evil_goat

So, my '91 jeep went about 200 miles with a full 20-gallon tank, and now the tank reads halfway, and It is also about 100 miles. I have not done any other tests. So this means that my jeep gets about 10 MPG. Maybe it is the roads I drive on, but it is still really bad. There is no check engine light, and no codes either. What could be the cause of this? What is the normal MPG?

4.3L XJ

How long since you have changed your O2 sensors?

evil_goat

I'm not sure but probably between 7 and 15 years ago. The car hasn't been driving much for the last five years. How long do the sensors usually last, and if they are broken shouldn't they throw a code?

4.3L XJ

Good question. The only codes for the O2 sensor is if the heater fails or the plug gets disconnected. The sensing part generates electrons and just like solar panels, which do the same thing, they wear out over time, or you could say they run out of free electrons. As the voltage gets lower, it causes the ECM to make the mixture richer because it thinks the fuel mixture is too lean

evil_goat

There is a hole on the end of my muffler on the tailpipe side. Would this contribute in any way? It is also possible that there are holes in other places too.

robsjeep

An exahust leak upstream of the oxygen sensor can cause rich running for sure, down stream leak is no problem.

Make sure your brakes are not dragging, that can be a big fuel economy killer.

fix this quick if indeed you is running rich, as rich running will ruin the catalyst quickly$$$$$$$$

evil_goat

How would I tell if I am running rich? Also is there a way to tell if the brakes are dragging without taking them apart? They don't smell like brakes like they do if you go down a steep hill for a while.

fb97xj1

Add tire pressures to that. That will also affect the mileage. As will plugs, wires, distributor cap, and rotor. Dirty injectors, fuel filter, air filter. All that stuff. Also, how you drive it plays a part. Like you stole it. Or like Miss Daisy. Short trips where the engine doesnt reach operating temps. Hilly terrain, etc.. Check the oil and transmission fluids, too. And coolant. Fuel pressure. In my experience, a simple tuneup or the tires needing air was all it needed.

Could be a tired engine. Anything, really.

4.3L XJ

OK, one other question. Do you have a crack in your exhaust manifold. That will cause it to run rich too

evil_goat

I don't think so. They usually make a lot of noise when it is cracked right? Since my jeep (1991) is the California model, how many o2 sensors does it have?

robsjeep

My 1990 has one O2 sensor, but it is different in many ways from yours suggest you buy a repair manual so all your specific needs can be answered. The factory manual is best, Haynes makes a decent starter manual that at minimum you should get.
An exhaust leak may not make a huge noise. Examine the manifold carefully for black soot, a sign of a leak, the gasket to head is a prime spot to check too. you will need a small mirror on a stick to see everything well. Or better still do a smoke machine test.
you should check general engine health with a vacuum gage test, a compression and/or leakdown test. If for instance you have bad vacuum signal, that will screw up the mamifold vacuum sensor input to the brain, a vacuum test will show this. you can also search for a vacuum leak with a flamable aerosol like carb cleaner, spary will at warmed up idle and listen for engine speed changes as a leak sucks in aerosol. of course you need a airly stable idle to perform this test. Examine all six spark plugs, they should all look same at the bussiness end, if not, if one is sooty, then maybe it has an injector that is leaky, or an intake leak near it if it is not sooty, and has signs of lean running, but the others are not. Make sure you got a strong spark, ignition wires good. If all plugs are sooty, then it likely is not an injector leak, as what are the odds for all six to leak? like wise a weak spark will give a different color, even a wet with fuel plug end.
Test fuel pressure, if you have a pressure regulator (1990 does) make sure it is functioning ok. do a leak down on the injectors, make sure none dribble fuel, leak, have ugly patterns of spray

Check the sensors, the manuals should have details on how to measure them with a volt/ohm meter.

Oxygen sensor is a consumable item, if lots of miles on it, replace it, doing so may save your catalyst. If more than 20 or 30 K miles on it, just get a new one to be safe.

Is power ok? or is she a dog at acceleration?

A diagnostic scan tool can be very helpful if these tests dont find the problem

You did assure the brakes are not dragging, right? Torque converter working, and locking up once at cruising speed on level? If it is slipping all the time you will get bad mpg, and hasten the destruction of the tranny

good luck

robsjeep

So two to ten years of regular use on the O2 sensor, and little use for five years. Well if indeed it is on the high end, ten years, at say a meager 5k miles per year, is 50Kmiles. That is high miles on an O2 sensor. Sometimes they die at 20K miles. Sometimes they fail slowly, gradually lagging in sensing, known as a "lazy" sensor, you can see that once warmed up they switch from high to low volts slowly rather than rapidly. sure they still run, but the mixture wont be right, and a lazy O2 sensor can kill a catalyst $$$$$. Even if only driven a lot for two years, that lot might be 20K or 30K miles, which is near the time these sensors might start going bad, not dead yet, but lazy. A new sensor is what Id be doing based on the possible miles on the existing one. Its like a spark plug as for replacement, you want to replace it regularly, BEFORE it fails, and you do that based on miles. If you have ten years with 10K a year on the jeep, thats 100kmiles! well past the "best buy date".

Very Red XJ

Indeed, the O2 sensors could be a problem. If replacing them doesn't improve your MPG, then assuming your air filter is clean, spark plugs, cap & rotor and wires are good, check for dangling wires grounding. Then open your shop manual and test your sensors: idle solenoid, MAP and TPS. If they are good and your throttle body is clean move to the injectors.

Testing - pins for the wires are a great help

Testing restance in cleaned injectors

robsjeep

ps, after doing the O2 sensor (soak its threads in a penatrating oil prior to removal) toss some injector cleaner in the fuel tank. I have found on some occassions that that can improve poor running, assuming the problem is a cleanable injector deposit. I do it as a preventative item on several of my cars every year or two after being proven it can clean up an injector and actually made the vehicle run better.

evil_goat

Here is what my spark plugs look like. Both photos are of the same one. Does this indicate that I am running rich? Also based on the condition should I replace my spark plugs?