Loss of Fuel Economy

Since I bought my XJ my gas mileage has been steadily getting worse and worse, I burned a half tank today going only 50 miles. All my tires have the proper amount of air in them and I don't drive like a lead-foot. I have a K&N Filter and I'm not sure how long it's been in there cause I can't tell if the stuff under the mexh is black or purple so how the heck do you clean these things? I'm gonna try running w/o the air-filter in place tommorrow to see if that helps any.

Any other ideas of what I could check/do to get better mileage? I've heard from a fellow XJer that the emission control system can be removed or by-passed for better mileage, how hard would it be to just by-pass or disable it?

Almost forgot, I'm also running 30" Goodyear All Terains...would front u-joints or ball joints going cause this? I'm no mechanic so they're probably unrelated, right?

Thanx 4 the help

Is it possible your catalytic converter is pluged up???? :wierd:

How do I check this? I'm perty sure the exhaust is less than 5k old tho...

Are you acually burning half a tank or is you fuel tank sending unit bad? Are there and other problems? Engine running well?

This morning if I'd hold the gas in any one position for longer than a second the whole Jeep would shake and it was doing it worse this afternoon.

When I started it this afternoon it hesitated like normal but was running really rough and shaking so I shut it off &pulled the air-cleaner out, started it back up and it was rough but not as bad, I noticed that my exhaust system was rattling really bad and when I got home my exhaust smelled like fuel and i thought I smelled something burning. I also noticed that when it ran w/the hood up I could hear some hissing or whooshing and for the last couple weeks I've noticed some hissing noises coming from the gas tank area while running.

Is it possible that I have a punctured fuel line?

Thanks for the help

It's a 1990 Cherokee Pioneer w/the 4.0 and the 5-spd and 180k

Your engine is running very rich and probably the commonest cause is a bad O2 sensor (or bad wiring to sensor). Check the voltage from the sensor signal wire. It should be close to 1.0v. If it's near 0.0v, either the sensor is dead or the wiring is shorted out.
Check all the vacuum connections to the intake manifold, especially the one going to the MAP sensor.

I have an ammeter so how do I check the voltage?

By any chance would an O2 sensor for an S10 Blazer work? I have one lying arround...

Dr. Dyno- Could you explain to me where those Vacuum hoses are, and also where the map sensor is? Thanx cause I'm no mechanic

The O2 sensor has four wires going to it. I don't know what colour they all are but probably two white, one gray, and one black. The two white ones are for the sensor heater element (one is 12v power, the other is ground), the gray wire is the O2 sensor ground, and the black one is the signal wire. Just probe that one and measure the voltage across that and a vehicle ground using a digital voltmeter.
If the sensor from the S10 Blazer is a 4-wire unit, it should work (most 4-wire sensors work the same way) but the connector to the wiring harness might be different.
The MAP (manifold absolute pressure) sensor is at the top of the firewall in the middle. It'll have a green 3-wire connector and a small diameter tube attached to it. The tube goes to the intake manifold. If this is cracked/broken, the MAP sensor will not sense vacuum, think that the engine is under load, and tell the computer to richen the A/F mixture.

You could also crack the gas cap while you're testing it, or remove it completely. These tanks use vented caps, and if the vent is blocked or broken, it will cause a vacuum in the tank (the hissing sound could be air rushing in thru any available orifice) and fuel starvation.
I second the vacuum line suggestion- these things were real rat's nests before they got the emissions and computer crap figured out, and a properly running engine DEPENDS on proper vacuum signals. Go thru them ALL, one at a time, checking for flexibility (but not squishyness), cracks, breaks, or disconnects.
If your K&N filter looks black or purple, it's time to clean it. Either order the Filtercharger kit from 4WD Hardware, or get one at your local Pep Boys or other speed shop. Follow the instructions EXPLICITLY, and re-oil. A properly cleaned K&N should have a deep red color to it.
Also check your manifold for leaks- while the engine is running, spritz the joints between the intake manifold and the head, as well as between the TPS and other intake sensors and the manifold, with brake cleaner or carb cleaner. If the idle changes, you have a vacuum leak you need to attend.
Good luck, let us know what you find.

The vacuum tube that goes to the map sensor WAS disconntected and the hissing in the engine bay I heard was coming from the side of the thing that the snorkel connects to. I saw that there are 2 holes on it as well as the plug at the end of the tube, I plugged it in and the idle speed dropped(sounded better) and the hissing stopped. Is there supposed to be noting on the top plug? i couldn't get the plug all the way in, is this a bad thing, also how could I get it in all the way w/o breaking the rubber piece?

About how much should the K&N cleaning stuff be, and am I killing the Jeep driving arround w/o an air filter?

Zoro said:

The vacuum tube that goes to the map sensor WAS disconntected and the hissing in the engine bay I heard was coming from the side of the thing that the snorkel connects to. I saw that there are 2 holes on it as well as the plug at the end of the tube, I plugged it in and the idle speed dropped(sounded better) and the hissing stopped. Is there supposed to be noting on the top plug? i couldn't get the plug all the way in, is this a bad thing, also how could I get it in all the way w/o breaking the rubber piece?

About how much should the K&N cleaning stuff be, and am I killing the Jeep driving arround w/o an air filter?

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Nah, most of the times the manufacturer puts them there for sh*t's and giggle, and so it'll look good.

The gas mileage and shaking problem are your o2 sensors. Mine did the EXACT same thing



















P.S. I'm being sarcastic, put the darn thing back on!

Zoro said:

The vacuum tube that goes to the map sensor WAS disconntected and the hissing in the engine bay I heard was coming from the side of the thing that the snorkel connects to. I saw that there are 2 holes on it as well as the plug at the end of the tube, I plugged it in and the idle speed dropped(sounded better) and the hissing stopped.

Click to expand...

It looks like you might have cured your problem. The gas mileage and shaking problem were due to an over rich-running engine. Now that your MAP sensor is seeing vacuum again, the gas mileage should improve dramatically. The drop in idle speed and the cessation of the hissing that you just saw is a good thing; you've cured the vacuum leak.

air filters are good, m'kay... Put it back on, you don't want dirt gettin up in your engine...The K&N cleaning kit should only be around $15 from your local car parts store. Got mine at Carquest for about that much.

Ok, i've had the exact same problems, and have checked all the vacuum lines that I can get to easy, even had my neighbor look over them with me as well. Can't find anything out of normal. Any other suggestions?

Also, I am planning on getting it up on a lift with in the next week or so to have a very detailed look at EVERYTHING underneath. I had a brief look yesterday, and everything looked surprisingly good, for having been up north most of it's life. Anything special or common I should look out for?

Careful with it on a lift in the Winter cause windshields like to break...mine did

I noticed today a definate improvement in gas mileage but it's still bad, if I replaced the O2 sensors that would help, right?

I also noticed a hissing coming from arround the gas tank area when the it's running, is this normal?

Zoro said:

I noticed today a definate improvement in gas mileage but it's still bad, if I replaced the O2 sensors that would help, right?

I also noticed a hissing coming from arround the gas tank area when the it's running, is this normal?

Click to expand...

Check the voltage from the upstream O2 sensor first before replacing it. You don't want to spend a bunch of $$$ replacing what might be a perfectly good sensor.
As for the hissing from the gas tank, that's the fuel pump telling you that it's working so it's normal.