MurderCityXJ

So I've slowly been doing a tune up and changing fluids since I bought my 98 classic, and I've noticed that the more I do, the worse the milage is getting. I am starting to think that I am going in the wrong direction in searching for the issue.

So far, I have done plugs, wires, oil change, both diff's, t case, and a seafoam treatment that produced a large amount of smoke. My plan for this weekend is to pull the TB and give that a good clean up and to also clean the IAC, which may fix my stuttering issue that happens if I don't let it warm up for a few minutes before pulling out of the driveway.

I was watching a video from EricTheCarGuy's channel about a vacuum leak possibly causing issues. I found that video after searching around here for possible reasoning for not having consistent heat in my XJ, and my vents not always switching.

My question is to anyone who knows a thing or two about the vacuum systems. Does it sound like I am following the symptoms to the proper cause? Could a vacuum leak somewhere be causing the jeep to over compensate for what it thinks is a lean condition by pushing extra fuel, which would be causing my mpg to have dropped to around 11mpg highway?

Any insight here would be excellent since I will be doing as much as I can in the driveway! Thanks!

Klutch

A vacuum leak can indeed cause issues with MPG as well as problems with the interior vents not switching from defrost, to vents to floor.

Other things which can cause MPG to suffer include a dirty air filter, a clogged catalytic converter, bad oxygen sensors, underinflated tires and leaking fuel injectors.

Are you getting a check engine light on the dash?

MurderCityXJ

I'm not getting any CEL or picking up any codes that haven't set it off either. The cat sounds to be flowing properly, the air filter is clean, and the tires are inflated properly as well. I am going to give this trick a go in the next few days if I can get home before the sun goes down. http://youtu.be/zMok2y05jNE

Maybe that will give me a clue as to where to look at least.

kdot

I'm getting terrible MPG in my remanned 4.0L (bored over .020) like about 11MPG..although I am driving a tad erratic- seems like its sucking gas too fast still.

Any thoughts? Any info I can present the mechanic with as we are working out the kinks on the remanned motor he installed?

jeepkid03

Have you been using 4x4? I get terrible mileage when I used 4 wheel a lot in the snow here. Its been snowing here a lot lately and I've been averaging 12.6 mpg. In the summer I can get 17-18 with the same driving.

Winter gas can also decrease mpg by up to 33%.

MurderCityXJ

I don't FEEL like the very little bit of 4x4 I have used should be causing that much loss. I also need to check the rear brakes, I can feel them rubbing a bit when I coast.

kdot

Yeah, ive been using 4x4 the past week or so due to questionable conditons and -20 degree's outside lol. But I definately kick it into 2wd as soon as the road allows me.

But 10MPG sucks for winter driving.

mnewxcv

10mpg? thats pretty good for these things.



really though, how long hasit beena problem?

DFWXJ

In order to get a better mpg, you might want to change your oxygen sensors or at least pull them out and see if they are fouled. If that doesn't help, I'd do a compression test and see if your motor is even healthy before you start dumping money in uneccessary places, but if you check your plugs right now, you can probably see which cylinder(s) giving you trouble. Thats usually where I start when doing a tune up. If you think you have a vacuum leak, get a vaccum check at a local shop if you don't know how to do it. It's cheaper than all the fluid you just changed. Hope this helps. Good luck.

cruiser54

Here ya go:

MoparNJeep

Map sensor might be bad too

cruiser54

Yup.

Jooman07

Mine is around 11 mpgs also. I have a vacuum leak and it's throwing the CEL also. I can hear it sometimes when I let of the throttle. It's more of a high pitched whistle coming from the firewall area.

I just changed both bank 2 sensors which I thought was making it run rich. It still does. Once this weather warms up a bit I'll be tracking down that leak. Some people like to use either carb cleaner it ether. Not sure which I'm gonna use just yet.

cruiser54

Use carb cleaner. Oxygen sensor safe stuff.

MurderCityXJ

Vacuum system all checked out as far as I could tell. Hopefully I can be easy on the skinny until its warm enough to start pulling off exhaust components to check for leaks there.

I pulled the IAC off and cleaned it up real nice and it seems to idle smoother, so there is a plus. Still noticing the same hesitation when I don't let it warm up.