1989 Cherokee getting 11mpg, why??

[Ecomike]

I've paid for a new O2 sensor in a single roadtrip before. Mass to central Pennsylvania and back, stuffed the new sensor in right before leaving and my mileage went from under 10 to around 20 (remember, this was all highway driving, I hit 23 driving like a granny on the highway in the summer but winter gas limits it to around 19-21 highway) instantly.

I wonder if that winter gas drop in mpg is more due to lower engine temperatures.

 

[kastein]

Could be, I hadn't really thought much about that. My engine does run fairly cool in the winter, it's still above the open/closed loop switchover but not by as much.

 

[cruiser54]

I wonder if that winter gas drop in mpg is more due to lower engine temperatures.

Probably the lower temps combined with winter blend fuel. Take longer to warm up and go into closed loop. Part of the reason 4 wire O2 sensors were developed.

I switched from a single wire sensor to a 4 wire in my supercharged Miata that has a narrowband AF ratio gauge and I could see that it goes into closed loop MUCH faster than before.

I was hoping to mod my Renix Jeeps the same way but since found out they have a frickin oddball sensor that works differently.......

 

[kastein]

yeah... RENIX has resistive O2 sensors while OBD I/II have voltage O2 sensors, right? I forget.

 

[cruiser54]

yeah... RENIX has resistive O2 sensors while OBD I/II have voltage O2 sensors, right? I forget.

Heck, I don't know. I just wish I could use a late model 4 wire sensor. Every once in a while, I hook up the DRB and make sure my O2 sensors are switching rich/lean/rich/lean.

 

[egotripp]

daman, youre worse than me....
i have 4.88s , 35s and lockers,a dn lots of exta weight...
renix 4.0 195K aw4 etc etc in my sig...

i have :
new 02 sensor MAP TPS(adjsuted properly) cap and rotor, plugs, coolant temp sensor, crankshaft sensor, fuel pump relay, cleaned MAT sensor, newer water pump and radiator air filter, oil change, new vaccuum lines, ford 19lb injectors, 2 guage ground wires , EGR is is great shape and solenoid is in good shape, new battery and 2 guage alternator cables, newer exhaust/intake manifold gasket
tons more to list


and i get around 13.5 MPG on the freeway and way worse in town.
i used to get around 17mpg with 33s and 3.55s...


any one getting better MPG with a similiar setup?

Have you changed out the speedometer gear in the TC? If not, You're actually putting more miles on the clock then are actually reading. Your MPG will be off. I'm running 4.88's and 33's and have yet to change my gear and still get around 15.

 

[Ecomike]

yeah... RENIX has resistive O2 sensors while OBD I/II have voltage O2 sensors, right? I forget.

Yes the Renix O2 sensor is a variable resistor, and operates as a voltage divider circuit with the ECU sending a 5 V signal to the O2 sensor.

The Renix O2 sensor is a 3 wire sensor, it has a heater just like the 4 wire sensors, only difference in the 4 wire is it has 2 ground wires, while the renix has a shared single ground wire. If that Renix ground wire on the harness has issues it can corrupt the O2 sensor reading and reduce the heater current.

The later 91-2002 O2 sensors are voltage producing sensors, low current, 0-1V and they went to an independent computer ground for the sensor signal in 1991.

 

[cruiser54]

All the more reason to refresh your grounds....................and maybe add some.

 

[Ecomike]

Probably the lower temps combined with winter blend fuel. Take longer to warm up and go into closed loop. Part of the reason 4 wire O2 sensors were developed.

I switched from a single wire sensor to a 4 wire in my supercharged Miata that has a narrowband AF ratio gauge and I could see that it goes into closed loop MUCH faster than before.

I was hoping to mod my Renix Jeeps the same way but since found out they have a frickin oddball sensor that works differently.......

I think it is more temperature related. I get warm spells in here in Houston all winter, and I run a 180 F T-stat (87 Renix), so I see the MPG drop when a northern drops in and stays for a week. I also see my MPG rise a little when I run the AC in the summer, as my engine runs hotter with the AC on, versus say a 75 F day (or week). I get peak mileage on the 87 in early August at 100 F, and worst in Feb at 30 F.

 

[Ecomike]

All the more reason to refresh your grounds....................and maybe add some.

Also a good reason to ohm test the sensor grounds, one by one!!!

 

[cruiser54]

How come you don't run a 195* stat year round? I do in Arizona.

 

[Ecomike]

Had too many overheating problems in the summer since 1976. I have never been found of 195 F T-Stats, long story. But I used a 165 F on the 87 for the first 4 years.

How come you don't run a 180* stat year round? I do in Houston.

 

[SonicCougar99]

OK thanks guys for the advice. It most likely will be a couple weeks before I can do a lot (got a previously planned vacation in a couple weeks that takes priority), but I will let everyone know how it goes. I filled up with gas the other night and am looking at 10.9mpg, that just won't cut it with gas prices going the way they are.

 

[master turner]

this post was alot of help i just went on a 12 hour trip to portland from the ca bay area and got on average 13 per gallon... long story short... ouch

 

[Ecomike]

I frequently forget to mention that people need to check for exhaust leaks, cracked exhaust pipes. The O2 sensor compares the exhaust gas O2 concentration to out door ambient air O2 concentrations, and if exhaust gas is blowing on the O2 sensor from a crack or gasket leak, it can throw off the measurements, and the computer will get miss led and waist fuel!!!!!!!!