Low power on highway + bad MPG + smog fail Help!!

I am stumped on this one, I've tried searching but cant seem to find symptoms that match mine. I hope someone can share some wisdom to get be back up an running!

99 XJ
4.0
mostly stock
Federal emissions
290k miles well maintained
Replaced CAT and entire exhaust including oxygen sensors 2 years ago
Did plugs wires and distributor 2 years ago
Recently replaced entire cooling system
Drained and filled all fluids before trip.
Just got back from ~1600 miles of highway + ~125 miles of trail trip.

Symptoms:
Drives fine on the trail
Terrible highway MPG 11-12 ( I normally get 18-21)
Sluggish on highway, shifts down on any type of grade to maintain 70-75mph
On steep grades in mountains I was struggling to hold 55mph getting dropped by big rigs, shifting into 2nd and doing 4000rpms!
I also noticed the temp starting to creep up on very long stretches of uphill
I recently failed CA smog with high NOX @ 15mph

I suspect the CAT might be partially plugged, how can I test this?
Could it be a fuel pressure problem? I've never replaced the fuel filter, on this model I think that they might be integrated into the tank or pump.

Anyone else had these problems? I don't mind spending a bit of cash to fix it, but I don't want to start throwing parts at it without first hearing from others with same symptoms.

Thanks All!

Unfortunately, I'd suspect the O2 sensors first. Do you happen to know what brand they are?

Also, what gear ratio and tire size is it? I consider mine mostly stock too, and it's got a 3" lift on 31's with the stock 3.73 gears. It dropped mileage pretty good when I did that. I need to regear to 4.10 and I should in theory gain some of that MPG loss back.

If you want to test the catalytic converter, I know of 3 ways.

1) use either a vacuum gauge or the MAP sensor reading. Snap the throttle. Manifold vacuum should go immediately from 18-20ish to near zero, and right back to 18-20ish within about a half second. If vacuum hangs out close to zero for a while before coming back, the cat is plugged.

2) use a pressure gauge in the upstream O2 sensor bung. I don't recommend this method unless you have an adapter fitting for that specific purpose.

3) loosen the bolts at the manifold and drop the down pipe a little. Go for a loud drive and see if power is back.

I don't suspect your cat is bad based on the symptoms you described, but you never know.

O2 sensors or O2 sensor wiring issues would be my first suspects.

Tps is always a good suspect too.

Thanks guys! I'll try the loud drive tomorrow! Because it sounds fun. I'll also try O2s and TPS to see if that makes any type of change. I'll let you know what I find before replacing the CAT.

O2 Sensors were Denso. Installed April 2018. CAT was Walker CARB compliant also April 2018.

Get the NTK sensors. They're like $25 each from RockAuto. There's been a known issue with other aftermarket brands having reliability problems.

Get NTK O2's

Agreen said:

Unfortunately, I'd suspect the O2 sensors first. Do you happen to know what brand they are?

Also, what gear ratio and tire size is it? I consider mine mostly stock too, and it's got a 3" lift on 31's with the stock 3.73 gears. It dropped mileage pretty good when I did that. I need to regear to 4.10 and I should in theory gain some of that MPG loss back.

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Did you regear the speedo gear? I would look at mileage from say MapQuest or such. Then calculate. You might lose mileage with 4.10s if open Highway. But not much.

Thanks everyone for the responses. I wanted to close the loop on this.

I did pass smog (barely) in the end, but the issues are not completely resolved. Here is what I did:

New CAT
New NTK O2s
New injectors
New fuel pump
New air filter

These parts made it pass, but just barely. The fuel pump and injectors improved power a little, but it still feels rather gutless on steep climbs, and fuel economy still isn't where it used to be. I am not sure what else could be causing this.

The problem is no longer severe, but the smog guy did think that it was still running a little bit lean. Any other pointers?

Thanks for all the help.

Obviously 300,000 miles is a lot, so it could be new engine time. It does not feel like that is the issue, because it is not burning significant oil. I have not done a compression test.

Most Cherokees have a cracked exhaust manifold.

IntrepidBear said:

Obviously 300,000 miles is a lot, so it could be new engine time. It does not feel like that is the issue, because it is not burning significant oil. I have not done a compression test.

Click to expand...

My XJ is sitting at 332,000+ miles and it still have a tremendous amount of power. I would assume the problem lies elsewhere in the fuel system. Check the fuel pressure and the filter.

Your location says Brazil. Are you using gasoline or ethanol in that thing. Working in Iowa here in the US, 30% ethanol caused a huge loss of power and 30% less mileage.