We purchased our 1990 2 wheel drive jeep last year from a mutual friend. We use it purely as a work vehicle for a 24 mile round trip daily. Up until recently we got very acceptably gas mileage and it ran great. It does have a small oil leak but nothing outrageous but, we changed the valve cover gasket to stop that. When we were changing that, we changed the plugs as well. Since then, it is running so rich u get a buzz standing behind it and the gas mileage is down to literaly2-3 miles per gallon...NUTS!!! No shorts or sparks on wires or cap. I'm extremely frustrated and am looking for any and all ideas.
Cant hardly afford to drive it as is........

 

you didn't specify what type of jeep or engine. however, in this case it might not matter. i'll share my own recent experience with you and hopefully, it will all apply to you as well. i developed a rough idle and averaged burning 1/2 of a 1/4 tank of gas per maybe 1 - 1 1/2 miles in my own neighborhood. my gas started running lean though. my jeep is an 97 grand cherokee laredo 4.0L inline 6 2 WD. obviously there are some part differences. however, trouble codes are same. is your check engine light coming on? just curious. anyways, you need a obd-1 scanner for your year jeep to check for codes..my problem started as soon as i changed my valve cover gasket as well. apparently with jeeps, anything ( and i mean anything ) you remove or bump out of place, will set a code in your computer. yes, it is ridiculous. my idle air control valve went bad ( from age, not the gasket change ) and like i said i developed a very, rough idle. the fuel was running lean all of a sudden. i didn't know this at first. just knew something wasn't right. the bad IAC caused excessive carbon to build up on my upstream O2 sensor and all my spark plugs from the lean gas mixture. you just changed your plugs. no offense, but, did you make sure they are gapped right? got to ask. i'm assuming you did and the plug wires are on the right plug locations. again, no offense. you just never know. check out your O2 sensor for sure. you probably have a real PCV ( positive crankcase ventilation ) valve on your jeep. did you change it out or make sure it's not clogged? that the hose to it is firmly on? all other hoses to valve cover on good? i assume all yeses on this. now, like i said, you didn't mention motor size or type of jeep. so if you don't have these parts/sensors on your jeep, ignore this. i'm pretty sure you do though. these cause some of your symptoms. the IAC i've already told you about as well as the O2 sensor. you have either a MAP (manifold absolute pressure) sensor or MAF (mass air flow or vane) sensor. these can cause your fuel to run too rich or too lean. mine are on my throttle body. i believe yours will be under your cowl panel.on top of your motor. these sensors help your computer to regulate fuel delivery & your ignition timing. you could just have a loose connector on one of your sensors or hoses. i'm assuming you've already checked them off your list. like i said before, if you don't disconnect your negative battery cable before you work on anything under that hood, you're just begging for some codes to be put in that computer. i would think that should help get you going. idle air control, O2 sensor, map or maf sensor. vacuum leak from cracked/ loose hose to valve cover etc., or loose connector to those sensors. if your near a auto parts store, they can check the jeep for codes i would imagine. i know they have obd-2's. i assume they have for personal use at least 1 obd-1 scanner. holler back with what you find out.

 

It's a 1990 Cherokee, it uses the Renix engine management system and will not store codes. You might have better luck posting this in the OEM Cherokee forum, there are a lot of Renix guys (Cruiser54) that can help you out over there.

 

What plugs did it have and what plugs did you replace them with?

Some motors are picky. if you run too hot of a plug, like Bosch +4 for example, they can run to hot and the management system dumps to much fuel trying to make up for the spark. If it had platinum plus, and you put coppers in, it can run rich.

 

So your putting gas in it twice a day? that is nuts. I would say at the very minimum the oil is so dilluted as to be worthless. First thing I would check would be the coolant temperature sensor.

 

Sorry i left out engine and jeep info. Straight 6 - 4.0 jeep larado. I did gap the plugs and they are the basic a/c delco plugs. wires are back where they came off of (did one at a time).
I do have concerns with vaccum issues but need a map for the vaccum lines. Anything that might have been on hood or firewall is gone.
I will try and get to autozone to get a code check but no engine light to this point.
Thanks for your input so far

 

IMHO, use Champion in Mopars, AC in GM, and Autolite in Ford. Also, check your CCV valve. It may have been damaged and leaking air after the gasket change.