Hey all,

I have a 98 xj 4.0 4x4 on 31s and stock gears. with about 205k on the clock. It seems like the car is not accelerating as it should. I know these are not race cars, but it seems a little sluggish. And when going up a long hill, it's hard to keep the car at highway speed (60+) and usually goes down to about 50-55mph. What usually happens is that the transmission down shifts and works hard to try to keep the car up to speed, and in turn the engine heats up to about 220-225 degrees or so, even with the electric fan going.

Car has new plugs (although they are not the greatest brand) wires, cap and rotor. New tps and map, throttle body is pretty clean, no codes, compression values are between 130 and 155 (a little wide but I don't think it's the issue). Coolant has been flushed as well. And new O2 sensors (not the same brand though, don't know if that makes a difference). Fuel pressure seems to hold around 50psi

Also worth mentioning is that I have a slight rough idle/random miss that doesn't seem to be coming from one cylinder in particular. I've gone through and tried unplugging injector wires to see which cylinder it is, but the idle just gets rougher when I pulled each one. No difference between them.

Additionally short term fuel trim when at idle warned up is from about +1 to -3% long term is about -5 to -8.5%.

Could it be that I just need to re gear? I feel like my Jeep should be able to go above 60 uphill. It's just really sad seeing literally all the cars pass me hahaha.

other thought Is clogged exhaust but I can't really tell. The cat passed California emissions less than a year ago so I don't think it's bad, but I'm not sure.

I'm pretty stumped and don't really want to re gear unless I have to. I live in the mountains so lots of roads have hills that I need to be able to go up with speed.

 

I welded in an extra O2 bung. Mostly it has a plug in it. When I'm trying to figure things out it has either a wide band O2 or a plug that is modified to lead to a pressure gauge.

That might be more than you want to do. For a quick and dirty, you can undo the exhaust after the front O2 and before the cat and see if that helps, but that would probably get you 25 to life in CA.

 

could be a clogged catalytic converter.
There is a vacuum gauge test that you can perform to verify.

http://cruiser54.com/?p=98

 

thanks! Would I be able to do this test by just monitoring the MAP sensor data? Or will I need to use a vacuum gauge?

 

Try it and let us know. Sounds like it might work.

 

You can absolutely use MAP data. That's how I always check for a bad cat.

FWIW, my 98 on 31" tires, stock 3.55 gears, and a 3" lift has trouble going up hill. I generally have to floor it and downshift to 4th to stay at 70. If the cat tests fine, do a compression test.

 

thanks man. I wish I could stay at 70 flooring it going uphill 😂. As mentioned I did a compression test and my values were between 130 and 155. A little spread out but not bad.

I'll reply back in a few days when I have the chance to do the test cruiser mentioned. I am really starting to think it's the cat. I believe the previous owner installed it in 2016. It's an aftermarket Cali legal one, at least it used to be California legal, but for some reason it got taken off the list. Maybe because it's a defective one... Who knows

 

Do you carry a lot of extra junk and/or have heavy bumpers/roof rack? Even with 31s and an automatic, you'll benefit from regearing. My experience has always been a 4.0 will tolerate higher rpms than you might think (my rig with 4.56s 33s and stick routinely stays around 2,800-3k rpm on the highway).

 

I have the same problem with my 98. 35's with 4.88 gears. On a level highway, I can do 70+ at around 2500 rpm with no problem. As soon as I hit a hill, it feels like I am being pulled back. Here in Colorado, some of the long passes are killer and I am lucky to pass most of the semi-trucks going up the pass. When it drops below 55, it downshifts hard and sits at over 4k rpm until it gets up a couple more mph. It hunts gears like crazy during those periods. I have over 250k on the clock on this rig and don't know the full engine/tranny history to know what has and hasn't been done to it, but it gets me to the trails and is a beast on them. It has a new Cat, so I do know it isn't a clogged Cat.

 

Did the test,

At idle my map sensor reads 35kpa. At 2500rmp, holding for 20 seconds, it reads 25 kpa (according to the test cruiser suggested, it should stabilize back at 35kpa, but it did not). Letting off, it drops to 15-17kpa then goes back up to 35kpa.

Makes me nervous that it did not stabilize back at 35, but what do you guess think of the result?

 

If you're heavy enough on the throttle to force a downshift, it might also be unlocking the torque converter which would add to the heat dumped into the radiator. Stick it in (3) on hills and see if that helps.

 

Not particularly. I've got a roof rack but other than that, not really anything that would add weight. I think you're right though. If it doesn't end up being the exhaust I will end up re gearing. Probably to 4.10 as I don't plan on going bigger than 31's

 

When I keep it in 3rd, I end up losing a lot of speed. Like I'll start the hill at 65 and end up at 50 or so

 

Before dropping the money on a new cat (which it probably is at this point) loosen the 2 bolts from the manifold to the main pipe. Enough so that the exhaust drops down. Yes, it's going to be loud. Take it for a drive and see how it does.

The test you did is different than what I normally do. I generally let the engine warm up fully, then watch the MAP as I snap the throttle open and closed as quick as I can. Full open to fully closed in under 1/2 second. The MAP should follow that throttle snap closely. If it lingers around atmospheric pressure after the blip for more than a second, it's the cat. Or the cat has dumped its guts in your muffler.

Do the exhaust drop test though. It's quick and free, so give it a whirl and see if anything changes.

 

So I finally got around to doing this. Took the two bolts off, but the pipe did not separate hardly at all from the header. It was not stuck on, as I could wiggle it back in fourth and stuff, but it would not come very far off the header. Anyways, drove up and down the mountain by my house and did not see a very noticable change.

I went ahead and bolted it back together and took out the upstream O2 sensor. Again, it did not seem to do anything... So I'm a little unsure of it is a clogged exhaust now.

Finally, I got everything back together as it should be, then let the car run to get hot, and held the throttle at 2500rpm while checking the cat temperature. The exit temp was definitely not as warm as the intake temperature, which leads me back to thinking my cat is bad....

At this point I'm just very confused. I figured if my cat was bad, causing a large exhaust leak upstream of the cat would make it have a little more power going uphill, but it did not. And the temperature readings on the cat suggest it is bad. I'm hesitant to just thrown on a new cat since I live in commiefornia and they are like 500-600 bucks.

Any other ideas???