Catalytic Converter, 02 sensor

In addition to my 99 4.0 XJ, I have a '98 grand 4.0. Starts beautifully but anywhere from 10-60 or so miles of highway driving engine looses power. If I try to give it any throttle it just gets worse. a couple times it has stalled. Most times it can stay barely running at idle, and sometime will go back to normal operation after coasting for 30 seconds or so. I can then go a few miles before it happens again. Other times best I can do is just coast at idle, get off the road, let it sit for a few minutes and then it starts like a champ only to go another mile or so till it happens again. The longer it sits after having a problem the further it will go next time. It has never happened when driving around town under 40mph for distances up to 15 miles. No CEL, but there is an old code for 02 sensor lean condition. Shop thinks it's a plugged catalytic converter that gets worse when heated up. I have the classic cracked exhaust manifold. They feel my converter is bad because the manifold is sucking air and causing the 02 sensor to tell the PCM to have the engine runner richer. I've driven this daily for 5+ years with the cracked manifold and no issues till now. Does this make sense? I thought people were driving forever with the cracked manifolds with no issues except some extra noise. The jeep is at a shop about 50 miles from home. I'm planning on going to get it this weekend. Drive as far as I can at lower speeds on back roads. Maybe I'll make it back. If not, next step is to pull the wires to the 02 sensor to force the engine to stay in open loop and continue the rest of the way. If that doesn't work I'll just sit it out till it cools down some and continue on a few miles and repeat. Any thoughts?

Fix all known faults and failures. Most likely, the O2's and cat are ruined by neglecting maintenance and repairs.

A cracked manifold lets the exhaust stream suck in fresh outside air. The PCM reads this as a lean condition and richens the fuel mixture. Fixing the manifold and installing fresh O2's should pay off in increased gas mpg's.

Buy genuine Jeep or NTK O2's, do not buy Bosch.

Wouldn't bad o2 and/or cat cause a CEL?
Do you agree disconnecting the downstream o2 would force engine to stay in open loop?

As mentioned before, just need to drive it 50 miles home. Think forcing open loop will prevent the stalling that happens after extended driving at highway speeds so I can limp home.

I don't think a cracked manifold would cause this condition. I bought my 97 XJ with a cracked manifold and replaced it because I didn't like the noise or the smell of exhaust. I also have never heard of anyone else with a cracked manifold having this problem. Crank sensors are notorious for stalling, but usually when the vehicle is at a traffic light, etc. Fuel pumps can act funny when they heat up. It may even be the coil pack (if your Jeep has one instead of a distributor, not sure when the Grands go them). Depending on what the O2 is doing, running rich can mean lower fuel economy while everything else seems to be just fine.

md21722 said:

I don't think a cracked manifold would cause this condition. I bought my 97 XJ with a cracked manifold and replaced it because I didn't like the noise or the smell of exhaust. I also have never heard of anyone else with a cracked manifold having this problem. Crank sensors are notorious for stalling, but usually when the vehicle is at a traffic light, etc. Fuel pumps can act funny when they heat up. It may even be the coil pack (if your Jeep has one instead of a distributor, not sure when the Grands go them). Depending on what the O2 is doing, running rich can mean lower fuel economy while everything else seems to be just fine.

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Fuel pump was my first thought when I first had the problem about a month ago. It sat overnight next day and started without issue and ran perfectly for 4 more weeks till this happened again. I was told by a service tech fuel pumps either work or they don't. Fuel pressure test at the rail was perfect after the problem a month ago. The manifold has been cracked for 8+ years and never had an issue. I just don't think that's the problem. I understand, it could suck some air in with the exhaust and give a false lean reading from the o2 sensor, but really 8 years with a good running engine and now that's a problem? So many 4.0s have manifold issue. Honestly 50% of same era 4.0 jeeps I'm near in traffic or hear start in a parking lot have cracked manifolds.

I'm leaning toward the fuel pump having a problem after heating up but again, tech says they either work or don't. I'm interested to hear more about fuel pump issues after they heat up.

Disconnecting the front O2 will probably make the PCM run in Limp Mode, and you could drive it that way. The CPS is heat sensitive, and if the exhaust manifold leak is blowing hot exhaust in the right direction it can affect the CPS, or the CPS could just be old and weak. The fuel pump is cooled by the fuel in the tank, so unless there is 1/8 tank or less, fuel pump temps shouldn't be an issue.

maybe the fuel filter? my dad had a similar problem with his YJ, he could get to work and back but then it would barely run, then it would be fine in the morning.he replaced the fuel pump but that didnt fix it. it was just the filter. hope that helps