Cherokee running rich I think, crappy gas mileage

I've got an 89 4.0 with a 3" lift and 31" tires, and I'm getting pisspoor gas mileage. It didn't used to be so bad, but lately and back with my 215's, I was getting 13.6 mpg, and I'm afraid to see what it is with the 31's. I've tried adjusting the TPS, but when I set it to 82% (3.8v), it idles at around 3000 rpm, so it's at 4.2v for now. I don't have a Check Emiss. light that would signify that the O2 Sensor is bad, but regardless my exhaust smells pretty gassy.

Would it be the IAC? My idle is kinda rough, around 800 rpm, and it'll fluctuate, sorta bog down for a second then come right back up.

What sensors would have to go bad for me to get such shitty mileage? I'm thinking it might be the IAC, but does anyone have the definitive way to adjust the TPS? I've been reading up on it, and I get a million different answers.

I had something similar happen to me last year. I had finally gotten everything adjusted and tuned where I was getting 22-24 mpg on my 88 4x4 XJ. Then over a two week period my gas milage droped down to 4-6 mpg. I was certain that I was going to have to rebuild or replace the engine.

I did a compression check on the cylinder, and tested the ignition system. All of that was fine. After reading through my haynes manual I realized that the mixture is set by the computer using the readings off of several sensors. Most of the sensors can be tested using a volt-meter. I don't have the manual handy so I cannot give you the exact porcedures for each sensor. I ended up having to replace my O2 sensor, Manifold Air Temperature (MAT) Sensor, and the Coolant Temperature Sensor. I also ran through a bottle of fuel injector cleaner. I figured it couldn't hurt since it had been running so rich.

That fixed my problem. I was back up to 22 mpg on the following tank. Good luck and I hope I pointed you in the right direction.

RyleyF said:

I've got an 89 4.0 with a 3" lift and 31" tires, and I'm getting pisspoor gas mileage. It didn't used to be so bad, but lately and back with my 215's, I was getting 13.6 mpg, and I'm afraid to see what it is with the 31's. I've tried adjusting the TPS, but when I set it to 82% (3.8v), it idles at around 3000 rpm, so it's at 4.2v for now. I don't have a Check Emiss. light that would signify that the O2 Sensor is bad, but regardless my exhaust smells pretty gassy.

Would it be the IAC? My idle is kinda rough, around 800 rpm, and it'll fluctuate, sorta bog down for a second then come right back up.

What sensors would have to go bad for me to get such shitty mileage? I'm thinking it might be the IAC, but does anyone have the definitive way to adjust the TPS? I've been reading up on it, and I get a million different answers.

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Inputs signals from the MAP and O2 sensors are used by the ECM to adjust fuel mixture. The 89 renix system don't have check engine lights.
You should have been around 18/21 stock, is your trans. shifting into overdrive or are you running in third all the time?

Ryley, it's a bad O2 sensor. If you want to check it first, locate the gray wire running in the loom under the FPR. That's the signal wire from the O2. Press a straight pin into it, and probe the pin with a voltmeter, with the other meter probe on an engine ground. With the engine idleing, the meter should alternate between 0v-1v, once every second, if the sensor is good.

Chances are you'll read 0v, which confirms a bad sensor.

Runnin'OnEmpty said:

Ryley, it's a bad O2 sensor. If you want to check it first, locate the gray wire running in the loom under the FPR. That's the signal wire from the O2. Press a straight pin into it, and probe the pin with a voltmeter, with the other meter probe on an engine ground. With the engine idleing, the meter should alternate between 0v-1v, once every second, if the sensor is good.

Chances are you'll read 0v, which confirms a bad sensor.

Click to expand...

I'll check it out, but I'm not too sure about the whole running rich thing since I checked my spark plugs and they look nice and gray. However, my exhaust is just plain STRONG, maybe I'm being hypersensitive to it, but sitting at a stoplight for a little bit, I can definitely start to smell it, and it doesn't smell quite right, a bit gassy.

Also, I've removed that cloth/spring hose that goes to the airbox near the bottom. It goes to that flapper door in the front of the airbox. It had holes in it already, and I think it's just for cold starting. Do I need to replace it?

Runnin'OnEmpty said:

Ryley, it's a bad O2 sensor. If you want to check it first, locate the gray wire running in the loom under the FPR. That's the signal wire from the O2. Press a straight pin into it, and probe the pin with a voltmeter, with the other meter probe on an engine ground. With the engine idleing, the meter should alternate between 0v-1v, once every second, if the sensor is good.

Chances are you'll read 0v, which confirms a bad sensor.

Click to expand...

The Renix O2 sensor is different from most O2 sensors and doesn't generate the 0v-1v signal. I provides variable resistance to modulate a +5V signal. The FSM provides a method to test the heating circuit for this sensor, but doesn't provide a test for the O2 sensor itself. Seems like I've read that the test Runnit'OnEmpty described can damage the Renix (titania) type O2 sensor.

I'm gonna go with a vacuum leak on your map sensor or bad map sensor. Just for fun.

Is the MAP sensor the big green sensor at the back of the engine bay above the coolant temp sensor? I may have gotten it wet a few times, so it might be bad.

RyleyF said:

Is the MAP sensor the big green sensor at the back of the engine bay above the coolant temp sensor? I may have gotten it wet a few times, so it might be bad.

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Yes but it's sealed. Stinky exaust would make me suspect the O2 sensor, whens the last time it was changed?

The Renix O2 sensor is different from most O2 sensors and doesn't generate the 0v-1v signal. I provides variable resistance to modulate a +5V signal. The FSM provides a method to test the heating circuit for this sensor, but doesn't provide a test for the O2 sensor itself. Seems like I've read that the test Runnit'OnEmpty described can damage the Renix (titania) type O2 sensor.

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Yep Max, you're right, I had the voltages wrong. The Titania will switch from about .5v to 4.5v, once every second. Checking the output voltage won't harm them, although checking the resistance with an ohmmeter might.
Here' some info on titania sensors.
Titania O2 Sensors

Ryley, it's almost certain the O2 sensor is at fault. Do the voltage check and confirm it's either good or bad. You don't even have to crawl under the vehicle or get dirty; takes about 5 minutes......

I don't know the last time it was changed, but I'm suspecting that it was quite some time ago. Can you give me some more details about changing the O2 Sensor? I can see it on the exhaust manifold, and followed the wires but I can't really see a gray wire that i'm supposed to test.

Okay, well I found the wire, in that plastic tubing right under the check valve on the pressure rail. I stuck a pick (like a tiny icepick, for misc. stuff) in there, put the pos on the pick ,and negative on the engine block. Not reading anything.

Ryley, if you did the test correctly, then the O2 is indeed bad.

As you say, the gray wire is in the loom underneath the fuel pressure regulator, and is checked for an alternating DC voltage with the engine idleing. Your polarity was correct, with the positive voltmeter probe on the wire, and the negative probe touching an engine ground. Voltmeter setting should be on the 10v DC setting.

To change the O2 sensor, you may have to use a 7/8" open-end wrench. I tried using an O2 socket on mine, but there wasn't room, and had to go to the open-end. The (Bosch) sensor is about $52 from AutoZone.

Okay, I installed a new O2 sensor, hopefully it'll run better now. It seems to anyway. I guess I'll find out when I hit 240 mi. and I need (or don't) gas.