mr66golfpro

Hey all, new to this forum, not new to jeeps, have had a few and they are by far the best rigs on the planet IMO.

Just purchased a new to me one, 97 XJ 4.0 auto. The thing is, it is getting the worse mileage I have seen for an XJ, that's not lifted or modded. 12 MPG!!!!

Motor is stock, no power adders, plugs were shot so I replaced them, wires are OEM mopar, look nice, air filter is new, I didn't check the cap and rotor yet, kinda short on funds at the moment but that's where I'm going next.

Runs great though, aside from it takes a bit for it to start, (pretty sure it's fuel pressure bleed off at the fuel pump)

Any ideas? Cat converter maybe? O2 sensor?

ZINGGG

There are hundreds of threads on this. But yes, replace the cap and rotor. Do the wires too while you're at it, your eyes may be deceiving when you think they're fine. And since you just got it, replace all the fluids. Also, clean the throttle body and replace the o2 sensors. Check for an exhaust leak, that could be a big cause for bad mileage too

lynskynbnd

Possabilities:
worn ignition components
O2 sensor
map sensor
clogged air filter
low tire pressure
dragging brakes
coolant temp sensor
too cold tstat
locked up fan clutch
too high vicosity oil
slipping trans
tcc problem
ecm problem
restricted exhaust
too much skinny peddle
etc...

mr66golfpro

Exhaust leak, it has the typical crack in the manifold, not terribly loud yet but it's there. That's on the list of things to do.....

lynskynbnd

Good place to start!

ZINGGG

Very good place to start!

gvns8

You say it is difficult to start, so yes it may possibly be the fuel pump check valve which could also decrease your mpgs. My check valve has recently gone bad and my gas milage went to crap.

The best way to troubleshoot problems that I have found is to get a hanes manual and look in the beginning of it in the trouble shooting section. Go down the list of different symptoms the manual lays out and circle each symptom you have (ie trouble starting cold, poor mpg). Then look at all the remedies they have listed for each of the symptoms and start first with the remedies that are mentioned the most frequently accross all of your symptoms you circled. Good luck.

damirsk8er87

helpful thread... i just got a jeep also.. 97 cherokee country.... all stock... 160,000 miles and i get 10 MPG.. yes i said 10.... im gonna check the stuff you guys mentioned... any other ideas?? and where does everyone get their parts??

mr66golfpro

I stopped by my cousins house who is a mechanic, I believe we are going to start with pulling the exhaust manifold and welding the crack, and going from there.

Per his identifix account first thing to look at is the O2 sensors, but on the scanner, my voltages look good and are fluctuating like they should be so hmmmm.....

86jeepmj

i just bought a 86 mj it was getting poor gas milage my brother told to replace the fuel filter man how much did that help when a fuel filtler is part time clog the competer tell the jets/injector to open wide open and keep dumping more fuel so it really is wasting fuel and u get bad fuel milage i at first dint want to belive him but he was telling me the truth

mr66golfpro

That's easy to do on an 86, and cheap, I know on the newer ones the fuel filter is in tank..... and about 100$ because it's the regulator as well.

Once I get some funds and some time together I'm gonna weld up the exhaust manifold and run some injector cleaner thru the fuel rail and see if that helps.