CCKen

There are a couple of sensors involved in fuel management, besides the Oxygen Sensors, that are essential. First, and foremost is the Manifold Air Pressure Sensor, then the Engine Coolant Temperature Sensor.

When in Open Loop, the PCM governs the engine without the O2S's in the loop and relies upon the inputs from the other sensors (main fuel management ones listed above). Without properly functioning O2S's you are probably in Open Loop all the time. So be it. You've removed the downstream O2S so that one is out of the loop, and may have damaged that portion of the PCM that depends upon the upstream O2S input (who knows). FYI, as with OBD II systems in most domestic vehicles the downstream O2S does contribute to the O2S goal voltage needed by the PCM. Read the data I've posted at the end of this post.

Anyway...

Verify you have a serviceable '97+ Engine Coolant Temp sensor installed in the T-Stat housing and that its connector and wiring is serviceable (not broken, water or oil soaked, and not corroded).

Verify the Manifold Air Pressure Sensor is installed with a serviceable rubber "L" coupling between the MAP sensor and the intake manifold. Verify the connector and wiring are serviceable, same as with the ECT sensor.

There are tests that can be performed on the ECT and MAP but I'm not going there now.

Here's some data taken from a Dodge truck website pertaining to OBD II O2S goal voltage activity. Don't let the fact that it came from a Dodge website throw you off the fact that it is discussing an OBD II system - like your '98 XJ.

A lot of people are dead set against believing that the downstream O2S does anything besides turn on the Check Engine light. I can't change their minds, but only provide data that shows that shows it does otherwise.

Upstream Oxygen Sensor Goal Voltage

From Star Center News #5, February 2000 - tests for those who have access to OBD II or OBD III code readers.
Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.
When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.
Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.
If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an effort to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.
If the downstream O2 sensor has a tendency to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.
Sometimes a service technician will reset the PCM and take it for a test drive. The PCM does a recalibration after the reset, and the problem may go away until the PCM has completed a relearn of the system and then begins to attempt to correct for the downstream sensor error. If the symptoms reappear after several days of driving after a service is performed, check the downstream O2 sensor, the exhaust system, the wiring, and the catalytic converter. Note: a worn out catalytic converter can produce symptoms similar to a bad downstream O2 sensor.

pmichel

Sounds like someone needs go to Dutch Mothers for a piece of pie.

854x4

I saw that you had the o2 wires mixed up, this can kill a sensor fast, they don't like to be hooked into a power feed like the heater circuit. Also running with one unhooked tends to make the engine run on the rich side which can foul out the sensor. I'm not going to get into an air/fuel intake argument so I'm leaving that one alone. One last thing why does #1
cylinder only have 90 psi?

CCKen

Lead Foot,

On a personal note, why does someone with a beast like you have get interested in gas mileage. Looks like it's just for offroad work/fun.

https://www.cherokeeforum.com/f46/fr...review-136123/

Ken

Lead Foot

So.... Guys.... I apologize for wasting your time. Moral of the story, when there is any doubt; change the sensor. I decided to go ahead and change the front O2 Sensor and reset the computer. Mileage according to the overhead MPG calculator went from 8-10 to 16-22.

DougyFresh

DIIIIIYYYUUUUUMMMMMMMM. I wish i got that kind of mileage

Lead Foot

I believe this is exactly what happened. No one confirmed that how I first had it wired would damage the O2 sensor so I just discarded that factor.

Like I mentioned in one of my first posts, I dont really care about the engine itself that much. This engine has served as a substitute in many of my other rigs just to get me by. It has been pulled in and out of vehicles without much regard to its condition; lack of oil changes, being overheated, running old fuel, ect. During most of this current build it was actually sitting outside, nose down, uncovered. Granted Yakima doesnt get much rain, when it did rain Im sure the water made its way into the intake runner and down into cylinder one. The cylinder wall is most like corroded and pitted.

This engine just needs to get me by for the next 4 months or so while I build my stroker out of the 98 engine I have sitting on the engine stand.

Lead Foot

HAHA. Thanks man. Well it is technically street legal. My wife will be driving it every now and then for errands and part time work.

More then anything, as sad as it is, we do not have a tow rig and trailer yet; so driving to and from the trails is a normal routine. Some of our favorite trails are 2-5 hours away.

Lead Foot

Im kinda curious what it would actually be. The overhead read outs are only so accurate you know? I will say that on the way there when I calculated 14mpg; the read out showed 12-16. On the way back when I calculated 8mpg; the read out showed 7-10. So it is decently accurate. I dont see why I couldnt expect 16-18mpg out of it with the deep gearing I have. Yes, I have low compression in cylinder 1; but the engine still feels plenty strong.