terzdesign

My check engine light came on. I went to advanced auto parts and had them read the code. It was P0138 which I guess is a downstream 02 sensor. After driving for another hour the check light went back off. Mind you it has been very cold here so I would expect ghosts in the system.

All that aside, I just did my first fuel economy test yesterday from fill up to empty. I drove like a granny and on top of that 2/3's of those miles on my last tank were straight highway. I am only getting 15.5 mpg! I don't expect miracles with a 99 Jeep but seems like it should be around the 18 - 20 mark. How much of this can I attribute to a quasi malfunctioning o2 sensor? should I just replace it and see what happens? I Would like to improve the economy to some level that others who drive stock see.

tjwalker

The downstream oxygen sensor does have some, but minimal effect on fuel mileage. I thought it had no effect until I saw some code from Jeep showing that it is used in optimizing fuel/air ratio. The downstream oxygen sensor's primary purpose however is to measure the efficiency of the catalytic converter.

This CEL/code is likely to return; you are on the front end of the failure. I replaced my oxygen sensors on my 99 at about 150k when I experienced the P0138 code.

My advice to you is to replace the downstream sensor and also replace the upstream sensor. With one failing, the other is not far behind. Do em both at the same time if you plan on keeping the vehicle.

I recommend using NTK replacement oxygen sensors. This is the brand that came with your XJ off the factory line. NTK is widely available and not that expensive. Stay away from el-cheapo oxygen sensors, especially the no-name universal crap that you often see on ebay.

BTW, 15.5 mpg is not that unusual in cold weather. I see that on my bone stock 99 in the dead of winter in MN. To optimize your gas mileage, get new oxygen sensors and also fresh tuneup hardware if it's been a while. Champion copper plugs gapped to .035, quality plug wires, distributor cap and rotor.

Enjoy your 99; it is a very good year for the XJ!

kjc

X2 what tjwalker said.. sage advice! I think most people ignore O2 sensors until they're well beyond their useful life. Reading these posts, I get the idea that a lot of us fail miserably when it comes to even the most basic 'scheduled maintenance' items. How many posts have you seen about "my coolant is brown" and i'm having overheating issues? Look at that maintenance schedule booklet just taking up space in the glove box. Change every two years!! HA! Who does that?? I'll tell ya who.. the guys with pretty green coolant and no worries runnin' the ac in the summer.. on the freeway.. constantly stopping for no apparent reason!! What the hell's wrong with you people up there?? it's a freeway!! get off the brakes!!! Why are you stopp.. Oh.. sorry.. I kinda went off track there for a minute. Where was I? Oh ya, what tjwalker said.. good advice!

terzdesign

whats the benefit to replacing the spark plug wires themselves? do they typically get shorts? Also, my jeep has coil packs. I'm not the most electrically inclined person, but that means there is no distributor correct? Should I replace those coils though?

tjwalker

Wow, your 99 has the coil pack and no distributor?? I've heard rumors of a few of the very, very late mfg. 99's possibly having this ignition setup but haven't seen much for confirmation. The vast majority of 99s have a conventional ignition with a distributor. You're sure there is no distributor?

If you have a coil pack ignition, then just change your plugs. I've had the best luck with NGK copper plugs for this ignition system. Be sure plug gap is verified at .035.

terzdesign

Isn't that rail piece that runs parallel to the valve cover on the right where the coil packs are?

tssguy123

Er, no.

If you look carefully you can see the top of the distributor as well as plug wires.

terzdesign

ha. Yup, I guess I just assumed. I've only been under the hood of this thing a couple of time so far. Still, what's the point in changing the wires?

tssguy123

The contacts at the end of the wires tend to get a bit corroded and just mechanically fail over time. Engine bays are subject to quite a lot of vibrations, and if you ever take the time to look inside of the end of a plug wire, you will notice that it's a thin piece of metal that clamps around the nubs on the distributor as well as the end of the spark plugs.

terzdesign

thank you TSSguy. Nice pic. I haven't opened the hood much since it's winter. Plus, I've only had the vehicle for a month. Come spring I will replace the sensors and such.

Aside from the cap, rotor, wires and plugs, (and sensors) what else would be good to 'tune up' after this b$(ch of a winter ends?

tssguy123

Do a few drains/fills of the Transmission fluid (make sure to use Dex III/Merc), oil change, diff fluid, coolant flush... possibly o2 sensors depending on how many miles the XJ has and if they've been replaced. I'd say go ahead and do new O2s if you're 120-150k+... Clean the IAC and throttle body... can't think of anything other than that, maybe somebody else could throw in some.