ThatPurpleXJ

I've got a 98 4.0 that is completely stock on 225 tires.

Just in the past 5000 miles I've replaced:
-upstream o2
-full tune up, plugs, wires, cap, rotor, air filter
-every fluid in the jeep is new
-oil changes with wix filters
-TPS
-cleaned TB, IAC and MAP
-VC Gasket

New things I've put in the past 5000 miles:
-Dorman exhaust manifold
-99+ intake
-703 injectors

I've also have run seafoam through the brake booster and gas tank. I'm about to order NGK spark plugs and see if they're any better than the autolites in there now.

So what's the deal? I've done pretty much everything others have said in other threads to no avail..

xjallseasons

You should wash it as it probably has a ton of mud on it based on that pic you have of it.

tssguy123

I'm in the same boat as you except I haven't tried fixing it yet. Getting 10mpg is taking away all of my money, so I can't afford to fix it! Kinda awful.

2doors

Sub'd before someone says "you drive a jeep, deal with it." I have the same problem. Not stock, but 11 mpg is a little excessive.

DFlintstone

Just tossen out a couple ideas. Is the engine light on?

The MAP sensor "roughs" out the mix, then the others fine tune it. If the map is to far off the others can't bring it back. Testing it's ground and feed from the Puter' is easy with any meter. Then with an old analogue meter with a needle and a vacuum pump, the "sweep" can be tested. You might check on checking that, at least the ground and feed. Any black smoke?

Also there is a pretty easy vacuum test for exhaust restriction. (plugged cat). Cruiser has some on that in my signature, or Google it.

Just tossen out a couple ideas! Best of luck.


Vacuum leaks suck. I go around with short little shots of starting fluid, if you hit one the sound of the engine will change. Small little shots! You don't want that stuff to accumulate. It's a fire hazard, but It evaporates right away. Also with a tube, or a section of hose you can listen for it. Don't forget the lower O rings on the injectors need to seal as well. (uppers would leak fuel).
The manifold bolts have a habit of loosening, especially that rear one. On anything like that you never want to tighten only one. It can warp/bend, even crack things. If I found that any that where easy to reach where loose, I'd pull the air cleaner and tighten them all, starting in the middle and working out towards the ends. I go over about three times. It's a bear of a spot to get a torque wrench on them all. Just don't ape on it. If its firm and not turning, no point in going on to break it, a REAL *****. The rearmost bolt underneath takes a little doing, but it can be done with the right extension. For that very back one by the firewall I use way long extensions (18"), with a swivel at the socket. Anyway, that's a good thing to check on any old Jeep.

alnielsen

Have you checked for ecu codes?
Is the cat plugged? Is it glowing red after it warms up?

sandfish88

First thing you do is get the auto lite plugs the **** out of your jeep.
Yes, chrysler did use autolite parts at one point, but they just dont perform in my opinion. And I would stay away from ngk. Fancy is not always better.

Go mopar plugs.
Change the fuel filter(s) as well




Need some more info
How many miles on the jeep?
what grade of gas do you use?
Manual or auto?
Hauling tools, camping gear, and a smart car in the back? Or just a daily driver hauling groceries occasionally?

ThatPurpleXJ

Thanks for the help guys. Just to add a bit more, Aw4, 166k, usually 20-25 miles round trip to school, sports and work. About 70 city, 30 highway. Haven't checked the cat yet but I'm getting no CEL even when checked with a scanner.

Honestly I'm not even that hard on it either. Usually I let it shift before 2500 rpm. Also I often try to get gas from shell or Sunoco.

DFlintstone

How's the temperature? Got a 195* T-stat and the heater blows hot?

I'm trien to improve my short trip mileage as well. After I finish this (second tank in this test), I'm swapping in a Mopar/OEM T-stat. Cruiser says he has had a number of others that were sluggish to get the temp up, and the OEM does the trick.

When you put your foot down the intake manifold vacuum drops as the throttle opens. The map senses the lower vacuum and orders more fuel. (a longer injector pulse width). Any other vacuum leak might trick it into delivering more fuel as well.

I guess your MAP is on your TB. Does it have a little elbow tube beneath it? If that were cracked or leaking, that would thank your mileage. Mine's different, not sure exactly how the 99 is set up.

ThatPurpleXJ

Ive still got a 195* t-stat but one question I have is I'm usually running just under 200*, around 198* or so, could that effect mpg at all?

All my vacuum hoses should be good. I put in the new intake last week and all the hoses seemed to be in good condition.

Slugger

I use the ngk copper plugs in my xj and they are working great.

xjallseasons

Just ran 12.2 gallons in a recent fill up and found I am getting 14.9 mpg. Thats with a 4" lift, brand new Goodyear Duratracs 33" x 12.50 on aluminum wheels.
I have installed the air intake hose in the cowl and plan on installing the 99 intake manifold after I buy a new exhaust manifold as it has a leak. Might as well do it all at once.

PocketsEmpty

^^ all of this. The torque ratings are 28 and 30 ftlbs for the middle section if I recall. I just tightened all of mine to 30. Also, make sure you take the bottom bolts out first, and then the top ones. And when putting it back together again, put the bottom bolts in first. Trust me on this one.

nakosd

a simple stupid question: does your job cause you to pile up loads of gear and crap that sits there for months? is your trunk filled with hundreds of pounds of gear?

ThatPurpleXJ

No gear in the jeep other then a small toolbox, shovel, recovery rope, subwoofer and myself. And occasionally my baseball bag