4500 rpm under heavy load, I saw up to 7.5 psi. I haven't seen anybody else testing back pressure under load like this, so I'm going to assume that value is okay.




Still unsure of what to make of these results as a whole. I'm thinking a compression test is probably the next best place to go, maybe followed by a fuel pressure test.

 

7.5 psi seems very high. The cat and muffler should not be causing that much back-pressure. I have heard of instances of clogged cats.

 

why would back pressure only be measurable while driving and not while sitting in park? Every video I’ve watched people are seeing ~3 psi at 3000 rpm while not under load. I haven’t found any info of what a normal measurement is while under load.

Maybe I’ll do that one again. Not measuring any back pressure in that situation doesn’t seem right.

 

Back pressure will depend not so much on air speed but on CFM. It only take a small amount of power (and thus a small amount of air consumption) to hold the engine 3500 rpm in neutral. A much greater amount of air flow is required when the motor is at 4500 rpm under heavy load.

Stock exhaust systems are not optimized strictly on making power, so there is going to be some back pressure, but 7.5 psi seems way too high.

A clogged cat (or somehow a clogged muffler) would explain both low power and the very hot exhaust. If you can swing, take it for a drive with cat removed. It will be insanely loud if you put your foot into it at all. Or if your exhaust has the band clamp setup, maybe you can have a test pipe made up to put in the place of the cat to try it out.

Or just remove the cat and take a look inside. Maybe the easiest thing to do.