Gas mileage... w00t!

[artsifrtsi]

Took my Jeep on a trip yesterday from Carthage, NY to Schenectady, NY and averaged 20.1 MPG! Things have been getting better and better on this rig. I started off with an almost outrageous 15 MPG, fixed the dragging brake and mileage went up to 17. Ran Seafoam through her, and in addition to the oil pressure increasing to ~75 on startup to ~65 at running temp (according to factory dash gauge) mileage went up to 18.7 mixed and now the big test at 20.1! I am loving this 210,000 mile rig more and more every day!

(Oh, and this rig is a 1990 2 door with 4.0 l and an AX15, has a 2 inch lift, 31's and original 3.55:1 gearing)

Now I cannot wait to get going with the next set of mods, and improving the body!

 

[Hackmeister]

I've gotten a quite nice 21.5mpg out of my last couple of tanks, with a mixture of mostly highway and city miles. I've got a stock '96 XJ sport with stock tires and whatnot, but I'm sure the manual tranny helps a lot for mileage... my last one got a whopping 15mpg stock

 

[Blaine B.]

Bla bla bla bragging. Ban.

 

[XJoshua]

All of it varies by the condition you drive in. In my ZJ I can do 32MPG all day if there is no wind and the speed limit is 55MPH.

XJ doesnt have a OBC so I cant say what I average in it at which speeds but 1/4 tank test gets my 17MPG city mostly 45MPH.

If I compair the two i do 15.6MPG in the ZJ in city doing 45MPH. But I believe the change is due to the XJ being light as a feather and the ZJ weighs as much as a caddy.

Hell a XJ could probally do 50MPG at 60MPH if you have constant wind on your hatch and a even road.

 

[89Daytona]

In my ZJ I can do 32MPG all day if there is no wind and the speed limit is 55MPH.

XJ doesnt have a OBC so I cant say what I average in it at which speeds but 1/4 tank test gets my 17MPG city mostly 45MPH.

If I compair the two i do 15.6MPG in the ZJ in city doing 45MPH.

So you get 15.6city/32 highway in a ZJ..... You must mean 23mpg highway, there is no way you can get 32mpg out of a jeep. If you are using the onboard computer display to calculate it, those can't be trusted.

 

[Jess]

The wife's 94 country (stock gears with 31's) now gets about 13 city and 16 hwy---and I don't know why!!! I just replaced the cat/muffler, 02 sensor, oil, tranny fluid, coolant flush, plugs/wires/cap/rotor less than 7K ago. Why is my mileage so bad? What else will help it to get better mileage?

 

[Blaine B.]

I was getting around 9.5 when I first got my 95 Country.

After Seafoam/Marvel and a complete tuneup, I get 14 in the city..stock gears and 30's.

So maybe you're right around perfect with 31's and all?

 

[XJoshua]

I wasnt talking highway. Our speed limit is 75 and gas milage goes to hell on all jeeps after you go faster then 55.

The computer is very trustworthy for the on the dot at that moment milage. This is where Im getting the info. Our main blvd for about five miles of straight road when its not windy going 55 I can sustain 32MPG. Once you add in other factors like traffic heat humidity altitude condition of engine tire pressure then the OBC is very inaccurate.

On average I think the ZJ does about 22MPG highway and I know if does 15.6 to 15.3 city.

The XJ after running empty and filling up did 17city and that all i use it for is city driving. Biggest mod was removing the valve cover and cleaning the head. Mucho build up that caused it to overheat on trips over 30miles in distance.

 

[darjevon]

MY jeep gets 60 mpg city, (infinite) highway WOOT WOOT WOOT WOOT now that I installed the Vortex Tornado. Best 20 bucks I ever spent! I also added gigantic rear spoiler to provide more downforce. What the hell is the point of this thread? ANY XJ OR ZJ OR TJ OR YJ GAS MILEAGE IS A LOAD OF CRAP. END OF STORY.

 

[Only In A Jeep XJ]

The wife's 94 country (stock gears with 31's) now gets about 13 city and 16 hwy---and I don't know why!!! I just replaced the cat/muffler, 02 sensor, oil, tranny fluid, coolant flush, plugs/wires/cap/rotor less than 7K ago. Why is my mileage so bad? What else will help it to get better mileage?

My guess would be the new cat and muffler. Various brands flow at different rates if you reduced the flow then it can reduce milage. Also Was the computer reset after all this work? If not then it is still could be going on the prvious settings still.

 

[artsifrtsi]

The trip has lots of big hills. The first leg was on highway running at about 60 MPH, then from Utica to Schenactady on the throughway at 70 MPH. If I were to slow the trip down a bit, then I'm sure the mileage would have been better.

I think that the stock gears with the 31's are keeping the rpm's low enough to get the good numbers.

 

[96XJ9705]

Have 133K on my 96 and got 24 mpg from southern OR down through Carson and into CA on 395 (elevation ran about 6000+ from carson on). This was after the factory recall work on my cat was performed. Millage in town still averages about 12 if I accellerate hard, or 15 if I go easy on the pedal. Oh, and on the trip, I had a yakima roof rack that was empty, theback was stuffed with all of my worldly possessions and had a bike on a rack on the rear hatch. This is also with 235 75 15's and stock gearing. 24 is the best that I have ever seen with this jeep. Usually got about 18-19 on the highways in North Idaho.

 

[RichP]

Got this from one of the guys over on blue mountain who got it from somewhere else, not sure of the validity of it but it sounds very plausable to me from being in the computer business.
posted on rof by elff


PCM Fast Learn Mode
I think most people know that the PCM performs several functions controlling Air Fuel ratios via Injector Pulse Width and Ignition Timing.

These “outputs” are controlled using the “inputs” received from all the sensors and then comparing these reading against a set of data tables burned into the PCM.

These “memory tables” are the brains that keep the engine running at low emissions and peak economy across the RPM band

What most people don’t know is that these memory tables change. They are “adaptive”, based on sensor readings, driving habits, engine performance and sensor tolerances.

Over time, the PCM Memory Tables become tuned to your engine.

Most folks know that disconnecting the battery for 15 to 20 minutes will reset the PCM to get rid of a Check Engine Light or Error Codes stored in the PCM.

However this DOES NOT reset the Adaptive Memory Tables in the PCM. Until recently I did not know how to do this.

This procedure first came to me from the folks at Avenger. I then verified it with a phone call to a Chrysler Engineer who had called me a year or so ago to ask about my experiences with a certain part on my TJ. I figured he owned me a favor. He had to check up on this for me. He called me back a week later and did indeed verify this procedure and what it does.

To the best of my knowledge it isn’t documented in any of the FSM.

Forced Flash

This very simple procedure will Erase the “Adaptive Memory” stored inside the TJ PCM and allow a new “Adaptive Memory” to be developed.

After performing this procedure the PCM will re-learn and store into Adaptive Memory your engines performance characteristics.

[L]Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory.
Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”[/L]

Adaptive memory has nor been flashed, or erased from the PCM.

When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory.

When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”.

The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.

Warm-up Cycle

A warm-up cycle happens when all of the following conditions exist.

Engine is running
A raise of 40F in engine temperature must occur ABOVE the engine temperature at start-up
Engine Coolant Temp must reach at least 160 F.

Once your engine has gone through 50 warm-up cycles in at least a 500-mile distance the PCM adaptive memory is set. It WILL NOT Change unless you flash it out and start all over again.

 

[Ba-Riedo]

and what will erasing your memory do? wont that hurt your engine if it is old?

 

[Blaine B.]

No, it won't hurt the engine.

Will this procedure work for OBD-1 systems? (like a 95 XJ.)

 

[88xjchief]

I get about 20- 23 for city driving. 88 all stock 4.0 auto.

 

[Ecomike]

there is no way you can get 32mpg out of a jeep. I

I beg to differ on that comment, I get an average of 34 MPG city with my 85 Jeep Cherokee Pioneer in the city.:sunshine:

Oh, it does have an SD22, Nissan diesel engine with a 5 speed manual tranny. Guess that might make a little difference, but Jeeps can get good mileage, especially downhill! :laugh3:

 

[Ecomike]

and what will erasing your memory do? wont that hurt your engine if it is old?

No it wont hurt it. If the data tables have data in them from old, bad, defective sensors that have sense been replaced it can do nothing but help the jeep run better and probably will improve gas mileage.

The clearing memory method below sounds logical, but I have two issues with the details. One is what the hell does turning the head lights on and off have to do with it. If the procedure is correct (and it might be), some Chrysler Geek/Nerd with a sense of humor, must have written a damn code into the PCM telling to clear the table memory on every third thursday if the head lights were turned on and then off at midnight.:bs:

I also don't know if I buy the part about the sensor data table being static after it is filled up on the 91 and later PCMs. MY FSM on the 87-90 Renix era says that the prior PCM (Called a Renix ECU) stored data in a table, but that the data was continuously overwritten and updated, but that it took a while for that to happen, like 200 start / stop ignition sequences.

The part that does make a lot of sense is disconecting the positive battery cable, shorting that cable to the ground cable and I would add turn the ignition on while shorting it to make sure all the capacitor driven memory is alive and hot to be grounded out, and do it for a minute, 60 seconds or more.

I am pretty sure the Chrysler diagnostics hardware had a way (message code to the PCM or a ground button) to clear the PCM memory as well on the OBD-I.

 

[RichP]

Don't shoot the messenger, clearing the tables with the + to ground does make sense, I have not delved into the electronics like some of you have so I have to take things with a grain of salt.

 

[Ecomike]

Don't shoot the messenger, clearing the tables with the + to ground does make sense, I have not delved into the electronics like some of you have so I have to take things with a grain of salt.

Not trying to shoot the messenger, in fact it's a great post,:worship:, I was just tickled at the part about the headlights, sounds like something a code programer and the hardware guys would actually dream up to reduce the routine bordom of the code writing job and to give them a secret back door to play with for clearing the data tables. Kinda like the secret decoder rings from the cereal boxes when we were kids!

Most memory is either cleared by removing the power, or grounding out and shorting some contacts, or with a powered deliberate re-write of the memory. Maybe the grounding step is clearing the codes and enabling the second memory sensor map clear the memory instruction step which is completed by an initialization signal from the headlights being powered up (something the PCM might monitor) then a line of code in the PCM program recognizes it as a clear the data table memory step, but only if the grounding step was done first to clear the codes, and enable the PCM to use the headlights power up as an instruction.

But it does sound a bit bizzare when you add the headlights part and makes me think of something a practical joker might do.

The real question is does it work?????

Any takers on trying it? I don't have an OBD-i or I would try it.

This is only for the OBD-1 generation 91 to 96 years, correct?

Then the next question is, how do we verify that it works?

If it does work it might solve some peoples gas mileage problems!!!!:woohoo: