XJ Hero

Hi,
So i have a 99 XJ sport, 4.0L. I got it stock, and have been going through it, upgrading/replacing worn/bad/****ty parts. When i first got the jeep, it was getting around 17mpg round town. After putting a ram intake, and throttle body spacer i gained another mile, averaging 18 in town.

A month or two after lifting it, 4.5" lift on 31" Micky Baja claws my mileage dropped down to around 11. Now right after the lift i was still getting 16-17 with the tires etc. I have just recently done a tune up, (oil change, cap, rotor, wires, plugs, coil, cleaned air filter), and have been watching my tire pressure to make sure their at the right psi, still only getting round 12 or so.
I have ran a scan tool, no misfires, o2 sensors were operating properly, no codes, no check engine light. Everything seemed to check out on the tool. Now im at the point where i cant think where else to look besides fuel, and i know next to nothing about fuel systems. I read on here about someone doing a injector swap from a Dodge stratus i believe it was and gained 5mpg! I dont know how to check for bad injectors, or how to adjust the fuel trim, if thats even possible without a chip/tuner(which i dont have).
A side note, not sure if this could be related but i have a coolant loss issue as well. Cant find any pools on the exterior of system, but can smell coolant around the front end of radiator most times i shut the engine off. Did a pressure test, lost 2-3 psi then stopped. Have not had a chance to do a cylinder leakdown to see if my head gasket is bad, have heard that a bad HG can ruin O2 sensors from burning the coolant. I have no clue yet if this is my case, but between the horrible mpg and coolant loss im having some good issues with my beast now. Any help would be greatly appreciated...Thanks

ZINGGG

As soon as you put your XJ higher in the air with larger shoes, your MPG will be negatively effected. So the drop in MPG that you saw is normal. There are still other things you can do/check for. Doing an injector swap is not a magical cure that will get you 5 more MPGs. Run some injector cleaner through the gas tank and call it a day. Also, run Seafoam through the intake and check your exhaust manifold for cracks.

How many miles are on it?

Darkvampmaster

Also check your fuel pressure regulator.... pull the vac hose off and smell it if u smell fuel regulator is bad. that alone will suck up fuel.

Radi

'99 fuel pressure regulator is back in the tank, not as easy to get at as older models.
Best way to check it is with a pressure gauge.

Are you compensating for the odometer error due to the larger tires? It's reading wrong now thanks to the larger tire diameter, your odometer thinks you drove fewer miles than you really did. Back of the hand math says the difference between a 29" and a 31" tire is roughly 10% at the odometer creating approximately a 2 MPG error. (certain assumptions made, number is ballpark)
That puts you in the 13-ish MPG neighborhood around town, not really out of line for a lifted XJ with oversize tires. For comparison my 100% stock, non-lifted '96 with 235 tires gets about 14-15 city. On a good day, downhill with a tailwind.

XJ Hero

Thanks for the replies guys. I forgot to mention I am compensating for the mileage, with that math method, i figured about the same...8-9% off the clock. My issue, in case i didnt make it clear was that I WAS getting 16-17mpg round town with the bigger tires, and lift. After about a month to two months I noticed a steep decline to 11mpg. After doing cap, rotor, plugs, wires, i gained about 1-1.5mpg(cap and rotor were cooked btw). Kinda was skeptical about that injector swap...would like to look more into it, or just replacing them, jeep has 186,600 on it today.
To check that exhaust manifold gasket, how would i go about doing that? Simple check? Ill see if i can get my hands on a fuel pressure tester, see if that helps out any too.

andrewmp6

Its a long shot but what brand gas do you run and has it been the same the whole time or not?

XJ Hero

Usually Hess, sometimes sunoco, havent noticed a difference between them, 87 octane.

Radi

I'd suspect an O2 sensor if all else checks out OK. It's possible for them to be off-calibration resulting in a rich mixture without throwing a code. Might see if your fuel trim numbers are reasonable using a scanner.

XJ Hero

That was my thought as well. Will be replacing those it looks like, pretty damn sure they're the original ones, so wouldnt hurt for sure. Anyone have any advice on a O2 sensor brand? Ive had luck with bosch in other vehicles, but ive heard some negative comments about their sensors...any suggestions? Thanks

salad

NTK 23151 for the upstream on your 99. Downstream won't affect fuel economy. You've cleaned your TB, right? Specifically the MAP? Is your engine running cooler than usual?

cherokee001

won't 4.10 gears fix that? I have the same issue.

letsgomuddin

This! I get about 15 average per week with mine, 5" and 31's
But i run stock 3.55's

XJ Hero

Alright, ill look into those. Wouldnt the downstream have at least some effect though? I thought they both worked together in order for the computer to adjust the A/F ratio.
I did clean my throttle body, plate, and as far into the manifold as i could get when i did my TBS. I dont think i cleaned my MAP sensor though...would you wanna use throttle body cleaner too on that or something else? And are you talking about cleaning the connector or something else? Engine temp has been consistent from what ive been noticing.
Side note again, found that coolant leak, not pin pointed yet but it ended up being in the left side fin/tank area on the radiator, very small, but hopefully can quick heal it with a radiator fix kit. Still no leakdown test done, but i would think that its safe to assume my head gasket is good then since i can account for the missing coolant.

salad

Taking a very long time to warm up or just running cooler than usual can be a symptom of running rich. DFlintstone went through this before realizing his O2 sensor was toast.

The Manifold Absolute Pressure sensor is the flat thing to the driver's side of the throttle body. I can't remember of TB cleaner is safe for this or not - I know there's a MAF cleaner but not sure if the MAP is special, too. Anyway it's possible that it's caked with goo or the vacuum seal isn't that great.

On all XJs with a single cat the downstream O2 sensor is there just to monitor catalytic converter performance. Earlier models didn't have downstream units at all - select ones actually came with a timer in a Magic Box under the dash that illuminated a maintenance light to have the emissions stuff checked... the dealer would replace it every 80,000 miles or something like that. As far as a Jeep is concerned emissions is an afterthought... Chrysler needed the fuel economy and power for marketing so they put in the upstream... then they needed to comply with the law so they added a downstream lol. 2000+ 4.0s with the 4 sensors may work differently, I'm not certain.

Also like 1996sportXJ mentioned check the exhaust manifold for cracks. Typically the stock one breaks at the collectors. If you can't see the hole you should be able to hear it with a bit of hose held up to your hear like a sort of stethoscope. You can also find exhaust leaks by plugging the tail pipe and giving the engine a shot of some light oil to make it smoke. Fix where the smoke comes out lol. You can also use the same trick as testing propane systems for leaks, get soapy water along the pipe and fire it up, bubbles will form at leaks.

Phantom1

Have you checked the throttle Body??