babibinie

I have a 98 jeep cherokee and it is running horrible and getting horrible gas mileage. It was throwing a few different codes and throwing one that was saying mf1. so i replaced the fuel injector,spark plaug, wires and just put a new distributer cap on it. also had the timing adjusted. its currently getting 12mpg and runs rough only at an idol. I need help!!! No one knows whats wrong with it! I have been told its the heads, a cracked valuve, a crank sensor, and many other things! I dont want to continue to dump money in it and not fix the problem! Any ideas?

tjwalker

You must start with a proper interpretation of the codes. They all need to be addressed and resolved and THEN see where you are at.

Post the EXACT CODE NUMBERS here. Not a description. Every code number. That is the only logical way to try to understand what is going on. Gotta use the technology provided to you by your Jeep.

We'll go from there.

babibinie

Well right now the only code its throwing is p1899 which is the park/neutral switch

tjwalker

That code is not going to be related to the symptoms you are seeing.

Start with fresh tuneup hardware. That includes spark plugs, plug wires, distributor cap and rotor. I know you installed some of this stuff new, you need to be sure that ALL of it is new.

For a 99, I recommend Champion copper plugs gapped to .035. Do NOT use splitfire or any other saturday morning TV show advertised plugs.

After that, you should verify fuel pressure with a gauge. For a 99, you should have 49 psi, plus or minus 5 psi.

Cleaning the throttle body and idle air control are also a good idea. Here's how.
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality

Radi

^^ Like


How was the timing "adjusted"?

Any sign of a plugged cat? How about O2 sensors? Ever been replaced? Not that I am suggesting you run right out and replace them necessarily- just looking for things that haven't been addressed yet.
Any other clues that might help? Any missing oil or coolant? Odd noises? Engine running at the correct temp?

Is that 12MPG city? Highway? Mixed? Still have stock tire sizes and such?

babibinie

Ok I will give that a try tomorrow as Im changing my radiator and fixing my transmission line leak I also having going on. Im to the point where I am just going to take it to a shop! Im tired of having dirty finger nails!!

babibinie

I'm not sure how it was adjusted, my cousin worked on it today because I am just so frustrated. The guy I bought it from assured me that it was just a vacuum leak when I bought the thing and knowing I am a single family income and working mother with two children I didnt have the time nor money to put into a car. I have thought about replacing the o2 sensors but they are kinda expensive for a fingers crossed fix. Truck is running hot, replacing the radiator tomorrow and gasket and thermastat. 12 mixed, still has stock tires and they are aired up with the right amount of pressure.

Radi

Perfect! Now we can start crossing things off the list. Don't let it frustrate you too much, chances are this is a simple fix, generally it's an O2 sensor, MAP sensor or general tune-up issue when gas mileage drops. Vacuum leak can certainly do it too. Did the PO happen to mention where the vacuum leak was? Can you hear a whistling noise anywhere under the hood? See any rubber vacuum lines that are broken or disconnected?

Do you know anyone with a OBDII scanner you can borrow? If you can find one that reads real-time data like fuel trim and O2 sensor output, might make short work of finding the problem.

incidentally... how hot is it running?

babibinie

checked for leak many many times, there was a broken ball on the right front end of the truck, told it was for cruise control. needless to say i dont have cruise control lol, found out today that its something to do with my ac. it has been plugged off since day one. dont sell in parts store will check junk yards next week when i get time to replace that piece, told i didnt really need it. We put it on a scanner tonight, gave us a few codes the park neutral switch and a engine distributor speed sensor? or something like that, replace the cap and rotor, cleared codes, check engine light came back on and put the scanner back on it. only code that came up was the park neutral code. did the live scan on it and everything seems to be ok from what my cousin said except the idol is really rough. The truck is overheating like in the red, dinging and check gauges light comes on as well as smoking. so im replacing the radiator and thermostat and gasket in the morning after i drop the kids off at school .

Radi

That ball in the front is the vacuum reservoir, it's used to store vacuum for the heater controls among other things. It isn't strictly necessary, but IMHO things work better with it operational. It shouldn't affect gas mileage unless it is leaking.
If you can post the exact codes you are getting that would be very helpful.

Tfox671

Take it to autozone or advance if you cant find a scanner. They read codes free.