19558077 Don't even Joke about removing the cats unless you are going to replace them. If you do this thread will be removed and you will earn an infraction and possible ban.

 

ok so keep all the cats..... check

 

99 had returnless fuel system... so no pressure regulator, fuel pressure is controlled by the PCM through the pump

 

Thanks I can mark that off the list of things to check. Would a messed up map sensor do this? I had one mess up in a ford and it wouldnt make it stumble like this it would just make it to where it couldnt go above 3k in rpms

 

Holy smokes, 4 O2 sensors and three cats. Harsh.

Lets keep this thread goin. i'm seeing lots of new ideas. things i would have never though of. I'm going to go look up right now how much my odometer is off with the 31s. actual measurement with a tape is only inch and a half differenece from the stock 225/75 15s.

bring on the info.

 

mine is off 1 mph for ever 10 mph what all my gps say anywat

 

Im on 30" tires. I add eleven (11) miles per one hundred for the difference. I only average 240 miles out of 15 gallons of gas (16 mpg) driving all city, under 45mph at all times. My last highway trip I averaged 20 mpg. My fuel gauge will show 100% empty after 250 miles. On my build sheet it tells me I have a 20 gallon tank. Something is not right.

So first off I hate the inaccuracy that seems to plague the xj cluster, its not any better then the idiot gauges, it still seems like a guess. I want to run an electric pump and a gm sending unit to a "Autometer" type guage to get a true reading.

I have replaced every fluid, done a complete tune-up, installed an afe intake, bored throttle body and had a new cat (old one was not functioning correctly) and dynomax muffler welded up. Exhaust mani is good. Tires are balanced and hold air just fine.

And now for the conclusion. Before and after tune-up I only gained one mpg city and two highway. What my story should tell you is these items do not effect your mileage as some would leave to believe. I would look deeper into whats going on inside. 10 mpg is crazy. I think mine are bad. This might be too simple but try resetting the ECM via disconnecting the battery. Then the ECM has to relearn, it might help.

Anyone know an easy way to measure air/fuel ratio? Is there a obd scanner hook up? Or do you have to throw down some serious coin?

 

Resetting the ECM:

Read this over on JF. Assuming this applies to OBD2

http://www.jeepforum.com/forum/f11/b...et-ecm-548305/

Exhaust mani:

I can hear an exhaust leak up high near the engine. Do these manifolds crack regularly? if so where and would that have an effect on MPG? An excuse to thrrow on a banks header wouldn't be all that bad haha. Thats how i justified the super turbo...

So far the dynomax sounds quieter than stock but still waiting on the "break in period." what exactly is this break in period? should i only expect any increase in MPG after the break in period?

 

I have read this a few times and I dont get it. Ok so for every 100 miles add 11. Got that. This means on your 250 on 20 gallons you went 277.5 miles. So 277.5/20=13.875 miles/gallon

Higher up you said out of 15 gallons you can go 266.4 miles on 15 gallons. So 266.4/15= 17.76

So my question is. how are you using 5 gallons of gas in 11. miles? (277.5-266.4=11.1)

X2 maybe not gm but something better.

 

Just hook the battery for about 15-20 mins. This will reset the ECM.



I apologize what I said may have been misleading. All city driving I get 240 miles out of 15 gallons (16 mpg). All Highway I get 300 miles out of 15 gallons (20mpg). I had already added the +11 miles into the 240 and 300 respectively.

 

Ok that makes sense. Sorry I was just like hmmmm I dont get it.

 

It was late when I was writing that. Do you have SMOG shops around you? Throw a tail sniffer in there and see what she is running like.

 

We dont have emmissions tests here in oklahoma. How would I know if it something with the heads? Other than compression is down?

 

Leak down test would help.

 

So is 10mpg just city bad? I get 14mpg average on a tank. Thats with at least 80 miles on the highway. My last tank had 275 of highway 25 miles of mountain and 50 miles city and I got about 19.8mpg. So am I normal? On all city I get about 10-11. I drive very aggresive. Jeep is all stock right now, 97 4.0.
Recently done
-Complete tune up
-seafoam to intake
-TB cleaning
-um...I bought it with no meow on it...I plan on putting one on mods...I swear...