Lead Foot

Story Of 14mpg to 8mpg In A Matter Of 2 Days and 600 Miles.

Hey guys, need your brains on a fuel mileage issue I am having. 1998 XJ, 4.0, AW4, NP231, 5.13 gears, 37.5" tires.

Engine was sitting for about 1 year outside, then was transplanted into my 98 during the build process. Had to swap some parts from the 98 block on to the 91 block for it to work with the 98 harness. Parts included throttle body, coolant temp sensor, intake are temperature sensor, distributor, coils, and fly wheel. The exhaust downpipe had a new 4 wire O2 sensor in it but hadnt taken the time to wire it up since I didnt have the wiring for the 98 harness. Also disconnected and plugged all vacuum except brake booster. I didnt have time to set up a cold air cowl intake so I threw a thick sock over the throttle body for now. It started on the first try, seems healthy. Driven it around the block a couple times.

Once my Jeep was done I took it on its first road/wheelin trip. About 280 miles from Lynden to Yakima to Rimrock. It didnt seem to have the power I remembered the 4.0 having (its been 2 years since I've driven a Jeep); I chalked it up to me being used to my BMW. I had a very slight coolant drip from the radiator but it wasnt enough to concern me (just made sure the overflow was topped off). I drove about 60mph there, 70mph according to my speedometer, and the Jeep ran about 210-215 degrees. Never the less when I filled up in Yakima I calculated that I got 14mpg; I was hoping for about 18 but whatever. While filling up I also added a can of Sea Foam in the gas tank. I wheeled all weekend and still felt I didnt have the power I should. I should add that I had to clean out the sock filter about 3 times a day as it didnt have much surface area and it was quite dusty.

After 2 days wheelin I started heading home. Since I was off the trail and the freeway really isnt too dusty I decided to take off the sock/filter. The radiator blew right as I got in to Yakima. Before fixing it I took the Jeep to the car wash and sprayed the whole thing down, including under the hood. The engine still ran fine and I didnt detect any misses during the drive to the auto parts store. I replaced the radiator and while I was at it grabbed a quart of Lucas Oil Treatment. I was only 1/4 quart low but I figured that the motor had been sitting for a long time and I cant remember the last time I did an oil change before I tore the last rig apart, couldnt hurt right.

While I was doing all this work to the Jeep I also took the time to try and research the O2 sensor wiring on the Jeep. I thought I had 2 black wires, a brown/yellow wire, and a green/white wire. The Bosch sensor has 2 white wires (heater positive and negative), a grey wire (signal ground), and a black wire (signal). Naturally I wired the 2 black wires to the 2 white wires since they are not polorized. I then wired the Green/white wire to black and the brown/yellow wire to grey. My O2 circuit fuse was blown due to the loose harness plug getting smashed by my axle truss during the weekend. I replaced the fuse and reset the computer by grounding the positve battery cable for 20 seconds.

I was then on my way. Right away I noticed the Jeep was running a little cooler, about 200 degrees compared to the previous 215. The 4.0 also felt like it had its power back, felt alot snappier like I remembered. I was doing about 70mph (80 according to my speedometer). After driving 95 miles I noticed I already used almost 1/2 a tank! I pulled over at the top of the pass. It didnt smell like the Jeep was running rich at all and couldnt hear or see anything out of the normal. I checked the O2 sensor fuse and it was still good. I decided that I must have gotten better mileage without it so I left the fuse out.

I drove another 75 miles and the engine still felt like it had the power it should but now the tank was on "E"! So no change in mileage. It was pretty late at night so I decided to just fill it up and head home and worry about it later. I calculated only 8mpg! and I used another 3/8 to 1/2 of a tank the last 80 miles of my trip!

I did some more research today and pulled back shielding on the O2 harness a little further. I found that both wires werent solid black. One was black with a green stripe. So according to my research, below is an image of how everything was hooked up.



So guys, after all this info; anyone got some ideas of what caused the mileage drop? Thanks for any help.

Kris Froehlich

Lead Foot

53 views and no ideas to throw out?

Radi

The problem is where do you start? '98 is heavily dependent on sensors telling the PCM what to do with fuel and spark and you are missing most of them. No MAP sensor? What about the rear O2?
That's compounded by an engine you don't recall the last oil change on and now full of grit from no real air filter.

Put it together the way it rolled out of the factory- all the sensors, filters, etc that should be there. Plugs, wires, cap, rotor, etc. as appropriate.
Change the oil, then change it again a few miles later. Then see where you are with fuel mileage.
You have to start from a known setup, then make any mods you feel are appropriate.

Lead Foot

Half the sensors? I'm
Missing one, the rear O2. Map sensor is on the side of the throttle body. (Transferred to the 91 block along with the TB). Rear O2 sensor doesn't do anything except trigger a CEL when the rear CAT is shot. The ECU doesn't use it to tune. Still none of this explains how I would get 14mpg one way then 8mpg back.

There isn't any sign of dirt inside the throttle body, I find it hard to believe enought dirt made it through to make a difference that quickly.

Something else that hasn't been explained is how the engine just randomly came back to life in Yakima on the way back.

Just a disclaimer here, I really don't care about this engine. I will be building the 98 block into a stroker with the 4.2 crank and rods I have. I just need it to last me a few months and hopefully not kill me on gas.

Radi

Ah. You listed the parts you transferred and MAP, Rear O2,etc wasn't listed so I assumed you didn't transfer them. My bad.
In '96, early OBDII the rear O2 only monitored cat function. If the rear sensor reading mirrored the front the PCM assumed the cat wasn't functioning. By '98 the rear O2 was considered for long term fuel trim as well.

That's the only difference between 24 MPG and 8, connecting the front sensor? If so the engine runs better on it's default fuel tables than by reading the sensors- one or more of them is likely lying to the PCM.
It'd be interesting to see what your numbers on a scan tool.

JeepCoMJ

for one, put the PCV system back on it if that's some of the vaacuum lines you deleted. building pressure in the crank case is a bad idea., will wreck seals.

is the evap system hooked up? it needs to be. and so it needs the intake on it so they PCV hoses can hook up to it.

belvedere

Looking at your spreadsheet, I'm wondering if you killed the O2 sensor. You had heater power hooked up to the signal wire of the sensor, which is trying to put out a small voltage of its own. I'm not positive that this would kill the sensor, but I wouldn't doubt it.

Lead Foot

PCV system is open to air. I can physically see crank gases coming out of the front port in the valve cover so I don't think it's building up pressure. (actually smells quite a bit like gas). Can't tell you anything about the port in the rear. I never had the PCV hooked up in the 91 either. Honestly don't know much about the system.

I don't know much about the evap systems in these things either. Isn't that all just emissions? The evap canister on these late models, is that the black box under the driver side rear seat? Had to remove that to make room for the rear 4 link. All that's left is one line that went to the fuel tank (breather?).

Still, neither of these explain 14 there and only 8 back.

Lead Foot

Now that I finally have a jeep that's OBDII I'm thinking of picking up one of those Bluetooth setups for the iPhone. Would be nice to see the live feed.

Changes made in order were:
-removed sock "filter" for the drive home.
-sprayed down the jeep and the engine at a car wash. Jeep ran fine after.
-replaced the radiator. Temp went from 210-215 on the way there to 195-200 on the way back.
-added 1 quart of Lucas Oil Treatment, noticed increase in oil pressure from 20 to 30.
-attempt to wire up the O2 sensor then reset the ECU by grounding positive cable to negative. I unplugged the O senor again at the top of the pass and still didn't see a difference in mileage.

Sometime between replacing the radiator and hooking up the O2 sensor I saw an increase in power, the jeep was running like it should when on the way there and up in the hills it was sluggish.

salad

So you ripped out a bunch of sensors, put in a non-OE O2, hacked the wiring, mangled the vacuum system, and now you're running rich? lol

I'm just not sure what you'd expect from a thread like this. There are so many things off all we can do is comment on what you already know.

Lead Foot

Thought about that. That's why I unplugged it half way there and reset the ECU. Didn't see a change in mileage the rest of the way. What about damage to the ECU? Any way to test this?

Odd thing is the jeep doesn't SMELL rich. I've had jeeps that ran rich. I could smell it and even sticking your hand behind the exhaust would leave a gas like residue on your hands. The only place I smell fuel gases is coming out of the front valve cover port.

Lead Foot

AGAIN WITH THE "BUNCH OF SENSORS"? A, they were not "ripped out". B, 2 sensors, 2 sensors were not hooked up. Downstream O2 which doesn't really matter and Upstream O2. What you are missing is the change in mileage from 14 there and 8 back!

The vacuum system is not "mangled". I deleted the unnecessary components. Many have done this without adverse effects. I have done this on all my other jeeps as well. The O2 wiring isn't necessarily "hacked" either. Just hooked up wrong the first time.

Lead Foot

#6 on left and #1 on the right.

s346k

the only thing modified for the return trip that would alter fuel mileage is the o2. given the info supplied I would start there.

Lead Foot

I have heard guys claiming a significant MPG drop when switching out to a lower temp thermostat. Would the temp dropping 15 degrees show a significant change?