Poor Milage, Clueless.

Hi All, So far I havent resolved much except for a crazy shifting transmission. This all fight all started when I finally got pissed off that my mileage is about 10 MPG, and that the tranny had a independent mind of her own. I decided that I would do everything that was obvious that could be affecting my mileage... So I changed my plugs, wires, cap, rotor, oxygen sensor and hasn't done damn thing yet. Then I was upset with the low rev shifting so I changed the TPS sensor re grounded it and set it properly... Got improvements,shifts perfect now. With all this done my mileage still sucks. Today I decided to remove one injector plug at a time to see if I was getting a misfire due to a blocked injector, all seemed normal. I have also changed the intake/ manifold seal because it was leaking. Now what could be the problem???????????????? 87 4.0 XJ

Are you running oversized tires/wheels ? If so, you may be getting better mileage than it would appear.

:cheers:

Im running 31 with 2 inch lift, rest is stock. I also forgot to say that when I'm at a dead stop and push the gas it hesitates for a second or so, seems to do it more once I've been driving around. Yesterday I held it at 1300 going through parkin lot and it was sputtery as I drove and it idles at about 400- 500. My temp also stays at 150, I put a new thermostat cause I over heated a few weeks ago, actually it blew a crack into the fill'er up bottle when I was Yankin a buddy out of a snow mound. thanks

check the catalylic converter it may be plugged

keep it simple engine needs 3 thing fuel, air, spark. if all three of thoses are good check tire presure, brake drag. also remember your driving a lifted jeep with 31s its not a tunner kepp it easy on the right pedal i know of a lot of people that get bad milage and dont know why, then you get in the truck with them and the gas pedal hits the floor.
Mr Xeace:

Isn't a 150 a bit low, seems it would stay in cold loop if that was a newer one. We picked up a 97ZJ 4.0, auto and it was getting 8-10mpg on the trip back home from where we bought it. Changed the Tstat, it came out in 3 pieces so I guess you could say it was stuck open , put in a 195 OEM stat, mileage went up to 20 right off the bat. Gets around 22mpg on the hiway now.
I don't know about the older ones like yours though, but I assume that may have something to do with it.

Sputtery, hesitation and poor mileage make me suspect the oxygen sensor. How long has it been in there? My 1990 (Renix like yours) gets about 18 mpg in town....., that's 15 mpg using the US gallon. This is stock, running on 235/75-15 tires or 215/75-15 tires (close to the same gas mileage).

I agree with Rich P. You must get the running temp of your engine up.
Did your engine run at 150 before the overheat? If you are in open loop, your mileage will suffer big-time. Don't know if that is the case or not but that engine temp has to come up.....:wave: Chase that first.

I changed the O2 sensor about 2 months ago, I did have a little mishap after about a month that I changed it. The catalyser fell apart on the inside and blocked my exhaust and I had to drive it home about 10 miles, so do you think it screwed up the O2 sensor by this happening??? I checked the voltage output on the O2 signal with a scanner and it bounces up and down from .2 - 5 volts, it fluctuates at random like that..I also drive very easy never give it hell unless I need to do a quick pass here in the city but its very seldom. Last tank I got about 10 MPG and I drove very reasonable.. so?? Is the O2 screwed ?

Sometimes the temp will come up to 195 but it always seems to stay low now 150 Maybe i got air in it, I have opened the bottle a few times to let the air out when under pressure and the temp hasn't changed. Before The old bottle busted which I knew it was going to happen it was leakin real slow, the temperature used to always go up and down never colder than 150 and never warmer than 200 so I dont know what the hell is a matter with it.

jeephick85 said:

I changed the O2 sensor about 2 months ago, I did have a little mishap after about a month that I changed it. The catalyser fell apart on the inside and blocked my exhaust and I had to drive it home about 10 miles, so do you think it screwed up the O2 sensor by this happening??? I checked the voltage output on the O2 signal with a scanner and it bounces up and down from .2 - 5 volts, it fluctuates at random like that..I also drive very easy never give it hell unless I need to do a quick pass here in the city but its very seldom. Last tank I got about 10 MPG and I drove very reasonable.. so?? Is the O2 screwed ?

Click to expand...

Take a look at RichP's post again. First thing that came to my mind when you said it's hanging around 150 is that it seems too low, and it's not coming out of cold loop mode just like Rich said, therefore running rich, and therefore the crappy mileage. Seems like there was a thread in just the last couple of days that was talking about when the computer came out of closed loop and into open loop, and it was mentioned the temp. at which that happens is around 160.

Since you've already done the o2 sensor, I doubt that's your problem. As for the readings on that sensor, that is normal for it to bounce like that. The first o2 sensor (pre cat) should be bouncing around all over the place, and the 2nd one (post cat) should still fluctuate in readings, but not nearly the amount as the first one....this is especially true if you're giving it gas while taking the readings.

Edit: After re-reading your last post, I would also take a look at the coolant temp sensor since you mentioned that before the temp would move bounce between 150 and 200.

If you are running a Renix rig, there is an air charge sensor behind the throttle body that clues in the computer about operating conditions. If it is bad, you will never go closed loop and your mileage will suck. You will run way too rich and get chugging and surging.

Could there be any possibility that I have air in the system. For the sensor behind the TB is there anyway to check these sensor. I also fired the truck up with out having the IAC plugged in and it wouldn't idle under 2100 and it was at 3200 and went down slowly to 2100. Also when I start it up in the morning today was about 15 degrees this morning and when it starts to run it idles high for about 5 seconds and then drops to about 900 then after it get warm maybe 20 mins, thats when it will start the hesitating...I will go check these other things once I know how.

Today I was fooling around checking out the power throughout the jeep and I noticed some weird stuff. When I test the ground from battery to a sensor on the truck there is a small amount of current going through them on the ground side. I general it varies from .05 to .8 volts in the ground wire. I checked the negative from battery to engine block it was 0, and battery to ground wire it was also zero and anywhere on the body it gives me "0". Can I just simply reground everything straight to the battery or is there a way to find out what is causing it to short out????????? HELP PLEASE

If you have a vampire draw somewhere, usually the radio's clock and presets or the alarm system the way you find it is by one fuse at a time, you pull the fuse and replace one at a time till the draw disappears then trace it from there.

Sounds to me like you still have some loose, dirty ground wire connector problems. Being an 87 renix, they are nortorious for this. Also sounds like the ground problem, or short, or crossed wires, may be in the TPS sensor wiring harness between the ECU and the TPS, or in the TPS harness ground wire. Also check the O2 sensor wiring harness and sensor wires for shorts, they frequently get too close to the exhaust manifold, burn and short, either of which would cause your 10 mpg problem. Also, check the the O2 sensor and TPS sensor connections, connector pins themselves as they get dirty and loose!!!!!!

The 87 renix can handle running colder and still go closed loop even at 100 F, and still get excellant mileage so don't worry about the thermostat for now, but you might see a small (like 1 mpg) improvement in mileage with a 180 F thermostat.

also, Are you sure the manifold leaks are sealed now??????? Could one of them been warped, and thus still leaking even with new gaskets?

Also, have you checked the compression yet to see if you have any bad cylinders. Be sure to check it cold and hot to see if any valves are leaking at just one temperature extreme.

Lastly, you should be getting a much narrower 2.0 to 3.0 volts on the O2 sensor at say a constant 2,500 rpm after only a one minute warm up, even if the coolant is only 100 F. The Renix (87-90 models) will switch to closed loop at very low engine temperatures. If you are not getting a significant narrowing of the 0 to 5 volt swing at 2,500 rpm within 60 seconds of a cold start, something in that circuit is bad. Mine runs 1 to 4 volts at idle and about 2.2 to 2.7 volts at 2,500 rpm and oscilates back and forth across the sweet spot (2.45 volts IIRC) once every second to half second.

Lastly, not sure exactly what, where, you measured from, but if it was a ground sensor wire that read 0.05 to .8 volts, IT IS YOUR PROBLEM!!!!!!!!! Just splice in a new ground wire from that sensor ground to the battery negative post and it might solve the last of your mileage and hesitation problems. It solved my variable idle speed problem on mine!!!!

old_man said:

If you are running a Renix rig, there is an air charge sensor behind the throttle body that clues in the computer about operating conditions. If it is bad, you will never go closed loop and your mileage will suck. You will run way too rich and get chugging and surging.

Click to expand...

Old_man,

You make an interesting point, but I think he has already tested that sensor. It seems to have several names, IAT, Intake Air Temperature, MAT, Manifold Air Temp... and ACS. If not he needs to dig up my prior multiple posts of the resistance versus temperature readings I have posted here a dozen times for that part.

Back to your point, I guess the question in some of these issues is just how the sensor fails. IF it shorts out or goes open circuit, I could see your point, but I wonder if the Jeep would even start. Mine failed about a year ago when I was tracking the ellusive fuel mileage demons in my 87 renix beast, but the way the MAT sensor failed on mine was that it locked in at fixed resistance that made it run perfectly at operating temperature, but made it a bitch to start on cold mornings (some mornings if never started) . So I guess my point is it may depend on how a parts fail, depends on what bad data it is sending to the ECU, as to what problems it is or can cause.

jeephick85 said:

Could there be any possibility that I have air in the system. For the sensor behind the TB is there anyway to check these sensor. I also fired the truck up with out having the IAC plugged in and it wouldn't idle under 2100 and it was at 3200 and went down slowly to 2100. Also when I start it up in the morning today was about 15 degrees this morning and when it starts to run it idles high for about 5 seconds and then drops to about 900 then after it get warm maybe 20 mins, thats when it will start the hesitating...I will go check these other things once I know how.

Click to expand...

Are you absolutely sure you elliminated all the vacuum leaks and manifold leaks even once it heats up?