I have a 98 Cherokee that I just cant seem to get running right. It starts fine, and drives fine for a little while, once I get on the expressway I get up to about 70mph and cruise, within about 2 minutes I lose throttle, jeep starts slowing down, dosnt die, but if i tap the throttle or pump it it will start backfiring and chugging, but if I put the pedal to the floor, all power is restored and it starts accelerating. As soon as I let off the throttle again and try going back to a normal throttle position, it continues to have problems. Seems like after the jeep is shut off for a while and started again, it is driveable again.

I have replaced the fuel pump, and used three different throttle position sensors, and two different map sensors and idle air control sensors. I have no idea what to look at next.

Any input is greatly appreciated!!

 

Is the check engine light on? If so, get the codes.

Driveability/power issues that resolve at WOT (wide open throttle) or for a while after cold startup can implicate your upstream O2 sensor(s). Which are ignored at WOT and for a while at cold startup. Faulty O2 sensors often cause check engine lights, but not always. A shop can troubleshoot them pretty quickly. You can too if you have a scanner capable of displaying their voltage values in near real time. Ultimately, any malfunction in the O2 sensors will generally show up in short/long term fuel trim numbers that are not where they should be, and are easily seen from most OBD2 scanners these days.

 

I ran the codes a while ago, Both o2's were throwing codes. The downstream one I knew was a problem because its currently zip tied to the frame due to it popping out of the exhaust. The upstream I just didn't worry about because I had no problem on the expressway or driving before. Why would it now be causing these issues?

 

The computer uses the data between the MAP and upstream O2 sensor to determine the adjustments needed to keep the air/fuel ratio where it should be. (Amount of oxygen coming into the engine vs. the amount of oxygen remaining after combustion). At wide open throttle, and at cold start-up the computer uses some "hard coded" values.

It could be that your upstream O2 sensor was marginal for a while (for example its heater circuit might have failed, but its readings were still generally accurate once the exhaust got up to operating temperature), but it might now be truly providing bad values to the computer. So the computer is sending the fuel trims to the roof (or to the floor).

 

That makes sense. Okay Ill try replacing that this week. Can I just replace the upstream and see if it makes a difference or should I replace both? If I can get away with just replacing the upstream at the moment that would be great because I need to have a new 02 bung welded into the exhaust for the downstream.

 

The upstream will definitely impact driveability (air/fuel ratio problems). The downstream will continue to give check engine light (codes) for emissions problems, but is not used by the computer for air/fuel changes.

Search this forum for the best O2 sensor to go with. The consensus is that XJs are particular about the O2 sensors, and I believe everyone recommends the NGK/NTK sensors.

Keep in mind, without testing, you are throwing parts at the problem. So I hope the O2 sensor helps, and certainly sounds like it could. But without data/testing its just a shot from the parts cannon. However, if you have over 100K miles, and that O2 sensor has never been replaced, then it is about time any way.

 

Okay cool then I will definitely start there. Unfortunately I don't know the miles on the 02's, but based off of the look of the other one Im going to assume its been a while. The jeep has 216,000 miles. Ive only put 600 miles on it since buying it and replacing the motor. I read up on the NTK/NGK, im going to purchase one of those. I hope this works because Im tired of chasing problems on this thing. But your theory seems like the likely cause because when I left for lunch today with a buddy, I started the jeep for ****s n giggles. Sure enough it started right up and ran fine.