moglide

96 4.0 bought a week ago,prior owner could not figure out hard starts,stalls, bad hesitation and bucking and sold it to me dirt cheap.Here is what he did before me new plugs,cap,wires,fuel filter,battery and cut cat out,bad idea.I put new fuel pump in and it runs fantastic.Sometimes you have to put your foot to the floor to start it and it starts right up and runs great.After I do this it idles higher than normal and can smell fuel slightly.Googled it and some say battery,sensors,disconnect battery to clear brain.I'll start with battery it's cheap but from there ideas would be appreciated,thanks. Here are codes before new pump,po108 map sensor/po141 o2 sensor,sensor 2/po138 o2 circuit high volt sensor 2,o2 codes may be since cat was taken out ,and last po171 system to lean,will run codes again,check engine light did come back on.

moglide

disconnecting battery didn't work

foreveryoung001

Just my opinion, but removing the cat will throw the whole fuel delivery system into confusion. Might be nothing more than your PCM can't figure out the proper mixture because of it.

scottmphoto

Put a cat back on there and a good O2 sensor. Without it, the computer doesn't know how much or how little fuel to put to the motor. On your MAP sensor, make sure that the vacuum line going to it is good. I had one that kept melting closed about every other week and would cause the engine to act funny. I replaced mine with a piece of steel brake line and haven't had a problem since.

skife

No it wont.
It'll throw a code for a downstream o2 sensor it doesn't do much more than let the ecu know if the cat is operating efficiently.

Your downstream o2 voltage high is usually a sign the sensor is bad.

Check map sensor, check for a fuel leak.

BlueRidgeMark

In addition to what skife said, I didn't see the coil mentioned in the list of stuff the PO did. That should be done as routine maintenance.

Also, check the stuff that was replaced. Is it OEM? Or aftermarket? (Autozone, etc.) The real pros on several Jeep boards strongly recommend OEM only on the sensors. Too many flaky units right out of the box for the aftermarket stuff. You did right getting the Bosch fuel pump, but check those recently replaced sensors. Any one of them might be junk.

skife

Don't worry about the parts that were thrown at the vehicle for now, right now we're diagnosing the reason the check engine light is on.
http://troubleshootmyvehicle.com/jee...e-map-sensor-1

read this, checking the map sensor isn't terribly hard.

XJ-92

My mj is straight piped and idles perfectly fine and has proper fuel mix. No cat or muffler.

skife

your MJ uses a completely different ECU than this guy's OBDII XJ

unless you swapped to OBDII electronics

BlueRidgeMark

So, you've never heard of a bad coil throwing a CEL?

skife

Do you see any misfire codes?
No.

But there is a map code and that is a huge thing here

XJRed96

Map sensor could definitely cause all the symptoms he is describing... hard start, rough running, etc. i would not eliminate the coil as a possibility but it's not his main concern right now considering he has faults pointing to other components. I would address the problems you know you have before you continue to toss parts at it like the battery and what not. although if you have the original terminals on it i would replace those and refresh the ground points simply for preventive purposes. Downstream 02 is used to verify that the cat is working and has a minimal effect on the way the ECM controls the fuel. the upstream is the sensor the computer uses to make most adjustments for fuel delivery amongst other sensors like the MAP sensor, Intake air temp, etc.